How do I raise a FileNotFoundError properly?

Question

I use a third-party library that\'s fine but does not handle inexistant files the way I would like. When giving it a non-existant file, instead of raising the good old

Answer 1

Pass in arguments:

import errno
import os

raise FileNotFoundError(
    errno.ENOENT, os.strerror(errno.ENOENT), filename)

FileNotFoundError is a subclass of OSError, which takes several arguments. The first is an error code from the errno module (file not found is always errno.ENOENT), the second the error message (use os.strerror() to obtain this), and pass in the filename as the 3rd.

The final string representation used in a traceback is built from those arguments:

>>> print(FileNotFoundError(errno.ENOENT, os.strerror(errno.ENOENT), 'foobar'))
[Errno 2] No such file or directory: 'foobar'