Seems to have been removed in revision 097f4fda61a4, for PEP 3151. The older EnvironmentError documentation talks about creating the exception with two and three constructor arguments, however I don’t see this in the new documentation. Is this usage meant to be deprecated, or still allowed? Either way, I think the documentation should mention it.

Here is a patch to redocument the constructor arguments. I am assuming that they were only removed by accident. The constructor arguments are already tested in ExceptionTests.testAttributes() at Lib/test/test_exceptions.py:248.

I did not restore the bit about more than three arguments being reflected in the “args” attribute, since this is not the current behaviour, and does not seem very important:

>>> OSError(1, 2, 3, 4).args
(1, 2)

Current behavior is more complicated.

>>> OSError(1, 2, 3, 4).args
(1, 2)
>>> OSError(1, 2, 3, 4, 5).args
(1, 2)
>>> OSError(1, 2, 3, 4, 5, 6).args
(1, 2, 3, 4, 5, 6)

If I remember correctly:

1 -- errno
2 -- strerror
3 -- filename
4 -- winerror (?)
5 -- filename2

Isn't it documented anywhere?

New patch with the following changes. Let me know what you think.

* Added extra markup for OSError attributes and constructor signature
* Explained how “winerror” works with and without Windows
* Added “filename2” argument
* Update tests for filename2 defaulting to None

For reference, the argument handling seems to be in oserror_parse_args(), at Objects/exceptions.c:724, and related functions.

It should be documented (if still not) that OSError() constructor can return an instance of OSError subclass, depending on errno value.

>>> OSError(errno.ENOENT, 'no such file')
FileNotFoundError(2, 'no such file')

Good point Serhiy about returning subclasses. I’ll see about making that more explicit if I can’t find it already documented somewhere.

Regarding the number of arguments, I resisted documenting what happens to extra arguments since the behaviour has changed over time, and it doesn’t seem that it would be very useful. IMO it would be better to say passing extra arguments is not supported. But if others disagree, I can have a go at documenting the existing and/or past behaviour.

Python 3.3.3 under Wine:
>>> OSError(None, "Not found", "file", 3).args 
(2, 'Not found', 'file', 3)
>>> OSError(ENOENT, "Not found", "file", None).args
(2, 'Not found', 'file', None)

Python 3.6.0a0 under Linux:
>>> OSError(ENOENT, "Not found", "file", None).args
(2, 'Not found')

New patch, clarifying that the constructor can raise a subclass.

If you still think I need to add something about extra arguments, or how the “args” attribute is set, let me know.

The patch LGTM. We should handle the problem with extra arguments in separate issue.

New changeset cb554248ce54 by Martin Panter in branch '3.4':
Issue #23391: Restore OSError constructor argument documentation
https://hg.python.org/cpython/rev/cb554248ce54

New changeset 7b1a9a12eb77 by Martin Panter in branch '3.5':
Issue #23391: Merge OSError doc from 3.4 into 3.5
https://hg.python.org/cpython/rev/7b1a9a12eb77

New changeset e5df201c0846 by Martin Panter in branch 'default':
Issue #23391: Merge OSError doc from 3.5
https://hg.python.org/cpython/rev/e5df201c0846

I will leave opening a bug for the args issue for someone else. But if you come up with a sensible solution or find out all the details, I am happy to help write up or review the documentation changes.

There is an outdated comment in Objects/exceptions.c that explains args hacking. It refers to no longer supported syntax:

    except OSError, (errno, strerror):