On Tue, May 31, 2011 at 3:46 PM, Eric Snow <ericsnowcurren...@gmail.com>wrote:
> Looking at the ABC code [1], I noticed that Mapping's __eq__ method can > return NotImplemented. This got me curious as to why you would return > NotImplemented and not raise a TypeError or a NotImplementedError. > > There was an issue regarding this last year [2] that indicated the reason > for Mapping's behavior. Also, I found a thread from python-dev [3] which > talks about the use of NotImplemented (the thread relates to numeric > coercion). I found some older documentation on informal coercion guidelines > that helped [4]. My understanding is that NotImplemented is used for binary > operators: the right hand side of the operator should be tried if the left > hand side returns NotImplemented. So it's more like > DoNotKnowHowToHandleArgumentOfThatType. I think the name may have added to > my confusion. (BTW, I learned that NotImplemented is a singleton, like > None). > > Is binary operators the only place that NotImplemented is used under the > hood? Is it all binary operators, and if not, where does it say which > operators use NotImplemented? This would have bearing on when I would need > to return it. > > In the python-dev thread [3], MRAB indicates NotImplemented is used instead > of exceptions for performance reasons. Is raising NotImplementedError or > TypeError that big a difference? I expect if you are using the operator on > a large loop it could matter, but how much? > > Guido indicates earlier in the thread that NotImplemented is used so that > you know that it came from the function that you directly called, and not > from another call inside that function. Why does it matter if it came > directly from the function or not? And couldn't the NotImplemented still > have come from a call inside the operator, rather than directly? > > As an aside, don't we have that same situation all over the place. For > instance, using __getattribute__, how do you know if an AttributeError means > that the attribute was not found on the object? It could mean that > __getattribute__ called something that raised the exception (and perhaps it > should have handled it). Does it matter? Is there a good way to tell the > difference, or would it be good practice to always handle explicitly in a > function any exception type that you may be raising there? > > Thanks, > > -eric > > [1] > http://hg.python.org/cpython/file/29e08a98281d/Lib/collections/abc.py#l398 > [2] http://bugs.python.org/issue8729 > [3] http://mail.python.org/pipermail/python-dev/2005-March/051835.html > [4] http://docs.python.org/release/2.5.2/ref/coercion-rules.html > I'm guessing that http://docs.python.org/reference/datamodel.html is the only place in the docs that talks about the use of NotImplemented. -eric
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