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

FileNotFoundError: [Errno 2] No such file or directory: 'nothing.txt'

it raises some obscure message:

OSError: Syntax error in file None (line 1)

I don’t want to handle the missing file, don’t want to catch nor handle the exception, don’t want to raise a custom exception, neither want I to open the file, nor to create it if it does not exist.

I only want to check it exists (os.path.isfile(filename) will do the trick) and if not, then just raise a proper FileNotFoundError.

I tried this:

#!/usr/bin/env python3

import os

if not os.path.isfile("nothing.txt"):
    raise FileNotFoundError

what only outputs:

Traceback (most recent call last):
  File "./test_script.py", line 6, in <module>
    raise FileNotFoundError
FileNotFoundError

This is better than a “Syntax error in file None”, but how is it possible to raise the “real” python exception with the proper message, without having to reimplement it?

Asked By: zezollo

||

Source

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'

Answered By: Martijn Pieters

Answer 2

In Python, a variable can refer to the type (class), or an object (instance of the class):

>>> x = FileNotFoundError
>>> print(type(x))
<class 'type'>

>>> x = FileNotFoundError()
>>> print(type(x))
<class 'FileNotFoundError'>

While it’s possible to also throw the type FileNotFoundError, you practically always want to throw an object that has been constructed from the class. The constructor accepts the same arguments as OSError. You can pass a standard POSIX and Windows error code, but it’s enough to pass an error message. (In your case the standard error message "No such file or directory" is not entirely accurate, since you also throw the error if a directory is found.)

if not os.path.isfile("nothing.txt"):
    raise FileNotFoundError("nothing.txt was not found or is a directory")

Answered By: Seppo Enarvi

How do I raise a FileNotFoundError properly?

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