# Copyright (c) 2003-2016 LOGILAB S.A. (Paris, FRANCE).
# http://www.logilab.fr/ -- mailto:contact@logilab.fr
# Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
# For details: https://github.com/PyCQA/pylint/blob/master/COPYING

"""basic checker for Python code"""

import collections
import itertools
import sys
import re

import six
from six.moves import zip  # pylint: disable=redefined-builtin

import astroid
import astroid.bases
import astroid.scoped_nodes
from astroid import are_exclusive, InferenceError

from pylint.interfaces import (IAstroidChecker, ITokenChecker, INFERENCE,
                               INFERENCE_FAILURE, HIGH)
from pylint.utils import EmptyReport, deprecated_option
from pylint.reporters import diff_string
from pylint.checkers import BaseChecker, BaseTokenChecker
from pylint.checkers.utils import (
    check_messages,
    clobber_in_except,
    is_builtin_object,
    is_inside_except,
    overrides_a_method,
    get_argument_from_call,
    node_frame_class,
    NoSuchArgumentError,
    error_of_type,
    unimplemented_abstract_methods,
    has_known_bases,
    safe_infer,
    is_comprehension
    )
from pylint.reporters.ureports.nodes import Table


# regex for class/function/variable/constant name
CLASS_NAME_RGX = re.compile('[A-Z_][a-zA-Z0-9]+$')
MOD_NAME_RGX = re.compile('(([a-z_][a-z0-9_]*)|([A-Z][a-zA-Z0-9]+))$')
CONST_NAME_RGX = re.compile('(([A-Z_][A-Z0-9_]*)|(__.*__))$')
COMP_VAR_RGX = re.compile('[A-Za-z_][A-Za-z0-9_]*$')
DEFAULT_NAME_RGX = re.compile('[a-z_][a-z0-9_]{2,30}$')
CLASS_ATTRIBUTE_RGX = re.compile(r'([A-Za-z_][A-Za-z0-9_]{2,30}|(__.*__))$')
# do not require a doc string on private/system methods
NO_REQUIRED_DOC_RGX = re.compile('^_')
REVERSED_PROTOCOL_METHOD = '__reversed__'
SEQUENCE_PROTOCOL_METHODS = ('__getitem__', '__len__')
REVERSED_METHODS = (SEQUENCE_PROTOCOL_METHODS,
                    (REVERSED_PROTOCOL_METHOD, ))
TYPECHECK_COMPARISON_OPERATORS = frozenset(('is', 'is not', '==',
                                            '!=', 'in', 'not in'))
LITERAL_NODE_TYPES = (astroid.Const, astroid.Dict, astroid.List, astroid.Set)
UNITTEST_CASE = 'unittest.case'
BUILTINS = six.moves.builtins.__name__
TYPE_QNAME = "%s.type" % BUILTINS
PY33 = sys.version_info >= (3, 3)
PY3K = sys.version_info >= (3, 0)
PY35 = sys.version_info >= (3, 5)

# Name categories that are always consistent with all naming conventions.
EXEMPT_NAME_CATEGORIES = set(('exempt', 'ignore'))

# A mapping from builtin-qname -> symbol, to be used when generating messages
# about dangerous default values as arguments
DEFAULT_ARGUMENT_SYMBOLS = dict(
    zip(['.'.join([BUILTINS, x]) for x in ('set', 'dict', 'list')],
        ['set()', '{}', '[]'])
)
REVERSED_COMPS = {'<': '>', '<=': '>=', '>': '<', '>=': '<='}

del re

def _redefines_import(node):
    """ Detect that the given node (AssName) is inside an
    exception handler and redefines an import from the tryexcept body.
    Returns True if the node redefines an import, False otherwise.
    """
    current = node
    while current and not isinstance(current.parent, astroid.ExceptHandler):
        current = current.parent
    if not current or not error_of_type(current.parent, ImportError):
        return False
    try_block = current.parent.parent
    for import_node in try_block.nodes_of_class((astroid.ImportFrom, astroid.Import)):
        for name, alias in import_node.names:
            if alias:
                if alias == node.name:
                    return True
            elif name == node.name:
                return True
    return False

def in_loop(node):
    """return True if the node is inside a kind of for loop"""
    parent = node.parent
    while parent is not None:
        if isinstance(parent, (astroid.For, astroid.ListComp, astroid.SetComp,
                               astroid.DictComp, astroid.GeneratorExp)):
            return True
        parent = parent.parent
    return False

def in_nested_list(nested_list, obj):
    """return true if the object is an element of <nested_list> or of a nested
    list
    """
    for elmt in nested_list:
        if isinstance(elmt, (list, tuple)):
            if in_nested_list(elmt, obj):
                return True
        elif elmt == obj:
            return True
    return False

def _loop_exits_early(loop):
    """Returns true if a loop has a break statement in its body."""
    loop_nodes = (astroid.For, astroid.While)
    # Loop over body explicitly to avoid matching break statements
    # in orelse.
    for child in loop.body:
        if isinstance(child, loop_nodes):
            # break statement may be in orelse of child loop.
            # pylint: disable=superfluous-parens
            for orelse in (child.orelse or ()):
                for _ in orelse.nodes_of_class(astroid.Break, skip_klass=loop_nodes):
                    return True
            continue
        for _ in child.nodes_of_class(astroid.Break, skip_klass=loop_nodes):
            return True
    return False

def _is_multi_naming_match(match, node_type, confidence):
    return (match is not None and
            match.lastgroup is not None and
            match.lastgroup not in EXEMPT_NAME_CATEGORIES
            and (node_type != 'method' or confidence != INFERENCE_FAILURE))


if sys.version_info < (3, 0):
    BUILTIN_PROPERTY = '__builtin__.property'
else:
    BUILTIN_PROPERTY = 'builtins.property'


def _get_properties(config):
    """Returns a tuple of property classes and names.

    Property classes are fully qualified, such as 'abc.abstractproperty' and
    property names are the actual names, such as 'abstract_property'.
    """
    property_classes = set((BUILTIN_PROPERTY,))
    property_names = set()  # Not returning 'property', it has its own check.
    if config is not None:
        property_classes.update(config.property_classes)
        property_names.update((prop.rsplit('.', 1)[-1]
                               for prop in config.property_classes))
    return property_classes, property_names


def _determine_function_name_type(node, config=None):
    """Determine the name type whose regex the a function's name should match.

    :param node: A function node.
    :type node: astroid.node_classes.NodeNG
    :param config: Configuration from which to pull additional property classes.
    :type config: :class:`optparse.Values`

    :returns: One of ('function', 'method', 'attr')
    :rtype: str
    """
    property_classes, property_names = _get_properties(config)
    if not node.is_method():
        return 'function'
    if node.decorators:
        decorators = node.decorators.nodes
    else:
        decorators = []
    for decorator in decorators:
        # If the function is a property (decorated with @property
        # or @abc.abstractproperty), the name type is 'attr'.
        if (isinstance(decorator, astroid.Name) or
                (isinstance(decorator, astroid.Attribute) and
                 decorator.attrname in property_names)):
            infered = safe_infer(decorator)
            if infered and infered.qname() in property_classes:
                return 'attr'
        # If the function is decorated using the prop_method.{setter,getter}
        # form, treat it like an attribute as well.
        elif (isinstance(decorator, astroid.Attribute) and
              decorator.attrname in ('setter', 'deleter')):
            return 'attr'
    return 'method'


def _is_none(node):
    return (node is None or
            (isinstance(node, astroid.Const) and node.value is None) or
            (isinstance(node, astroid.Name)  and node.name == 'None')
           )


def _has_abstract_methods(node):
    """
    Determine if the given `node` has abstract methods.

    The methods should be made abstract by decorating them
    with `abc` decorators.
    """
    return len(unimplemented_abstract_methods(node)) > 0


def report_by_type_stats(sect, stats, old_stats):
    """make a report of

    * percentage of different types documented
    * percentage of different types with a bad name
    """
    # percentage of different types documented and/or with a bad name
    nice_stats = {}
    for node_type in ('module', 'class', 'method', 'function'):
        try:
            total = stats[node_type]
        except KeyError:
            raise EmptyReport()
        nice_stats[node_type] = {}
        if total != 0:
            try:
                documented = total - stats['undocumented_'+node_type]
                percent = (documented * 100.) / total
                nice_stats[node_type]['percent_documented'] = '%.2f' % percent
            except KeyError:
                nice_stats[node_type]['percent_documented'] = 'NC'
            try:
                percent = (stats['badname_'+node_type] * 100.) / total
                nice_stats[node_type]['percent_badname'] = '%.2f' % percent
            except KeyError:
                nice_stats[node_type]['percent_badname'] = 'NC'
    lines = ('type', 'number', 'old number', 'difference',
             '%documented', '%badname')
    for node_type in ('module', 'class', 'method', 'function'):
        new = stats[node_type]
        old = old_stats.get(node_type, None)
        if old is not None:
            diff_str = diff_string(old, new)
        else:
            old, diff_str = 'NC', 'NC'
        lines += (node_type, str(new), str(old), diff_str,
                  nice_stats[node_type].get('percent_documented', '0'),
                  nice_stats[node_type].get('percent_badname', '0'))
    sect.append(Table(children=lines, cols=6, rheaders=1))

def redefined_by_decorator(node):
    """return True if the object is a method redefined via decorator.

