static method is better than new builtin function, agree. On Wed, Dec 26, 2012 at 12:03 AM, Benjamin Peterson wrote:

2012/12/25 Andrew Svetlov :

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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.

Maybe adding builtin like exception_from_errno(errno, filename=None) make some value? Function returns exception instance, concrete class depends of errno value

I think a static method on OSError like .from_errno would be good.

-- Regards, Benjamin

-- Thanks, Andrew Svetlov

On Wed, Dec 26, 2012 at 6:50 PM, Serhiy Storchaka wrote:

On 25.12.12 23:55, Andrew Svetlov wrote:

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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.

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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@gmail.com | Brisbane, Australia

On Wed, 26 Dec 2012 13:37:13 +0200 Andrew Svetlov wrote:

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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.

Sorry. Looks like OSError.__new__ requires at least two arguments for executing subclass search mechanism:

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OSError(errno.ENOENT) OSError(2,) OSError(errno.ENOENT, 'error msg') FileNotFoundError(2, 'error msg')

Indeed, it does. I did this for consistency, because calling OSError with only one argument doesn't set the "errno" attribute at all:

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e = OSError(5) e.errno

Regards Antoine.