What is the difference between __reduce__ and __reduce_ex__ ?

Question

I understand that these methods are for pickling/unpickling and have no relation to the reduce built-in function, but what's the difference between the 2 and why do we need both?

Answer 1

The docs say that

If provided, at pickling time __reduce__() will be called with no arguments, and it must return either a string or a tuple.

On the other hand,

It is sometimes useful to know the protocol version when implementing __reduce__. This can be done by implementing a method named __reduce_ex__ instead of __reduce__. __reduce_ex__, when it exists, is called in preference over __reduce__ (you may still provide __reduce__ for backwards compatibility). The __reduce_ex__ method will be called with a single integer argument, the protocol version.

On the gripping hand, Guido says that this is an area that could be cleaned up.

Answer 2

__reduce_ex__ is what __reduce__ should have been but never became. __reduce_ex__ works like __reduce__ but the pickle protocol is passed.