Check if a file exists.

[Pedroski55]

I need to look for a file in this path:

pathToAnswersHW = '/home/pedro/' + termName[0] + '/' + theClass[0] + '/correctAnswersHW/'

but sometimes this file

theAnswersHW = open(pathToAnswersHW + 'correctAnswersHW' + theClass[0] + answersWeeknumber[0] + '.txt', 'r')

will not exist.

I think there must be some 'os.if_not_exists(file)' I can just return from the function.

What is the best way to check if the file exists?

def openAnswersHW():
    pathToAnswersHW = '/home/pedro/' + termName[0] + '/' + theClass[0] + '/correctAnswersHW/'
    # HW get the correct HW answers with answerDataHW = theAnswersHW.readlines()
    theAnswersHW = open(pathToAnswersHW + 'correctAnswersHW' + theClass[0] + answersWeeknumber[0] + '.txt', 'r')
    # check if the file exists
    
    #put a return in here if the file does not exist

    answerDataHW = theAnswersHW.readlines()
    theAnswersHW.close()
    # HW standardize the apostrophes
    aposSorted = changeApostrophe(answerDataHW)
    echoMessage('Apostrophes sorted ... ')
    for word in aposSorted:            
        answersHW.append(word)        
    echoMessage('answersHW is: ' + str(len(answersHW)) + ' long.')
    echoMessage('Now click "tidy student answers CW" or "tidy student answers HW"')

[bowlofred]

It's pretty common to instead attempt the open and then catch if it fails due to nonexistence.

try:
    theAnswersHW = open(pathToAnswersHW + 'correctAnswersHW' + theClass[0] + answersWeeknumber[0] + '.txt', 'r')
except FileNotFoundError:
    return None # return immediately if file doesn't exist
# program continues here if open succeeds

[buran]

As @bowlofred explained - the canonic way is to try to open and handle the exception. Checking if file exists and then try to open it is prone to race condition - the file may be deleted/renamed between your check and your attempt to work with it.

As a side note - it's better using with context manager when open the file

[snippsat]

Some style issues that make this not so good to look at(for a Python guy),it's Python you are writing and not Java
To give a example with some cleaning done.

path_hw = f'/home/pedro/{term_name[0]}/{the_class[0]}/correct_hw/'
try:
    with open(f'{path_hw}correct_hw{the_class[0]}{week_number[0]}.txt', 'r') as answers_hw:
        data_hw = answers_hw.readlines()
except FileNotFoundError as error:
    print(error)

So f-string,no camlCase(in Python use snake_case) and with open as mention.
Using as as error we cacth what FileNotFoundError return.
Example:

Error:
[Errno 2] No such file or directory: 'data.txt'

[perfringo]

I think there must be some 'os.if_not_exists(file)' I can just return from the function.

Yes, of course there is.

import os

if os.path.exists('/directory/subdirectory/filename'):
     # do something 
else:
     # do something else

One can be more specific as well (is it a file?):

if os.path.isfile('/directory/subdirectory/filename'):
    # do something
else:
    # do something else

Of course, you can reverse it with 'not': if not os.path.isfile('/directory/subdirectory/filename'):

Always keep in mind buran's wise words about race condition.

[Pedroski55]

Thank you everyone! I am the wiser!