CODEWITHSUNDEEP
pythonexception
abyx
abyx

Reputation: 73028

Why return NotImplemented instead of raising NotImplementedError

Python has a singleton called NotImplemented.

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?

Upvotes: 453

Views: 121492

Answers (5)

Ataf
Ataf

Reputation: 37

If a special method supporting a binary operation is not implemented it should return NotImplemented. On the other hand, NotImplementedError should be raised from abstract methods inside user defined base classes to indicate that derived classes should override those methods.

Basically, NotImplemented helps the interpreter support a binary operation, whereas the NotImplementedError throws an exception. You can find a general usage here.

Upvotes: -1

ahsan naeem
ahsan naeem

Reputation: 275

Returning NotImplemented by a function is something like declaring that the function is unable to process the inputs but instead of raising exception, the control is transferred to another function known as Reflection Function with a hope that the Reflection Function might be able to process the inputs.

Firstly, the Reflection Functions associated with any functions are predefined. Secondly when the original function returns NotImplemented, interpreter runs the Reflection Function but on the flipped order of input arguments.

You can find detailed examples here

Upvotes: 19

RichieHindle
RichieHindle

Reputation: 281875

One reason is performance. In a situation like rich comparisons, where you could be doing lots of operations in a short time, setting up and handling lots of exceptions could take a lot longer than simply returning a NotImplemented value.

Upvotes: 24

Paolo
Paolo

Reputation: 21136

Because they have different use cases.

Quoting the docs (Python 3.6):

NotImplemented

should be returned by the binary special methods (e.g. __eq__(), __lt__(), __add__(), __rsub__(), etc.) to indicate that the operation is not implemented with respect to the other type

exception NotImplementedError

[...] In user defined base classes, abstract methods should raise this exception when they require derived classes to override the method, or while the class is being developed to indicate that the real implementation still needs to be added.

See the links for details.

Upvotes: 304

SpliFF
SpliFF

Reputation: 39014

It's because __lt__() and related comparison methods are 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 !=)."

Upvotes: 416

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