    For example:
        @property
        def x(self): return self._x
        @x.setter
        def x(self, value): self._x = value
    """
    if node.decorators:
        for decorator in node.decorators.nodes:
            if (isinstance(decorator, astroid.Attribute) and
                    getattr(decorator.expr, 'name', None) == node.name):
                return True
    return False


def _node_type(node):
    """Return the inferred type for `node`

    If there is more than one possible type, or if inferred type is YES or None,
    return None
    """
    # check there is only one possible type for the assign node. Else we
    # don't handle it for now
    types = set()
    try:
        for var_type in node.infer():
            if var_type == astroid.YES or _is_none(var_type):
                continue
            types.add(var_type)
            if len(types) > 1:
                return
    except InferenceError:
        return
    return types.pop() if types else None


class _BasicChecker(BaseChecker):
    __implements__ = IAstroidChecker
    name = 'basic'

class BasicErrorChecker(_BasicChecker):
    msgs = {
        'E0100': ('__init__ method is a generator',
                  'init-is-generator',
                  'Used when the special class method __init__ is turned into a '
                  'generator by a yield in its body.'),
        'E0101': ('Explicit return in __init__',
                  'return-in-init',
                  'Used when the special class method __init__ has an explicit '
                  'return value.'),
        'E0102': ('%s already defined line %s',
                  'function-redefined',
                  'Used when a function / class / method is redefined.'),
        'E0103': ('%r not properly in loop',
                  'not-in-loop',
                  'Used when break or continue keywords are used outside a loop.'),
        'E0104': ('Return outside function',
                  'return-outside-function',
                  'Used when a "return" statement is found outside a function or '
                  'method.'),
        'E0105': ('Yield outside function',
                  'yield-outside-function',
                  'Used when a "yield" statement is found outside a function or '
                  'method.'),
        'E0106': ('Return with argument inside generator',
                  'return-arg-in-generator',
                  'Used when a "return" statement with an argument is found '
                  'outside in a generator function or method (e.g. with some '
                  '"yield" statements).',
                  {'maxversion': (3, 3)}),
        'E0107': ("Use of the non-existent %s operator",
                  'nonexistent-operator',
                  "Used when you attempt to use the C-style pre-increment or"
                  "pre-decrement operator -- and ++, which doesn't exist in Python."),
        'E0108': ('Duplicate argument name %s in function definition',
                  'duplicate-argument-name',
                  'Duplicate argument names in function definitions are syntax'
                  ' errors.'),
        'E0110': ('Abstract class %r with abstract methods instantiated',
                  'abstract-class-instantiated',
                  'Used when an abstract class with `abc.ABCMeta` as metaclass '
                  'has abstract methods and is instantiated.'),
        'W0120': ('Else clause on loop without a break statement',
                  'useless-else-on-loop',
                  'Loops should only have an else clause if they can exit early '
                  'with a break statement, otherwise the statements under else '
                  'should be on the same scope as the loop itself.'),
        'E0112': ('More than one starred expression in assignment',
                  'too-many-star-expressions',
                  'Emitted when there are more than one starred '
                  'expressions (`*x`) in an assignment. This is a SyntaxError.',
                  {'minversion': (3, 0)}),
        'E0113': ('Starred assignment target must be in a list or tuple',
                  'invalid-star-assignment-target',
                  'Emitted when a star expression is used as a starred '
                  'assignment target.',
                  {'minversion': (3, 0)}),
        'E0114': ('Can use starred expression only in assignment target',
                  'star-needs-assignment-target',
                  'Emitted when a star expression is not used in an '
                  'assignment target.',
                  {'minversion': (3, 0)}),
        'E0115': ('Name %r is nonlocal and global',
                  'nonlocal-and-global',
                  'Emitted when a name is both nonlocal and global.',
                  {'minversion': (3, 0)}),
        'E0116': ("'continue' not supported inside 'finally' clause",
                  'continue-in-finally',
                  'Emitted when the `continue` keyword is found '
                  'inside a finally clause, which is a SyntaxError.'),
        'E0117': ("nonlocal name %s found without binding",
                  'nonlocal-without-binding',
                  'Emitted when a nonlocal variable does not have an attached '
                  'name somewhere in the parent scopes',
                  {'minversion': (3, 0)}),
        }

    @check_messages('function-redefined')
    def visit_classdef(self, node):
        self._check_redefinition('class', node)

    @check_messages('too-many-star-expressions',
                    'invalid-star-assignment-target')
    def visit_assign(self, node):
        starred = list(node.targets[0].nodes_of_class(astroid.Starred))
        if len(starred) > 1:
            self.add_message('too-many-star-expressions', node=node)

        # Check *a = b
        if isinstance(node.targets[0], astroid.Starred):
            self.add_message('invalid-star-assignment-target', node=node)

    @check_messages('star-needs-assignment-target')
    def visit_starred(self, node):
        """Check that a Starred expression is used in an assignment target."""
        if isinstance(node.parent, astroid.Call):
            # f(*args) is converted to Call(args=[Starred]), so ignore
            # them for this check.
            return
        if PY35 and isinstance(node.parent,
                               (astroid.List, astroid.Tuple,
                                astroid.Set, astroid.Dict)):
            # PEP 448 unpacking.
            return

        stmt = node.statement()
        if not isinstance(stmt, astroid.Assign):
            return

        if stmt.value is node or stmt.value.parent_of(node):
            self.add_message('star-needs-assignment-target', node=node)

    @check_messages('init-is-generator', 'return-in-init',
                    'function-redefined', 'return-arg-in-generator',
                    'duplicate-argument-name', 'nonlocal-and-global')
    def visit_functiondef(self, node):
        self._check_nonlocal_and_global(node)
        if not redefined_by_decorator(node):
            self._check_redefinition(node.is_method() and 'method' or 'function', node)
        # checks for max returns, branch, return in __init__
        returns = node.nodes_of_class(astroid.Return,
                                      skip_klass=(astroid.FunctionDef,
                                                  astroid.ClassDef))
        if node.is_method() and node.name == '__init__':
            if node.is_generator():
                self.add_message('init-is-generator', node=node)
            else:
                values = [r.value for r in returns]
                # Are we returning anything but None from constructors
                if [v for v in values if not _is_none(v)]:
                    self.add_message('return-in-init', node=node)
        elif node.is_generator():
            # make sure we don't mix non-None returns and yields
            if not PY33:
                for retnode in returns:
                    if isinstance(retnode.value, astroid.Const) and \
                           retnode.value.value is not None:
                        self.add_message('return-arg-in-generator', node=node,
                                         line=retnode.fromlineno)
        # Check for duplicate names
        args = set()
        for name in node.argnames():
            if name in args:
                self.add_message('duplicate-argument-name', node=node, args=(name,))
            else:
                args.add(name)

    visit_asyncfunctiondef = visit_functiondef

    def _check_nonlocal_and_global(self, node):
        """Check that a name is both nonlocal and global."""
        def same_scope(current):
            return current.scope() is node

        from_iter = itertools.chain.from_iterable
        nonlocals = set(from_iter(
            child.names for child in node.nodes_of_class(astroid.Nonlocal)
            if same_scope(child)))
        global_vars = set(from_iter(
            child.names for child in node.nodes_of_class(astroid.Global)
            if same_scope(child)))
        for name in nonlocals.intersection(global_vars):
            self.add_message('nonlocal-and-global',
                             args=(name, ), node=node)

    @check_messages('return-outside-function')
    def visit_return(self, node):
        if not isinstance(node.frame(), astroid.FunctionDef):
            self.add_message('return-outside-function', node=node)

    @check_messages('yield-outside-function')
    def visit_yield(self, node):
        self._check_yield_outside_func(node)

    @check_messages('yield-outside-function')
    def visit_yieldfrom(self, node):
        self._check_yield_outside_func(node)

    @check_messages('not-in-loop', 'continue-in-finally')
    def visit_continue(self, node):
        self._check_in_loop(node, 'continue')

    @check_messages('not-in-loop')
    def visit_break(self, node):
        self._check_in_loop(node, 'break')

    @check_messages('useless-else-on-loop')
    def visit_for(self, node):
        self._check_else_on_loop(node)

    @check_messages('useless-else-on-loop')
    def visit_while(self, node):
        self._check_else_on_loop(node)

