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Re: returning NotImplemented

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  Re: returning NotImplemented Ethan Furman <ethan@stoneleaf.us> - 2011-05-31 15:18 -0700

#6760 — Re: returning NotImplemented

Eric Snow 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.  

My understanding is that if your object does not know how to perform the 
desired action you should return NotImplemented; Python will then give 
the other object a chance to perform the operation (after all, it may 
know how), and if the other object also returns NotImplemented then 
Python itself will raise a TypeError.

If the first object were to raise TypeError (or any exception), the 
second object would not get the chance to try.

~Ethan~

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