Flake8 Not Recognizing built-in exception

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

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.

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