    @check_messages('nonexistent-operator')
    def visit_unaryop(self, node):
        """check use of the non-existent ++ and -- operator operator"""
        if ((node.op in '+-') and
                isinstance(node.operand, astroid.UnaryOp) and
                (node.operand.op == node.op)):
            self.add_message('nonexistent-operator', node=node, args=node.op*2)

    def _check_nonlocal_without_binding(self, node, name):
        current_scope = node.scope()
        while True:
            if current_scope.parent is None:
                break

            if not isinstance(current_scope, astroid.FunctionDef):
                self.add_message('nonlocal-without-binding', args=(name, ),
                                 node=node)
                return
            else:
                if name not in current_scope.locals:
                    current_scope = current_scope.parent.scope()
                    continue
                else:
                    # Okay, found it.
                    return

        self.add_message('nonlocal-without-binding', args=(name, ), node=node)

    @check_messages('nonlocal-without-binding')
    def visit_nonlocal(self, node):
        for name in node.names:
            self._check_nonlocal_without_binding(node, name)

    @check_messages('abstract-class-instantiated')
    def visit_call(self, node):
        """ Check instantiating abstract class with
        abc.ABCMeta as metaclass.
        """
        try:
            infered = next(node.func.infer())
        except astroid.InferenceError:
            return

        if not isinstance(infered, astroid.ClassDef):
            return

        klass = node_frame_class(node)
        if klass is infered:
            # Don't emit the warning if the class is instantiated
            # in its own body or if the call is not an instance
            # creation. If the class is instantiated into its own
            # body, we're expecting that it knows what it is doing.
            return

        # __init__ was called
        metaclass = infered.metaclass()
        abstract_methods = _has_abstract_methods(infered)
        if metaclass is None:
            # Python 3.4 has `abc.ABC`, which won't be detected
            # by ClassNode.metaclass()
            for ancestor in infered.ancestors():
                if ancestor.qname() == 'abc.ABC' and abstract_methods:
                    self.add_message('abstract-class-instantiated',
                                     args=(infered.name, ),
                                     node=node)
                    break
            return
        if metaclass.qname() == 'abc.ABCMeta' and abstract_methods:
            self.add_message('abstract-class-instantiated',
                             args=(infered.name, ),
                             node=node)

    def _check_yield_outside_func(self, node):
        if not isinstance(node.frame(), (astroid.FunctionDef, astroid.Lambda)):
            self.add_message('yield-outside-function', node=node)

    def _check_else_on_loop(self, node):
        """Check that any loop with an else clause has a break statement."""
        if node.orelse and not _loop_exits_early(node):
            self.add_message('useless-else-on-loop', node=node,
                             # This is not optimal, but the line previous
                             # to the first statement in the else clause
                             # will usually be the one that contains the else:.
                             line=node.orelse[0].lineno - 1)

    def _check_in_loop(self, node, node_name):
        """check that a node is inside a for or while loop"""
        _node = node.parent
        while _node:
            if isinstance(_node, (astroid.For, astroid.While)):
                if node not in _node.orelse:
                    return

            if isinstance(_node, (astroid.ClassDef, astroid.FunctionDef)):
                break
            if (isinstance(_node, astroid.TryFinally)
                    and node in _node.finalbody
                    and isinstance(node, astroid.Continue)):
                self.add_message('continue-in-finally', node=node)

            _node = _node.parent

        self.add_message('not-in-loop', node=node, args=node_name)

    def _check_redefinition(self, redeftype, node):
        """check for redefinition of a function / method / class name"""
        defined_self = node.parent.frame()[node.name]
        if defined_self is not node and not are_exclusive(node, defined_self):
            self.add_message('function-redefined', node=node,
                             args=(redeftype, defined_self.fromlineno))



class BasicChecker(_BasicChecker):
    """checks for :
    * doc strings
    * number of arguments, local variables, branches, returns and statements in
functions, methods
    * required module attributes
    * dangerous default values as arguments
    * redefinition of function / method / class
    * uses of the global statement
    """

    __implements__ = IAstroidChecker

    name = 'basic'
    msgs = {
        'W0101': ('Unreachable code',
                  'unreachable',
                  'Used when there is some code behind a "return" or "raise" '
                  'statement, which will never be accessed.'),
        'W0102': ('Dangerous default value %s as argument',
                  'dangerous-default-value',
                  'Used when a mutable value as list or dictionary is detected in '
                  'a default value for an argument.'),
        'W0104': ('Statement seems to have no effect',
                  'pointless-statement',
                  'Used when a statement doesn\'t have (or at least seems to) '
                  'any effect.'),
        'W0105': ('String statement has no effect',
                  'pointless-string-statement',
                  'Used when a string is used as a statement (which of course '
                  'has no effect). This is a particular case of W0104 with its '
                  'own message so you can easily disable it if you\'re using '
                  'those strings as documentation, instead of comments.'),
        'W0106': ('Expression "%s" is assigned to nothing',
                  'expression-not-assigned',
                  'Used when an expression that is not a function call is assigned '
                  'to nothing. Probably something else was intended.'),
        'W0108': ('Lambda may not be necessary',
                  'unnecessary-lambda',
                  'Used when the body of a lambda expression is a function call '
                  'on the same argument list as the lambda itself; such lambda '
                  'expressions are in all but a few cases replaceable with the '
                  'function being called in the body of the lambda.'),
        'W0109': ("Duplicate key %r in dictionary",
                  'duplicate-key',
                  'Used when a dictionary expression binds the same key multiple '
                  'times.'),
        'W0122': ('Use of exec',
                  'exec-used',
                  'Used when you use the "exec" statement (function for Python '
                  '3), to discourage its usage. That doesn\'t '
                  'mean you can not use it !'),
        'W0123': ('Use of eval',
                  'eval-used',
                  'Used when you use the "eval" function, to discourage its '
                  'usage. Consider using `ast.literal_eval` for safely evaluating '
                  'strings containing Python expressions '
                  'from untrusted sources. '),
        'W0150': ("%s statement in finally block may swallow exception",
                  'lost-exception',
                  'Used when a break or a return statement is found inside the '
                  'finally clause of a try...finally block: the exceptions raised '
                  'in the try clause will be silently swallowed instead of being '
                  're-raised.'),
        'W0199': ('Assert called on a 2-uple. Did you mean \'assert x,y\'?',
                  'assert-on-tuple',
                  'A call of assert on a tuple will always evaluate to true if '
                  'the tuple is not empty, and will always evaluate to false if '
                  'it is.'),
        'W0124': ('Following "as" with another context manager looks like a tuple.',
                  'confusing-with-statement',
                  'Emitted when a `with` statement component returns multiple values '
                  'and uses name binding with `as` only for a part of those values, '
                  'as in with ctx() as a, b. This can be misleading, since it\'s not '
                  'clear if the context manager returns a tuple or if the node without '
                  'a name binding is another context manager.'),
        'W0125': ('Using a conditional statement with a constant value',
                  'using-constant-test',
                  'Emitted when a conditional statement (If or ternary if) '
                  'uses a constant value for its test. This might not be what '
                  'the user intended to do.'),
        'E0111': ('The first reversed() argument is not a sequence',
                  'bad-reversed-sequence',
                  'Used when the first argument to reversed() builtin '
                  'isn\'t a sequence (does not implement __reversed__, '
                  'nor __getitem__ and __len__'),

    }

    options = (('required-attributes',
                deprecated_option(opt_type='csv',
                                  help_msg="Required attributes for module. "
                                           "This option is obsolete.",
                                  deprecation_msg="This option %r will be removed in Pylint 2.0")),
              )
    reports = (('RP0101', 'Statistics by type', report_by_type_stats),)

    def __init__(self, linter):
        _BasicChecker.__init__(self, linter)
        self.stats = None
        self._tryfinallys = None

    def open(self):
        """initialize visit variables and statistics
        """
        self._tryfinallys = []
        self.stats = self.linter.add_stats(module=0, function=0,
                                           method=0, class_=0)

    @check_messages('using-constant-test')
    def visit_if(self, node):
        self._check_using_constant_test(node, node.test)

    @check_messages('using-constant-test')
    def visit_ifexp(self, node):
        self._check_using_constant_test(node, node.test)

    @check_messages('using-constant-test')
    def visit_comprehension(self, node):
        if node.ifs:
            for if_test in node.ifs:
                self._check_using_constant_test(node, if_test)

    def _check_using_constant_test(self, node, test):
        const_nodes = (
            astroid.Module,
            astroid.scoped_nodes.GeneratorExp,
            astroid.Lambda, astroid.FunctionDef, astroid.ClassDef,
            astroid.bases.Generator, astroid.UnboundMethod,
            astroid.BoundMethod, astroid.Module)
        structs = (astroid.Dict, astroid.Tuple, astroid.Set)

        # These nodes are excepted, since they are not constant
        # values, requiring a computation to happen. The only type
        # of node in this list which doesn't have this property is
        # Getattr, which is excepted because the conditional statement
        # can be used to verify that the attribute was set inside a class,
        # which is definitely a valid use case.
        except_nodes = (astroid.Attribute, astroid.Call,
                        astroid.BinOp, astroid.BoolOp, astroid.UnaryOp,
                        astroid.Subscript)
        inferred = None
        emit = isinstance(test, (astroid.Const, ) + structs + const_nodes)
        if not isinstance(test, except_nodes):
            inferred = safe_infer(test)

        if emit or isinstance(inferred, const_nodes):
            self.add_message('using-constant-test', node=node)

    def visit_module(self, _):
        """check module name, docstring and required arguments
        """
        self.stats['module'] += 1

    def visit_classdef(self, node): # pylint: disable=unused-argument
        """check module name, docstring and redefinition
        increment branch counter
        """
        self.stats['class'] += 1

