# The (future of the) Black code style

## Preview style

(labels/preview-style)=

Experimental, potentially disruptive style changes are gathered under the `--preview`
CLI flag. At the end of each year, these changes may be adopted into the default style,
as described in [The Black Code Style](index.md). Because the functionality is
experimental, feedback and issue reports are highly encouraged!

(labels/preview-features)=

Currently, the following features are included in the preview style:

- `wrap_comprehension_in`: Wrap the `in` clause of list and dictionary comprehensions
  across lines if it would otherwise exceed the maximum line length.
  ([see below](labels/wrap-comprehension-in))
- `simplify_power_operator_hugging`: Use a simpler implementation of the power operator
  "hugging" logic (removing whitespace around `**` in simple expressions), which applies
  also in the rare case the exponentiation is split into separate lines.
  ([see below](labels/simplify-power-operator))
- `wrap_long_dict_values_in_parens`: Add parentheses around long values in dictionaries.
  ([see below](labels/wrap-long-dict-values))
- `fix_if_guard_explosion_in_case_statement`: fixed exploding of the if guard in case
  patterns which have trailing commas in them, even if the guard expression fits in one
  line
- `pyi_overload_group_blank_lines`: In `.pyi` stub files, improve heuristics around when
  blank lines should appear before, after and within decorated function groups.
  ([see below](labels/pyi-overload-group))
- `pyi_blank_line_before_decorated_class`: In `.pyi` stub files, enforce a blank line
  before a decorated class definition when it follows a function definition.
  ([see below](labels/pyi-blank-line-before-decorated-class))
- `fix_unnecessary_parens_in_indexed_assignment`: Remove unnecessary parentheses around
  the right-hand side of indexed assignments (e.g. `x[key] = expr`) when the subscripted
  target is too long to fit on one line and the expression fits on the tail line.
  ([see below](labels/fix-unnecessary-parens-indexed-assignment))
- `pyi_blank_line_after_function_docstring`: In `.pyi` stub files, enforce a blank line
  after a function or method body that consists of a docstring.
  ([see below](labels/pyi-blank-line-after-function-docstring))
- `hug_comparator`: Don't break a comparator (`not in`, `==`, `is`, ...) away from its
  left operand when the right operand is a bracketed expression that has to break
  anyway; let the bracket explode instead. ([see below](labels/hug-comparator))
- `parenthesize_tuple_in_yield`: Add parentheses around tuple expressions in `yield`
  statements.

(labels/wrap-comprehension-in)=

### Wrapping long comprehension `in` clauses

When a list or dictionary comprehension has a long `in` clause that would exceed the
maximum line length, Black will wrap it across multiple lines for better readability.
This helps keep comprehensions readable when the iterable expression is complex or
lengthy.

For example:

```python
# Before
result = [
    very_very_very_very_very_long_item
    for very_very_very_very_very_long_item in some_very_very_very_very_very_very_long_function_name
]
```

will be formatted to:

```python
# After
result = [
    very_very_very_very_very_long_item
    for very_very_very_very_very_long_item in (
        some_very_very_very_very_very_very_long_function_name
    )
]
```

This also applies to dictionary comprehensions:

```python
# Before
mapping = {
    very_long_key: very_very_very_long_item
    for very_long_key, very_very_very_long_item in very_very_very_very_long_function_name
}
```

will be formatted to:

```python
# After
mapping = {
    very_long_key: very_very_very_long_item
    for very_long_key, very_very_very_long_item in (
        very_very_very_very_long_function_name
    )
}
```

(labels/simplify-power-operator)=

### Simplified power operator whitespace handling

Black's power operator "hugging" logic removes whitespace around `**` in simple
expressions (e.g., `x**2` instead of `x ** 2`). This feature uses a simpler, more
consistent implementation that also applies when exponentiation is split across lines.

For example:

```python
# Simple expressions - whitespace is removed
result = x**2 + y**3
value = base**exponent
```

When the exponentiation is split across lines (rare), the simplified logic ensures
consistent formatting:

```python
# Complex expression split across lines
result = (
    some_very_long_base_expression
    **some_very_long_exponent_expression
    **some_very_long_third_expression
)
```

This feature primarily improves the internal consistency of Black's formatting logic
rather than making dramatic visual changes to most code.

