Flake8 Not Recognizing built-in exception

Question

I have a strict pre-commit hook set up with flake8, so I can't commit until its happy. This is normally a good thing, but it is causing some problems with what seems like no problem at all.

I can't get Flake8 to recognize a FileNotFoundError.

example.py

try:     pass except FileNotFoundError:     pass 

This is enough code to get Flake8 to generate the error

$ flake8 example.py example.py:3:8: F821 undefined name 'FileNotFoundError' $ python example.py # no error $ python3 example.py # no error 

I checked the python docs, and FileNotFoundError is a 'built-in' exception, so I don't think I should have to import it from anywhere, and my python interpreters have not complained about it, just seems like an issue with flake8.

Answer 1

I found a couple of mentions of this issue on the Python Code Quality tools repo. Specifically Issue #75.

Two workarounds were listed. You can use the --builtins flag to specify a comma separated list of known builtins that flake8 is flagging.

$ flake8 example.py example.py:3:8: F821 undefined name 'FileNotFoundError' $ flake8 --builtins=FileNotFoundError,... example.py $  

The other workaround is running flake8 under python3 instead of python2.

$ /usr/bin/python3.5 -m pyflakes example.py  $  

Hopefully one of these two solutions will work out for you, as twisting your code up to work around a syntax check tool is counterproductive.

Answer 2

Running python 2.7.8 I get the follwoing:

 except FileNotFoundError:  NameError: name 'FileNotFoundError' is not defined 

If I change to OSError it works correctly on python 3

except OSError: 

FileNotFoundError is a subclass of OSError in python3

Using OSError will catch more than just the FileNotFound error in python 3 so you may want to add additional checks.

For python 2 it is an IOError:

You can catch the specific error:

import __builtin__  exc = getattr(__builtin__,"IOError","FileNotFoundError")  try:     (open("bad_file")) except exc:     pass 

Sure there are betters ways but for python 3:

try:     f = (open("bad_file")) except OSError as e:     if isinstance(e, FileNotFoundError):         pass     else:         raise e