    @check_messages('pointless-statement', 'pointless-string-statement',
                    'expression-not-assigned')
    def visit_expr(self, node):
        """check for various kind of statements without effect"""
        expr = node.value
        if isinstance(expr, astroid.Const) and isinstance(expr.value,
                                                          six.string_types):
            # treat string statement in a separated message
            # Handle PEP-257 attribute docstrings.
            # An attribute docstring is defined as being a string right after
            # an assignment at the module level, class level or __init__ level.
            scope = expr.scope()
            if isinstance(scope, (astroid.ClassDef, astroid.Module, astroid.FunctionDef)):
                if isinstance(scope, astroid.FunctionDef) and scope.name != '__init__':
                    pass
                else:
                    sibling = expr.previous_sibling()
                    if (sibling is not None and sibling.scope() is scope and
                            isinstance(sibling, astroid.Assign)):
                        return
            self.add_message('pointless-string-statement', node=node)
            return
        # ignore if this is :
        # * a direct function call
        # * the unique child of a try/except body
        # * a yield (which are wrapped by a discard node in _ast XXX)
        # warn W0106 if we have any underlying function call (we can't predict
        # side effects), else pointless-statement
        if (isinstance(expr, (astroid.Yield, astroid.Await, astroid.Call)) or
                (isinstance(node.parent, astroid.TryExcept) and
                 node.parent.body == [node])):
            return
        if any(expr.nodes_of_class(astroid.Call)):
            self.add_message('expression-not-assigned', node=node,
                             args=expr.as_string())
        else:
            self.add_message('pointless-statement', node=node)

    @staticmethod
    def _filter_vararg(node, call_args):
        # Return the arguments for the given call which are
        # not passed as vararg.
        for arg in call_args:
            if isinstance(arg, astroid.Starred):
                if (isinstance(arg.value, astroid.Name)
                        and arg.value.name != node.args.vararg):
                    yield arg
            else:
                yield arg

    @staticmethod
    def _has_variadic_argument(args, variadic_name):
        if not args:
            return True
        for arg in args:
            if isinstance(arg.value, astroid.Name):
                if arg.value.name != variadic_name:
                    return True
            else:
                return True
        return False

    @check_messages('unnecessary-lambda')
    def visit_lambda(self, node):
        """check whether or not the lambda is suspicious
        """
        # if the body of the lambda is a call expression with the same
        # argument list as the lambda itself, then the lambda is
        # possibly unnecessary and at least suspicious.
        if node.args.defaults:
            # If the arguments of the lambda include defaults, then a
            # judgment cannot be made because there is no way to check
            # that the defaults defined by the lambda are the same as
            # the defaults defined by the function called in the body
            # of the lambda.
            return
        call = node.body
        if not isinstance(call, astroid.Call):
            # The body of the lambda must be a function call expression
            # for the lambda to be unnecessary.
            return
        if (isinstance(node.body.func, astroid.Attribute) and
                isinstance(node.body.func.expr, astroid.Call)):
            # Chained call, the intermediate call might
            # return something else (but we don't check that, yet).
            return

        ordinary_args = list(node.args.args)
        new_call_args = list(self._filter_vararg(node, call.args))
        if node.args.kwarg:
            if self._has_variadic_argument(call.kwargs, node.args.kwarg):
                return
        elif call.kwargs or call.keywords:
            return

        if node.args.vararg:
            if self._has_variadic_argument(call.starargs, node.args.vararg):
                return
        elif call.starargs:
            return

        # The "ordinary" arguments must be in a correspondence such that:
        # ordinary_args[i].name == call.args[i].name.
        if len(ordinary_args) != len(new_call_args):
            return
        for arg, passed_arg in zip(ordinary_args, new_call_args):
            if not isinstance(passed_arg, astroid.Name):
                return
            if arg.name != passed_arg.name:
                return

        self.add_message('unnecessary-lambda', line=node.fromlineno,
                         node=node)

    @check_messages('dangerous-default-value')
    def visit_functiondef(self, node):
        """check function name, docstring, arguments, redefinition,
        variable names, max locals
        """
        self.stats[node.is_method() and 'method' or 'function'] += 1
        self._check_dangerous_default(node)

    visit_asyncfunctiondef = visit_functiondef

    def _check_dangerous_default(self, node):
        # check for dangerous default values as arguments
        is_iterable = lambda n: isinstance(n, (astroid.List,
                                               astroid.Set,
                                               astroid.Dict))
        for default in node.args.defaults:
            try:
                value = next(default.infer())
            except astroid.InferenceError:
                continue

            if (isinstance(value, astroid.Instance) and
                    value.qname() in DEFAULT_ARGUMENT_SYMBOLS):

                if value is default:
                    msg = DEFAULT_ARGUMENT_SYMBOLS[value.qname()]
                elif isinstance(value, astroid.Instance) or is_iterable(value):
                    # We are here in the following situation(s):
                    #   * a dict/set/list/tuple call which wasn't inferred
                    #     to a syntax node ({}, () etc.). This can happen
                    #     when the arguments are invalid or unknown to
                    #     the inference.
                    #   * a variable from somewhere else, which turns out to be a list
                    #     or a dict.
                    if is_iterable(default):
                        msg = value.pytype()
                    elif isinstance(default, astroid.Call):
                        msg = '%s() (%s)' % (value.name, value.qname())
                    else:
                        msg = '%s (%s)' % (default.as_string(), value.qname())
                else:
                    # this argument is a name
                    msg = '%s (%s)' % (default.as_string(),
                                       DEFAULT_ARGUMENT_SYMBOLS[value.qname()])
                self.add_message('dangerous-default-value',
                                 node=node,
                                 args=(msg, ))

    @check_messages('unreachable', 'lost-exception')
    def visit_return(self, node):
        """1 - check is the node has a right sibling (if so, that's some
        unreachable code)
        2 - check is the node is inside the finally clause of a try...finally
        block
        """
        self._check_unreachable(node)
        # Is it inside final body of a try...finally bloc ?
        self._check_not_in_finally(node, 'return', (astroid.FunctionDef,))

    @check_messages('unreachable')
    def visit_continue(self, node):
        """check is the node has a right sibling (if so, that's some unreachable
        code)
        """
        self._check_unreachable(node)

    @check_messages('unreachable', 'lost-exception')
    def visit_break(self, node):
        """1 - check is the node has a right sibling (if so, that's some
        unreachable code)
        2 - check is the node is inside the finally clause of a try...finally
        block
        """
        # 1 - Is it right sibling ?
        self._check_unreachable(node)
        # 2 - Is it inside final body of a try...finally bloc ?
        self._check_not_in_finally(node, 'break', (astroid.For, astroid.While,))

    @check_messages('unreachable')
    def visit_raise(self, node):
        """check if the node has a right sibling (if so, that's some unreachable
        code)
        """
        self._check_unreachable(node)

    @check_messages('exec-used')
    def visit_exec(self, node):
        """just print a warning on exec statements"""
        self.add_message('exec-used', node=node)

    @check_messages('eval-used', 'exec-used', 'bad-reversed-sequence')
    def visit_call(self, node):
        """visit a CallFunc node -> check if this is not a blacklisted builtin
        call and check for * or ** use
        """
        if isinstance(node.func, astroid.Name):
            name = node.func.name
            # ignore the name if it's not a builtin (i.e. not defined in the
            # locals nor globals scope)
            if not (name in node.frame() or
                    name in node.root()):
                if name == 'exec':
                    self.add_message('exec-used', node=node)
                elif name == 'reversed':
                    self._check_reversed(node)
                elif name == 'eval':
                    self.add_message('eval-used', node=node)

    @check_messages('assert-on-tuple')
    def visit_assert(self, node):
        """check the use of an assert statement on a tuple."""
        if node.fail is None and isinstance(node.test, astroid.Tuple) and \
                len(node.test.elts) == 2:
            self.add_message('assert-on-tuple', node=node)

