remove __reduce__ method from Mutability

[mjungmath]

The class Mutability used to be a mixin class for mutable objects. However, not every class inheriting from it uses pickling and even then it must be overloaded manually in most cases anyway.

We suggest to remove that method from Mutability.

CC: @tscrim @mkoeppe @egourgoulhon

Component: misc

Author: Michael Jung

Branch/Commit: f0230f0

Reviewer: Travis Scrimshaw, Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/30281

[mjungmath]

Author: Michael Jung

[mjungmath]

Branch: u/gh-mjungmath/remove___reduce___method_from_mutability

[mjungmath]

New commits:

f0230f0

Trac #30281: `__reduce__` method removed

[mjungmath]

Commit: f0230f0

[mkoeppe]

comment:5

Is this for a ticket where the existence of this method is a creating a problem?

[mjungmath]

comment:6

Yes. See comment:10 in #30261 and all the following by Travis.

[mjungmath]

comment:7

It seems that inheriting from Mutability causes a pickling test. Probably that is the reason why it is endowed with this method.

[mkoeppe]

comment:8

Yes. When there is a __reduce__ method, then it is tested.
The correct fix is to implement pickling for your class in #30261.

[mjungmath]

comment:9

Replying to @mkoeppe:

Yes. When there is a __reduce__ method, then it is tested.
The correct fix is to implement pickling for your class in #30261.

This is exactly the reason why we remove __reduce__ from Mutability. But even then, pickling is tested (see #30280).

[tscrim]

comment:10

This __reduce__ method is bad because it it tries to unpickle anything as a Mutability object. So a Python class that inherits from this must then contend with this, which puts more of burden on the implementer. Perhaps there is a better way to do this, but it is not immediate to me how to do this...

[tscrim]

Reviewer: Travis Scrimshaw

[tscrim]

comment:11

I really think this should be removed. However, we need to make sure that when we pickle and unpickle an immutable object X it remains immutable.

[mkoeppe]

comment:12

Fine with me too

[mkoeppe]

Changed reviewer from Travis Scrimshaw to Travis Scrimshaw, Matthias Koeppe

[vbraun]

Changed branch from u/gh-mjungmath/remove___reduce___method_from_mutability to f0230f0