abyx abyx - 4 months ago 13
Python Question

Why return NotImplemented instead of raising NotImplementedError

Python has a singleton called

.

Why would someone want to ever return
NotImplemented
instead of raising the
NotImplementedError
exception? Won't it just make it harder to find bugs, such as code that executes invalid methods?

Answer

It's because __lt__() and related comparison methods is quite commonly used indirectly in list sorts and such. Sometimes the algorithm will choose to try another way or pick a default winner. Raising an exception would break out of the sort unless caught, whereas NotImplemented doesn't get raised and can be used in further tests.

http://jcalderone.livejournal.com/32837.html

To summarise that link:

"NotImplemented signals to the runtime that it should ask someone else to satisfy the operation. In the expression a == b, if a.__eq__(b) returns NotImplemented, then Python tries b.__eq__(a). If b knows enough to return True or False, then the expression can succeed. If it doesn't, then the runtime will fall back to the built-in behavior (which is based on identity for == and !=)."

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