    @check_messages('duplicate-key')
    def visit_dict(self, node):
        """check duplicate key in dictionary"""
        keys = set()
        for k, _ in node.items:
            if isinstance(k, astroid.Const):
                key = k.value
                if key in keys:
                    self.add_message('duplicate-key', node=node, args=key)
                keys.add(key)

    def visit_tryfinally(self, node):
        """update try...finally flag"""
        self._tryfinallys.append(node)

    def leave_tryfinally(self, node): # pylint: disable=unused-argument
        """update try...finally flag"""
        self._tryfinallys.pop()

    def _check_unreachable(self, node):
        """check unreachable code"""
        unreach_stmt = node.next_sibling()
        if unreach_stmt is not None:
            self.add_message('unreachable', node=unreach_stmt)

    def _check_not_in_finally(self, node, node_name, breaker_classes=()):
        """check that a node is not inside a finally clause of a
        try...finally statement.
        If we found before a try...finally bloc a parent which its type is
        in breaker_classes, we skip the whole check."""
        # if self._tryfinallys is empty, we're not a in try...finally bloc
        if not self._tryfinallys:
            return
        # the node could be a grand-grand...-children of the try...finally
        _parent = node.parent
        _node = node
        while _parent and not isinstance(_parent, breaker_classes):
            if hasattr(_parent, 'finalbody') and _node in _parent.finalbody:
                self.add_message('lost-exception', node=node, args=node_name)
                return
            _node = _parent
            _parent = _node.parent

    def _check_reversed(self, node):
        """ check that the argument to `reversed` is a sequence """
        try:
            argument = safe_infer(get_argument_from_call(node, position=0))
        except NoSuchArgumentError:
            pass
        else:
            if argument is astroid.YES:
                return
            if argument is None:
                # Nothing was infered.
                # Try to see if we have iter().
                if isinstance(node.args[0], astroid.Call):
                    try:
                        func = next(node.args[0].func.infer())
                    except InferenceError:
                        return
                    if (getattr(func, 'name', None) == 'iter' and
                            is_builtin_object(func)):
                        self.add_message('bad-reversed-sequence', node=node)
                return

            if isinstance(argument, astroid.Instance):
                if (argument._proxied.name == 'dict' and
                        is_builtin_object(argument._proxied)):
                    self.add_message('bad-reversed-sequence', node=node)
                    return
                elif any(ancestor.name == 'dict' and is_builtin_object(ancestor)
                         for ancestor in argument._proxied.ancestors()):
                    # Mappings aren't accepted by reversed(), unless
                    # they provide explicitly a __reversed__ method.
                    try:
                        argument.locals[REVERSED_PROTOCOL_METHOD]
                    except KeyError:
                        self.add_message('bad-reversed-sequence', node=node)
                    return

                for methods in REVERSED_METHODS:
                    for meth in methods:
                        try:
                            argument.getattr(meth)
                        except astroid.NotFoundError:
                            break
                    else:
                        break
                else:
                    self.add_message('bad-reversed-sequence', node=node)
            elif not isinstance(argument, (astroid.List, astroid.Tuple)):
                # everything else is not a proper sequence for reversed()
                self.add_message('bad-reversed-sequence', node=node)

    @check_messages('confusing-with-statement')
    def visit_with(self, node):
        if not PY3K:
            # in Python 2 a "with" statement with multiple managers coresponds
            # to multiple nested AST "With" nodes
            pairs = []
            parent_node = node.parent
            if isinstance(parent_node, astroid.With):
                # we only care about the direct parent, since this method
                # gets called for each with node anyway
                pairs.extend(parent_node.items)
            pairs.extend(node.items)
        else:
            # in PY3K a "with" statement with multiple managers coresponds
            # to one AST "With" node with multiple items
            pairs = node.items
        if pairs:
            for prev_pair, pair in zip(pairs, pairs[1:]):
                if (isinstance(prev_pair[1], astroid.AssignName) and
                        (pair[1] is None and not isinstance(pair[0], astroid.Call))):
                    # don't emit a message if the second is a function call
                    # there's no way that can be mistaken for a name assignment
                    if PY3K or node.lineno == node.parent.lineno:
                        # if the line number doesn't match
                        # we assume it's a nested "with"
                        self.add_message('confusing-with-statement', node=node)


_NAME_TYPES = {
    'module': (MOD_NAME_RGX, 'module'),
    'const': (CONST_NAME_RGX, 'constant'),
    'class': (CLASS_NAME_RGX, 'class'),
    'function': (DEFAULT_NAME_RGX, 'function'),
    'method': (DEFAULT_NAME_RGX, 'method'),
    'attr': (DEFAULT_NAME_RGX, 'attribute'),
    'argument': (DEFAULT_NAME_RGX, 'argument'),
    'variable': (DEFAULT_NAME_RGX, 'variable'),
    'class_attribute': (CLASS_ATTRIBUTE_RGX, 'class attribute'),
    'inlinevar': (COMP_VAR_RGX, 'inline iteration'),
}

def _create_naming_options():
    name_options = []
    for name_type, (rgx, human_readable_name) in six.iteritems(_NAME_TYPES):
        name_type = name_type.replace('_', '-')
        name_options.append((
            '%s-rgx' % (name_type,),
            {'default': rgx, 'type': 'regexp', 'metavar': '<regexp>',
             'help': 'Regular expression matching correct %s names' % (human_readable_name,)}))
        name_options.append((
            '%s-name-hint' % (name_type,),
            {'default': rgx.pattern, 'type': 'string', 'metavar': '<string>',
             'help': 'Naming hint for %s names' % (human_readable_name,)}))
    return tuple(name_options)

class NameChecker(_BasicChecker):
    msgs = {
        'C0102': ('Black listed name "%s"',
                  'blacklisted-name',
                  'Used when the name is listed in the black list (unauthorized '
                  'names).'),
        'C0103': ('Invalid %s name "%s"%s',
                  'invalid-name',
                  'Used when the name doesn\'t match the regular expression '
                  'associated to its type (constant, variable, class...).'),
    }

    options = (('good-names',
                {'default' : ('i', 'j', 'k', 'ex', 'Run', '_'),
                 'type' :'csv', 'metavar' : '<names>',
                 'help' : 'Good variable names which should always be accepted,'
                          ' separated by a comma'}
               ),
               ('bad-names',
                {'default' : ('foo', 'bar', 'baz', 'toto', 'tutu', 'tata'),
                 'type' :'csv', 'metavar' : '<names>',
                 'help' : 'Bad variable names which should always be refused, '
                          'separated by a comma'}
               ),
               ('name-group',
                {'default' : (),
                 'type' :'csv', 'metavar' : '<name1:name2>',
                 'help' : ('Colon-delimited sets of names that determine each'
                           ' other\'s naming style when the name regexes'
                           ' allow several styles.')}
               ),
               ('include-naming-hint',
                {'default': False, 'type' : 'yn', 'metavar' : '<y_or_n>',
                 'help': 'Include a hint for the correct naming format with invalid-name'}
               ),
               ('property-classes',
                {'default': ('abc.abstractproperty',),
                 'type': 'csv',
                 'metavar': '<decorator names>',
                 'help': 'List of decorators that produce properties, such as '
                         'abc.abstractproperty. Add to this list to register '
                         'other decorators that produce valid properties.'}
               ),
              ) + _create_naming_options()


    def __init__(self, linter):
        _BasicChecker.__init__(self, linter)
        self._name_category = {}
        self._name_group = {}
        self._bad_names = {}

    def open(self):
        self.stats = self.linter.add_stats(badname_module=0,
                                           badname_class=0, badname_function=0,
                                           badname_method=0, badname_attr=0,
                                           badname_const=0,
                                           badname_variable=0,
                                           badname_inlinevar=0,
                                           badname_argument=0,
                                           badname_class_attribute=0)
        for group in self.config.name_group:
            for name_type in group.split(':'):
                self._name_group[name_type] = 'group_%s' % (group,)

    @check_messages('blacklisted-name', 'invalid-name')
    def visit_module(self, node):
        self._check_name('module', node.name.split('.')[-1], node)
        self._bad_names = {}

    def leave_module(self, node): # pylint: disable=unused-argument
        for all_groups in six.itervalues(self._bad_names):
            if len(all_groups) < 2:
                continue
            groups = collections.defaultdict(list)
            min_warnings = sys.maxsize
            for group in six.itervalues(all_groups):
                groups[len(group)].append(group)
                min_warnings = min(len(group), min_warnings)
            if len(groups[min_warnings]) > 1:
                by_line = sorted(groups[min_warnings],
                                 key=lambda group: min(warning[0].lineno for warning in group))
                warnings = itertools.chain(*by_line[1:])
            else:
                warnings = groups[min_warnings][0]
            for args in warnings:
                self._raise_name_warning(*args)

    @check_messages('blacklisted-name', 'invalid-name')
    def visit_classdef(self, node):
        self._check_name('class', node.name, node)
        for attr, anodes in six.iteritems(node.instance_attrs):
            if not any(node.instance_attr_ancestors(attr)):
                self._check_name('attr', attr, anodes[0])

    @check_messages('blacklisted-name', 'invalid-name')
    def visit_functiondef(self, node):
        # Do not emit any warnings if the method is just an implementation
        # of a base class method.
        confidence = HIGH
        if node.is_method():
            if overrides_a_method(node.parent.frame(), node.name):
                return
            confidence = (INFERENCE if has_known_bases(node.parent.frame())
                          else INFERENCE_FAILURE)

        self._check_name(_determine_function_name_type(node,
                                                       config=self.config),
                         node.name, node, confidence)
        # Check argument names
        args = node.args.args
        if args is not None:
            self._recursive_check_names(args, node)

    visit_asyncfunctiondef = visit_functiondef

    @check_messages('blacklisted-name', 'invalid-name')
    def visit_global(self, node):
        for name in node.names:
            self._check_name('const', name, node)