(labels/wrap-long-dict-values)=

### Improved parentheses management in dicts

For dict literals with long values, they are now wrapped in parentheses. Unnecessary
parentheses are now removed. For example:

```python
my_dict = {
    "a key in my dict": a_very_long_variable
    * and_a_very_long_function_call()
    / 100000.0,
    "another key": (short_value),
}
```

will be changed to:

```python
my_dict = {
    "a key in my dict": (
        a_very_long_variable * and_a_very_long_function_call() / 100000.0
    ),
    "another key": short_value,
}
```

(labels/pyi-overload-group)=

### Improved overload groups in stub files

In `.pyi` stub files, Black now has improved heuristics regarding when blank lines
should appear before, after or within groups of decorated functions that share the same
name (such as `@overload` groups). Two rules are applied when a decorated function is
determined to be part of a series of >=2 decorated functions with the same name:

1. **Before the decorated function**: a blank line is always inserted, unless the
   preceding statement is a same-name decorated function (i.e. part of an `@overload`
   group) or the function is the first statement in its block.
2. **After the decorated function**: a blank line is always inserted, unless the
   following statement is a same-name decorated function.

These rules apply regardless of what the adjacent statement is — whether it's another
function definition, a variable annotation, or any other statement.

Previously, Black could insert unwanted blank lines _within_ an overload group when one
of the overloads had a docstring, and did not consistently enforce blank lines at the
boundaries of overload groups:

```python
# Before

@overload
def foo(x: int) -> int:
    """Docs."""

@overload                    # unwanted blank line within group
def foo(x: str) -> str: ...
def bar(x): ...              # no blank line after group
```

With this feature enabled, the group is kept together and clearly separated from
surrounding code:

```python
# After (with --preview)

@overload
def foo(x: int) -> int:
    """Docs."""
@overload
def foo(x: str) -> str: ...

def bar(x): ...
```

(labels/pyi-blank-line-before-decorated-class)=

### Blank line before decorated classes in stub files

In `.pyi` stub files, Black already enforces blank lines around class definitions in
many situations. However, when a class has a decorator, Black previously failed to
enforce this, instead _removing_ the blank line between a function and the decorator:

```diff
  # Before

  def foo(): ...
-
  @decorator
  class Bar: ...

  def baz(): ...
  @decorator
  class Spam: ...
```

With this feature enabled, a blank line is enforced, consistent with how undecorated
classes are handled:

```diff
  # After (with --preview)

  def foo(): ...

  @decorator
  class Bar: ...

  def baz(): ...
+
  @decorator
  class Spam: ...
```

(labels/fix-unnecessary-parens-indexed-assignment)=

### Unnecessary parentheses in indexed assignments

When an assignment target ends with a subscript (e.g. `x[key] = expr`) and is too long
to fit on one line, Black has to split at the subscript's brackets. Previously it would
additionally wrap the right-hand side expression in parentheses, even when the
expression fits on the closing line. With this feature enabled, Black omits those
unnecessary parentheses.

For example:

```python
# Before
dictionary_of_arrays["long_key_name_for_the_example"][
    very_long_index_name, index_zero
] = (10 - 5)
```

will be formatted to:

```python
# After (with --preview)
dictionary_of_arrays["long_key_name_for_the_example"][
    very_long_index_name, index_zero
] = 10 - 5
```

Assignments whose target fits on one line are not affected: wrapping the right-hand side
in parentheses remains the preferred style there.

(labels/pyi-blank-line-after-function-docstring)=

### Blank line after function docstrings in stub files

In `.pyi` stub files, functions and methods sometimes use a docstring as their whole
body. Black already separated these definitions from a following function definition,
but did not consistently do the same before a following comment, conditional block,
variable annotation, or other statement:

```python
# Before

class Example:
    def method(self) -> None:
        """Documentation."""
    # comment for the next member
    attr: int
```

With this feature enabled, the docstring-only function body is consistently separated
from the next comment or statement:

```python
# After (with --preview)

class Example:
    def method(self) -> None:
        """Documentation."""

    # comment for the next member
    attr: int
```

Black still keeps same-name decorated functions, such as `@overload` groups and property
setters, together without inserting blank lines between them.

