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[Python-Dev] Raising OSError concrete classes from errno code

Nick Coghlan ncoghlan at gmail.com
Wed Dec 26 11:16:23 CET 2012 
On Wed, Dec 26, 2012 at 6:50 PM, Serhiy Storchaka <storchaka at gmail.com> wrote:
> On 25.12.12 23:55, Andrew Svetlov wrote:
>>
>> Currently we have exception tree of classes inherited from OSError
>> When we use C API we can call PyErr_SetFromErrno and
>> PyErr_SetFromErrnoWithFilename[Object] functions.
>> This ones raise concrete exception class (FileNotFoundError for
>> example) looking on implicit errno value.
>> I cannot see the way to do it from python.
>
>
>>>> raise OSError(errno.ENOENT, 'No such file or directory', 'qwerty')
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> FileNotFoundError: [Errno 2] No such file or directory: 'qwerty'

As Serhiy's example shows, this mapping of error numbers to subclasses
is implemented directly in OSError.__new__. We did this so that code
could catch the new exceptions, even when dealing with old code that
raises the legacy exception types.

http://docs.python.org/3/library/exceptions#OSError could probably do
with an example like the one quoted in order to make this clearer

Cheers,
Nick.


-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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