    @check_messages('blacklisted-name', 'invalid-name')
    def visit_assignname(self, node):
        """check module level assigned names"""
        frame = node.frame()
        ass_type = node.assign_type()
        if isinstance(ass_type, astroid.Comprehension):
            self._check_name('inlinevar', node.name, node)
        elif isinstance(frame, astroid.Module):
            if isinstance(ass_type, astroid.Assign) and not in_loop(ass_type):
                if isinstance(safe_infer(ass_type.value), astroid.ClassDef):
                    self._check_name('class', node.name, node)
                else:
                    if not _redefines_import(node):
                        # Don't emit if the name redefines an import
                        # in an ImportError except handler.
                        self._check_name('const', node.name, node)
            elif isinstance(ass_type, astroid.ExceptHandler):
                self._check_name('variable', node.name, node)
        elif isinstance(frame, astroid.FunctionDef):
            # global introduced variable aren't in the function locals
            if node.name in frame and node.name not in frame.argnames():
                if not _redefines_import(node):
                    self._check_name('variable', node.name, node)
        elif isinstance(frame, astroid.ClassDef):
            if not list(frame.local_attr_ancestors(node.name)):
                self._check_name('class_attribute', node.name, node)

    def _recursive_check_names(self, args, node):
        """check names in a possibly recursive list <arg>"""
        for arg in args:
            if isinstance(arg, astroid.AssignName):
                self._check_name('argument', arg.name, node)
            else:
                self._recursive_check_names(arg.elts, node)

    def _find_name_group(self, node_type):
        return self._name_group.get(node_type, node_type)

    def _raise_name_warning(self, node, node_type, name, confidence):
        type_label = _NAME_TYPES[node_type][1]
        hint = ''
        if self.config.include_naming_hint:
            hint = ' (hint: %s)' % (getattr(self.config, node_type + '_name_hint'))
        self.add_message('invalid-name', node=node, args=(type_label, name, hint),
                         confidence=confidence)
        self.stats['badname_' + node_type] += 1

    def _check_name(self, node_type, name, node, confidence=HIGH):
        """check for a name using the type's regexp"""
        if is_inside_except(node):
            clobbering, _ = clobber_in_except(node)
            if clobbering:
                return
        if name in self.config.good_names:
            return
        if name in self.config.bad_names:
            self.stats['badname_' + node_type] += 1
            self.add_message('blacklisted-name', node=node, args=name)
            return
        regexp = getattr(self.config, node_type + '_rgx')
        match = regexp.match(name)

        if _is_multi_naming_match(match, node_type, confidence):
            name_group = self._find_name_group(node_type)
            bad_name_group = self._bad_names.setdefault(name_group, {})
            warnings = bad_name_group.setdefault(match.lastgroup, [])
            warnings.append((node, node_type, name, confidence))

        if match is None:
            self._raise_name_warning(node, node_type, name, confidence)


class DocStringChecker(_BasicChecker):
    msgs = {
        'C0111': ('Missing %s docstring', # W0131
                  'missing-docstring',
                  'Used when a module, function, class or method has no docstring.'
                  'Some special methods like __init__ doesn\'t necessary require a '
                  'docstring.'),
        'C0112': ('Empty %s docstring', # W0132
                  'empty-docstring',
                  'Used when a module, function, class or method has an empty '
                  'docstring (it would be too easy ;).'),
        }
    options = (('no-docstring-rgx',
                {'default' : NO_REQUIRED_DOC_RGX,
                 'type' : 'regexp', 'metavar' : '<regexp>',
                 'help' : 'Regular expression which should only match '
                          'function or class names that do not require a '
                          'docstring.'}
               ),
               ('docstring-min-length',
                {'default' : -1,
                 'type' : 'int', 'metavar' : '<int>',
                 'help': ('Minimum line length for functions/classes that'
                          ' require docstrings, shorter ones are exempt.')}
               ),
              )


    def open(self):
        self.stats = self.linter.add_stats(undocumented_module=0,
                                           undocumented_function=0,
                                           undocumented_method=0,
                                           undocumented_class=0)
    @check_messages('missing-docstring', 'empty-docstring')
    def visit_module(self, node):
        self._check_docstring('module', node)

    @check_messages('missing-docstring', 'empty-docstring')
    def visit_classdef(self, node):
        if self.config.no_docstring_rgx.match(node.name) is None:
            self._check_docstring('class', node)

    @staticmethod
    def _is_setter_or_deleter(node):
        names = {'setter', 'deleter'}
        for decorator in node.decorators.nodes:
            if (isinstance(decorator, astroid.Attribute)
                    and decorator.attrname in names):
                return True
        return False

    @check_messages('missing-docstring', 'empty-docstring')
    def visit_functiondef(self, node):
        if self.config.no_docstring_rgx.match(node.name) is None:
            ftype = node.is_method() and 'method' or 'function'
            if node.decorators and self._is_setter_or_deleter(node):
                return

            if isinstance(node.parent.frame(), astroid.ClassDef):
                overridden = False
                confidence = (INFERENCE if has_known_bases(node.parent.frame())
                              else INFERENCE_FAILURE)
                # check if node is from a method overridden by its ancestor
                for ancestor in node.parent.frame().ancestors():
                    if node.name in ancestor and \
                       isinstance(ancestor[node.name], astroid.FunctionDef):
                        overridden = True
                        break
                self._check_docstring(ftype, node,
                                      report_missing=not overridden,
                                      confidence=confidence)
            else:
                self._check_docstring(ftype, node)

    visit_asyncfunctiondef = visit_functiondef

    def _check_docstring(self, node_type, node, report_missing=True,
                         confidence=HIGH):
        """check the node has a non empty docstring"""
        docstring = node.doc
        if docstring is None:
            if not report_missing:
                return
            if node.body:
                lines = node.body[-1].lineno - node.body[0].lineno + 1
            else:
                lines = 0

            if node_type == 'module' and not lines:
                # If the module has no body, there's no reason
                # to require a docstring.
                return
            max_lines = self.config.docstring_min_length

            if node_type != 'module' and max_lines > -1 and lines < max_lines:
                return
            self.stats['undocumented_'+node_type] += 1
            if (node.body and isinstance(node.body[0], astroid.Expr) and
                    isinstance(node.body[0].value, astroid.Call)):
                # Most likely a string with a format call. Let's see.
                func = safe_infer(node.body[0].value.func)
                if (isinstance(func, astroid.BoundMethod)
                        and isinstance(func.bound, astroid.Instance)):
                    # Strings in Python 3, others in Python 2.
                    if PY3K and func.bound.name == 'str':
                        return
                    elif func.bound.name in ('str', 'unicode', 'bytes'):
                        return
            self.add_message('missing-docstring', node=node, args=(node_type,),
                             confidence=confidence)
        elif not docstring.strip():
            self.stats['undocumented_'+node_type] += 1
            self.add_message('empty-docstring', node=node, args=(node_type,),
                             confidence=confidence)


class PassChecker(_BasicChecker):
    """check if the pass statement is really necessary"""
    msgs = {'W0107': ('Unnecessary pass statement',
                      'unnecessary-pass',
                      'Used when a "pass" statement that can be avoided is '
                      'encountered.'),
           }
    @check_messages('unnecessary-pass')
    def visit_pass(self, node):
        if len(node.parent.child_sequence(node)) > 1:
            self.add_message('unnecessary-pass', node=node)


class LambdaForComprehensionChecker(_BasicChecker):
    """check for using a lambda where a comprehension would do.

    See <http://www.artima.com/weblogs/viewpost.jsp?thread=98196>
    where GvR says comprehensions would be clearer.
    """

    msgs = {'W0110': ('map/filter on lambda could be replaced by comprehension',
                      'deprecated-lambda',
                      'Used when a lambda is the first argument to "map" or '
                      '"filter". It could be clearer as a list '
                      'comprehension or generator expression.',
                      {'maxversion': (3, 0)}),
           }

    @check_messages('deprecated-lambda')
    def visit_call(self, node):
        """visit a CallFunc node, check if map or filter are called with a
        lambda
        """
        if not node.args:
            return
        if not isinstance(node.args[0], astroid.Lambda):
            return
        infered = safe_infer(node.func)
        if (is_builtin_object(infered)
                and infered.name in ['map', 'filter']):
            self.add_message('deprecated-lambda', node=node)


class RecommandationChecker(_BasicChecker):
    msgs = {'C0200': ('Consider using enumerate instead of iterating with range and len',
                      'consider-using-enumerate',
                      'Emitted when code that iterates with range and len is '
                      'encountered. Such code can be simplified by using the '
                      'enumerate builtin.'),
            'C0201': ('Consider iterating the dictionary directly instead of calling .keys()',
                      'consider-iterating-dictionary',
                      'Emitted when the keys of a dictionary are iterated through the .keys() '
                      'method. It is enough to just iterate through the dictionary itself, as '
                      'in "for key in dictionary".'),
           }

    @staticmethod
    def _is_builtin(node, function):
        inferred = safe_infer(node)
        if not inferred:
            return False
        return is_builtin_object(inferred) and inferred.name == function

    @check_messages('consider-iterating-dictionary')
    def visit_call(self, node):
        inferred = safe_infer(node.func)
        if inferred in (astroid.YES, None):
            return

        if not isinstance(inferred, astroid.BoundMethod):
            return
        if not isinstance(inferred.bound, astroid.Dict) or inferred.name != 'keys':
            return

        # Check if the statement is what we're expecting to have.
        statement = node.statement()
        if isinstance(statement, astroid.Expr):
            statement = statement.value

        if isinstance(statement, astroid.For) or is_comprehension(statement):
            self.add_message('consider-iterating-dictionary', node=node)

    @check_messages('consider-using-enumerate')
    def visit_for(self, node):
        """Emit a convention whenever range and len are used for indexing."""
        # Verify that we have a `range(len(...))` call and that the object
        # which is iterated is used as a subscript in the body of the for.

