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up vote 3 down vote favorite
2

I am writing a script that does something to a text file (what it does is irrelevant for my question though). So before I do something to the file I want to check if the file exists. I can do this, no problem, but the issue is more that of aesthetics.

Here is my code, implementing the same thing in two different ways.

def modify_file(filename):
    assert os.path.isfile(filename), 'file does NOT exist.'


Traceback (most recent call last):
  File "clean_files.py", line 15, in <module>
    print(clean_file('tes3t.txt'))
  File "clean_files.py", line 8, in clean_file
    assert os.path.isfile(filename), 'file does NOT exist.'
AssertionError: file does NOT exist.

or:

def modify_file(filename):
    if not os.path.isfile(filename):
        return 'file does NOT exist.'


file does NOT exist.

The first method produces an output that is mostly trivial, the only thing I care about is that the file does not exist.

The second method returns a string, it is simple.

My questions is: which method is better for letting the user know that the file does not exist? Using the assert method seems somehow more pythonic.

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2 Answers 2

up vote 16 down vote accepted

You'd go with a third option instead: use raise and a specific exception. This can be one of the built-in exceptions, or you can create a custom exception for the job.

In this case, I'd use IOError, but a ValueError might also fit:

def modify_file(filename):
    if not os.path.isfile(filename):
        raise IOError('file does NOT exist.')

Using a specific exception allows you to raise other exceptions for different exceptional circumstances, and lets the caller handle the exception gracefully.

Of course, many file operations (like open()) themselves raise OSError already; explicitly first testing if the file exists may be redundant here.

Don't use assert; if you run python with the -O flag, all assertions are stripped from the code.

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up vote 2 down vote

assert is intended for cases where the programmer calling the function made a mistake, as opposed to the user. Using assert under that circumstance allows you to make sure a programmer is using your function correctly during testing, but then strip it out in production.

Its value is somewhat limited since you have to ensure you exercise that path through the code, and you often want to additionally handle the problem with a separate if statement in production. assert is most useful in situations like, "I want to helpfully work around this problem if a user hits it, but if a developer hits it, I want it to crash hard so he will fix the code that calls this function incorrectly."

In your particular case, a missing file is almost certainly a user error, and should be handled by raising an exception.

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