returning NotImplemented

[Eric Snow]

On Tue, May 31, 2011 at 3:46 PM, Eric Snow <ericsnowcurrently at 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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