        # Is it a proper range call?
        if not isinstance(node.iter, astroid.Call):
            return
        if not self._is_builtin(node.iter.func, 'range'):
            return
        if len(node.iter.args) != 1:
            return

        # Is it a proper len call?
        if not isinstance(node.iter.args[0], astroid.Call):
            return
        second_func = node.iter.args[0].func
        if not self._is_builtin(second_func, 'len'):
            return
        len_args = node.iter.args[0].args
        if not len_args or len(len_args) != 1:
            return
        iterating_object = len_args[0]
        if not isinstance(iterating_object, astroid.Name):
            return

        # Verify that the body of the for loop uses a subscript
        # with the object that was iterated. This uses some heuristics
        # in order to make sure that the same object is used in the
        # for body.
        for child in node.body:
            for subscript in child.nodes_of_class(astroid.Subscript):
                if not isinstance(subscript.value, astroid.Name):
                    continue
                if not isinstance(subscript.slice, astroid.Index):
                    continue
                if not isinstance(subscript.slice.value, astroid.Name):
                    continue
                if subscript.slice.value.name != node.target.name:
                    continue
                if iterating_object.name != subscript.value.name:
                    continue
                if subscript.value.scope() != node.scope():
                    # Ignore this subscript if it's not in the same
                    # scope. This means that in the body of the for
                    # loop, another scope was created, where the same
                    # name for the iterating object was used.
                    continue
                self.add_message('consider-using-enumerate', node=node)
                return


def _is_one_arg_pos_call(call):
    """Is this a call with exactly 1 argument,
    where that argument is positional?
    """
    return (isinstance(call, astroid.Call)
            and len(call.args) == 1 and not call.keywords)


class ComparisonChecker(_BasicChecker):
    """Checks for comparisons

    - singleton comparison: 'expr == True', 'expr == False' and 'expr == None'
    - yoda condition: 'const "comp" right' where comp can be '==', '!=', '<',
      '<=', '>' or '>=', and right can be a variable, an attribute, a method or
      a function
    """
    msgs = {'C0121': ('Comparison to %s should be %s',
                      'singleton-comparison',
                      'Used when an expression is compared to singleton '
                      'values like True, False or None.'),
            'C0122': ('Comparison should be %s',
                      'misplaced-comparison-constant',
                      'Used when the constant is placed on the left side'
                      'of a comparison. It is usually clearer in intent to '
                      'place it in the right hand side of the comparison.'),
            'C0123': ('Using type() instead of isinstance() for a typecheck.',
                      'unidiomatic-typecheck',
                      'The idiomatic way to perform an explicit typecheck in '
                      'Python is to use isinstance(x, Y) rather than '
                      'type(x) == Y, type(x) is Y. Though there are unusual '
                      'situations where these give different results.',
                      {'old_names': [('W0154', 'unidiomatic-typecheck')]}),
           }

    def _check_singleton_comparison(self, singleton, root_node):
        if singleton.value is True:
            suggestion = "just 'expr' or 'expr is True'"
            self.add_message('singleton-comparison',
                             node=root_node,
                             args=(True, suggestion))
        elif singleton.value is False:
            suggestion = "'not expr' or 'expr is False'"
            self.add_message('singleton-comparison',
                             node=root_node,
                             args=(False, suggestion))
        elif singleton.value is None:
            self.add_message('singleton-comparison',
                             node=root_node,
                             args=(None, "'expr is None'"))

    def _check_misplaced_constant(self, node, left, right, operator):
        if isinstance(right, astroid.Const):
            return
        operator = REVERSED_COMPS.get(operator, operator)
        suggestion = '%s %s %r' % (right.as_string(), operator, left.value)
        self.add_message('misplaced-comparison-constant', node=node,
                         args=(suggestion,))

    @check_messages('singleton-comparison', 'misplaced-comparison-constant',
                    'unidiomatic-typecheck')
    def visit_compare(self, node):
        self._check_unidiomatic_typecheck(node)
        # NOTE: this checker only works with binary comparisons like 'x == 42'
        # but not 'x == y == 42'
        if len(node.ops) != 1:
            return
        left = node.left
        operator, right = node.ops[0]
        if (operator in ('<', '<=', '>', '>=', '!=', '==')
                and isinstance(left, astroid.Const)):
            self._check_misplaced_constant(node, left, right, operator)

        if operator == '==':
            if isinstance(left, astroid.Const):
                self._check_singleton_comparison(left, node)
            elif isinstance(right, astroid.Const):
                self._check_singleton_comparison(right, node)

    def _check_unidiomatic_typecheck(self, node):
        operator, right = node.ops[0]
        if operator in TYPECHECK_COMPARISON_OPERATORS:
            left = node.left
            if _is_one_arg_pos_call(left):
                self._check_type_x_is_y(node, left, operator, right)

    def _check_type_x_is_y(self, node, left, operator, right):
        """Check for expressions like type(x) == Y."""
        left_func = safe_infer(left.func)
        if not (isinstance(left_func, astroid.ClassDef)
                and left_func.qname() == TYPE_QNAME):
            return

        if operator in ('is', 'is not') and _is_one_arg_pos_call(right):
            right_func = safe_infer(right.func)
            if (isinstance(right_func, astroid.ClassDef)
                    and right_func.qname() == TYPE_QNAME):
                # type(x) == type(a)
                right_arg = safe_infer(right.args[0])
                if not isinstance(right_arg, LITERAL_NODE_TYPES):
                    # not e.g. type(x) == type([])
                    return
        self.add_message('unidiomatic-typecheck', node=node)


class ElifChecker(BaseTokenChecker):
    """Checks needing to distinguish "else if" from "elif"

    This checker mixes the astroid and the token approaches in order to create
    knowledge about whether a "else if" node is a true "else if" node, or a
    "elif" node.

    The following checks depend on this implementation:
        - check for too many nested blocks (if/elif structures aren't considered
          as nested)
        - to be continued
    """
    __implements__ = (ITokenChecker, IAstroidChecker)
    name = 'elif'
    msgs = {'R0101': ('Too many nested blocks (%s/%s)',
                      'too-many-nested-blocks',
                      'Used when a function or a method has too many nested '
                      'blocks. This makes the code less understandable and '
                      'maintainable.'),
            'R0102': ('The if statement can be replaced with %s',
                      'simplifiable-if-statement',
                      'Used when an if statement can be replaced with '
                      '\'bool(test)\'. '),
           }
    options = (('max-nested-blocks',
                {'default' : 5, 'type' : 'int', 'metavar' : '<int>',
                 'help': 'Maximum number of nested blocks for function / '
                         'method body'}
               ),)

    def __init__(self, linter=None):
        BaseTokenChecker.__init__(self, linter)
        self._init()

    def _init(self):
        self._nested_blocks = []
        self._elifs = []
        self._if_counter = 0
        self._nested_blocks_msg = None

    @staticmethod
    def _is_bool_const(node):
        return (isinstance(node.value, astroid.Const)
                and isinstance(node.value.value, bool))

    def _is_actual_elif(self, node):
        """Check if the given node is an actual elif

        This is a problem we're having with the builtin ast module,
        which splits `elif` branches into a separate if statement.
        Unfortunately we need to know the exact type in certain
        cases.
        """

        if isinstance(node.parent, astroid.If):
            orelse = node.parent.orelse
            # current if node must directly follow a "else"
            if orelse and orelse == [node]:
                if self._elifs[self._if_counter]:
                    return True
        return False

    def _check_simplifiable_if(self, node):
        """Check if the given if node can be simplified.