(labels/hug-comparator)=

### Keep comparators next to their left operand

When a comparator (`not in`, `==`, `is`, ...) sits inside another bracketed construct
and its right operand is a bracketed expression that has to break (either via a magic
trailing comma or because the line is too long), Black used to split the line right
before the comparator. The left operand ended up alone on its own line, visually
disconnected from the operator and the operand that explains it:

```python
# Before

x = [
    t
    for t in y
    if t
    not in {
        LongNameOne,
        LongNameTwo,
        LongNameThree,
    }
]
```

With this feature enabled, Black skips that comparator split and lets the right-hand
bracket explode instead, which it would have done anyway:

```python
# After (with --preview)

x = [
    t
    for t in y
    if t not in {
        LongNameOne,
        LongNameTwo,
        LongNameThree,
    }
]
```

The fix is not limited to comprehensions. The same shape appears inside `if`/`elif`
chains, `assert` statements, and parenthesized expressions, and gets the same treatment:

```python
# Before

if (
    is_scalar(value)
    and self.dtype
    in (np.dtype("float64"), np.dtype("float32"), np.dtype("object"))
    and (limit is not None or inplace)
):
    ...

assert (
    bool
    is _AnnotationExtractor(attr.fields(C).x.converter.__call__).get_return_type()
)
```

```python
# After (with --preview)

if (
    is_scalar(value)
    and self.dtype in (
        np.dtype("float64"),
        np.dtype("float32"),
        np.dtype("object"),
    )
    and (limit is not None or inplace)
):
    ...

assert (
    bool is _AnnotationExtractor(
        attr.fields(C).x.converter.__call__
    ).get_return_type()
)
```

## Unstable style

(labels/unstable-style)=

In the past, the preview style included some features with known bugs, so that we were
unable to move these features to the stable style. Therefore, such features are now
moved to the `--unstable` style. All features in the `--preview` style are expected to
make it to next year's stable style; features in the `--unstable` style will be
stabilized only if issues with them are fixed. If bugs are discovered in a `--preview`
feature, it is demoted to the `--unstable` style. To avoid thrash when a feature is
demoted from the `--preview` to the `--unstable` style, users can use the
`--enable-unstable-feature` flag to enable specific unstable features.

(labels/unstable-features)=

The unstable style additionally includes the following features:

- `hug_parens_with_braces_and_square_brackets`: More compact formatting of nested
  brackets. ([see below](labels/hug-parens))
- `string_processing`: Split long string literals and related changes.
  ([see below](labels/string-processing))

(labels/hug-parens)=

### Improved multiline dictionary and list indentation for sole function parameter

For better readability and less verticality, _Black_ now pairs parentheses ("(", ")")
with braces ("{", "}") and square brackets ("[", "]") on the same line. For example:

```python
foo(
    [
        1,
        2,
        3,
    ]
)

nested_array = [
    [
        1,
        2,
        3,
    ]
]
```

will be changed to:

```python
foo([
    1,
    2,
    3,
])

nested_array = [[
    1,
    2,
    3,
]]
```

This also applies to list and dictionary unpacking:

```python
foo(
    *[
        a_long_function_name(a_long_variable_name)
        for a_long_variable_name in some_generator
    ]
)
```

will become:

```python
foo(*[
    a_long_function_name(a_long_variable_name)
    for a_long_variable_name in some_generator
])
```

You can use a magic trailing comma to avoid this compacting behavior; by default,
_Black_ will not reformat the following code:

```python
foo(
    [
        1,
        2,
        3,
    ],
)
```

(labels/string-processing)=

### Improved string processing

_Black_ will split long string literals and merge short ones. Parentheses are used where
appropriate. When split, parts of f-strings that don't need formatting are converted to
plain strings. f-strings will not be merged if they contain internal quotes and it would
change their quotation mark style. Line continuation backslashes are converted into
parenthesized strings. Unnecessary parentheses are stripped. The stability and status of
this feature is tracked in [this issue](https://github.com/psf/black/issues/2188).