        The if statement can be reduced to a boolean expression
        in some cases. For instance, if there are two branches
        and both of them return a boolean value that depends on
        the result of the statement's test, then this can be reduced
        to `bool(test)` without losing any functionality.
        """

        if self._is_actual_elif(node):
            # Not interested in if statements with multiple branches.
            return
        if len(node.orelse) != 1 or len(node.body) != 1:
            return

        # Check if both branches can be reduced.
        first_branch = node.body[0]
        else_branch = node.orelse[0]
        if isinstance(first_branch, astroid.Return):
            if not isinstance(else_branch, astroid.Return):
                return
            first_branch_is_bool = self._is_bool_const(first_branch)
            else_branch_is_bool = self._is_bool_const(else_branch)
            reduced_to = "'return bool(test)'"
        elif isinstance(first_branch, astroid.Assign):
            if not isinstance(else_branch, astroid.Assign):
                return
            first_branch_is_bool = self._is_bool_const(first_branch)
            else_branch_is_bool = self._is_bool_const(else_branch)
            reduced_to = "'var = bool(test)'"
        else:
            return

        if not first_branch_is_bool or not else_branch_is_bool:
            return
        if not first_branch.value.value:
            # This is a case that can't be easily simplified and
            # if it can be simplified, it will usually result in a
            # code that's harder to understand and comprehend.
            # Let's take for instance `arg and arg <= 3`. This could theoretically be
            # reduced to `not arg or arg > 3`, but the net result is that now the
            # condition is harder to understand, because it requires understanding of
            # an extra clause:
            #   * first, there is the negation of truthness with `not arg`
            #   * the second clause is `arg > 3`, which occurs when arg has a
            #     a truth value, but it implies that `arg > 3` is equivalent
            #     with `arg and arg > 3`, which means that the user must
            #     think about this assumption when evaluating `arg > 3`.
            #     The original form is easier to grasp.
            return

        self.add_message('simplifiable-if-statement', node=node,
                         args=(reduced_to, ))

    def process_tokens(self, tokens):
        # Process tokens and look for 'if' or 'elif'
        for _, token, _, _, _ in tokens:
            if token == 'elif':
                self._elifs.append(True)
            elif token == 'if':
                self._elifs.append(False)

    def leave_module(self, _):
        self._init()

    @check_messages('too-many-nested-blocks')
    def visit_tryexcept(self, node):
        self._check_nested_blocks(node)

    visit_tryfinally = visit_tryexcept
    visit_while = visit_tryexcept
    visit_for = visit_while

    def visit_ifexp(self, _):
        self._if_counter += 1

    def visit_comprehension(self, node):
        self._if_counter += len(node.ifs)

    @check_messages('too-many-nested-blocks', 'simplifiable-if-statement')
    def visit_if(self, node):
        self._check_simplifiable_if(node)
        self._check_nested_blocks(node)
        self._if_counter += 1

    @check_messages('too-many-nested-blocks')
    def leave_functiondef(self, _):
        # new scope = reinitialize the stack of nested blocks
        self._nested_blocks = []
        # if there is a waiting message left, send it
        if self._nested_blocks_msg:
            self.add_message('too-many-nested-blocks',
                             node=self._nested_blocks_msg[0],
                             args=self._nested_blocks_msg[1])
            self._nested_blocks_msg = None

    def _check_nested_blocks(self, node):
        """Update and check the number of nested blocks
        """
        # only check block levels inside functions or methods
        if not isinstance(node.scope(), astroid.FunctionDef):
            return
        # messages are triggered on leaving the nested block. Here we save the
        # stack in case the current node isn't nested in the previous one
        nested_blocks = self._nested_blocks[:]
        if node.parent == node.scope():
            self._nested_blocks = [node]
        else:
            # go through ancestors from the most nested to the less
            for ancestor_node in reversed(self._nested_blocks):
                if ancestor_node == node.parent:
                    break
                self._nested_blocks.pop()
            # if the node is a elif, this should not be another nesting level
            if isinstance(node, astroid.If) and self._elifs[self._if_counter]:
                if self._nested_blocks:
                    self._nested_blocks.pop()
            self._nested_blocks.append(node)
        # send message only once per group of nested blocks
        if len(nested_blocks) > self.config.max_nested_blocks:
            if len(nested_blocks) > len(self._nested_blocks):
                self.add_message('too-many-nested-blocks', node=nested_blocks[0],
                                 args=(len(nested_blocks),
                                       self.config.max_nested_blocks))
                self._nested_blocks_msg = None
            else:
                # if time has not come yet to send the message (ie the stack of
                # nested nodes is still increasing), save it in case the
                # current node is the last one of the function
                self._nested_blocks_msg = (self._nested_blocks[0],
                                           (len(self._nested_blocks),
                                            self.config.max_nested_blocks))

class NotChecker(_BasicChecker):
    """checks for too many not in comparison expressions

    - "not not" should trigger a warning
    - "not" followed by a comparison should trigger a warning
    """
    msgs = {'C0113': ('Consider changing "%s" to "%s"',
                      'unneeded-not',
                      'Used when a boolean expression contains an unneeded '
                      'negation.'),
           }

    reverse_op = {'<': '>=', '<=': '>', '>': '<=', '>=': '<', '==': '!=',
                  '!=': '==', 'in': 'not in', 'is': 'is not'}
    # sets are not ordered, so for example "not set(LEFT_VALS) <= set(RIGHT_VALS)" is
    # not equivalent to "set(LEFT_VALS) > set(RIGHT_VALS)"
    skipped_nodes = (astroid.Set, )
    # 'builtins' py3, '__builtin__' py2
    skipped_classnames = ['%s.%s' % (six.moves.builtins.__name__, qname)
                          for qname in ('set', 'frozenset')]

    @check_messages('unneeded-not')
    def visit_unaryop(self, node):
        if node.op != 'not':
            return
        operand = node.operand

        if isinstance(operand, astroid.UnaryOp) and operand.op == 'not':
            self.add_message('unneeded-not', node=node,
                             args=(node.as_string(),
                                   operand.operand.as_string()))
        elif isinstance(operand, astroid.Compare):
            left = operand.left
            # ignore multiple comparisons
            if len(operand.ops) > 1:
                return
            operator, right = operand.ops[0]
            if operator not in self.reverse_op:
                return
            # Ignore __ne__ as function of __eq__
            frame = node.frame()
            if frame.name == '__ne__' and operator == '==':
                return
            for _type in (_node_type(left), _node_type(right)):
                if not _type:
                    return
                if isinstance(_type, self.skipped_nodes):
                    return
                if (isinstance(_type, astroid.Instance) and
                        _type.qname() in self.skipped_classnames):
                    return
            suggestion = '%s %s %s' % (left.as_string(),
                                       self.reverse_op[operator],
                                       right.as_string())
            self.add_message('unneeded-not', node=node,
                             args=(node.as_string(), suggestion))


class MultipleTypesChecker(BaseChecker):
    """Checks for variable type redefinitions (NoneType excepted)

    At a function, method, class or module scope

    This rule could be improved:
    - Currently, if an attribute is set to different types in 2 methods of a
      same class, it won't be detected (see functional test)
    - One could improve the support for inference on assignment with tuples,
      ifexpr, etc. Also it would be great to have support for inference on
      str.split()
    """
    __implements__ = IAstroidChecker

    name = 'multiple_types'
    msgs = {'R0204': ('Redefinition of %s type from %s to %s',
                      'redefined-variable-type',
                      'Used when the type of a variable changes inside a '
                      'method or a function.'
                     ),
           }

    def visit_classdef(self, _):
        self._assigns.append({})

    @check_messages('redefined-variable-type')
    def leave_classdef(self, _):
        self._check_and_add_messages()

    visit_functiondef = visit_classdef
    leave_functiondef = leave_module = leave_classdef

    def visit_module(self, _):
        self._assigns = [{}]

    def _check_and_add_messages(self):
        assigns = self._assigns.pop()
        for name, args in assigns.items():
            if len(args) <= 1:
                continue
            _, orig_type = args[0]
            # Check if there is a type in the following nodes that would be
            # different from orig_type.
            for redef_node, redef_type in args[1:]:
                if redef_type != orig_type:
                    orig_type = orig_type.replace(BUILTINS + ".", '')
                    redef_type = redef_type.replace(BUILTINS + ".", '')
                    self.add_message('redefined-variable-type', node=redef_node,
                                     args=(name, orig_type, redef_type))
                    break

    def visit_assign(self, node):
        # we don't handle multiple assignment nor slice assignment
        target = node.targets[0]
        if isinstance(target, (astroid.Tuple, astroid.Subscript)):
            return
        # ignore NoneType
        if _is_none(node):
            return
        _type = _node_type(node.value)
        if _type:
            self._assigns[-1].setdefault(target.as_string(), []).append(
                (node, _type.pytype()))


def register(linter):
    """required method to auto register this checker"""
    linter.register_checker(BasicErrorChecker(linter))
    linter.register_checker(BasicChecker(linter))
    linter.register_checker(NameChecker(linter))
    linter.register_checker(DocStringChecker(linter))
    linter.register_checker(PassChecker(linter))
    linter.register_checker(LambdaForComprehensionChecker(linter))
    linter.register_checker(ComparisonChecker(linter))
    linter.register_checker(NotChecker(linter))
    linter.register_checker(RecommandationChecker(linter))
    linter.register_checker(ElifChecker(linter))
    linter.register_checker(MultipleTypesChecker(linter))
