B909 improvements

Note Rule B909 originally was B038, but after some discussion, it was decided that we make it optional and it's B909 now (see #456).

I have not updated the content of this issue, so please read "B038" as "B909".

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Description

With the introduction of B038 (#445), I initially introduced a bug causing many false positives, which should be fixed with #453.
#453 implements a check for the methods listed in Python Builit-in Mutable Sequence Types documentation, but not the operators.
However, #453 left out some mutations for the sake of getting a fix to the flood of false positives (#451) out asap.

In this issue, I will propose further changes to harder B038, to detect more (hopefully all) mutations to iterables that the python builtins allow.

Assignment Operations

This is a very obvious one that was not implemented yet, and I wonder how I could've overseen it.
This part will cover the following operations:

for _ in foo:
  foo = bar
  foo *= bar
  foo += bar
  foo[i] = bar
  foo[i:j] = bar
  foo[i:j:k] = bar
  foo |= bar
  foo &= bar
  foo -= bar
  foo ^= bar

The implementation will check if any ast.Assign and ast.AugAssign have the name or a subscript of the name of the iterable as target.

Further, to also check for annotated assignments, the plan is to inspect ast.AnnAssign nodes that are not simple (ie have simple = 0).
simple = 1 are nodes of the form foo: bar, whereas simple = 0 have any of the following forms:

for _ in foo:
  foo: t = bar
  foo: t *= bar
  foo: t += bar
  foo[i]: t = bar
  foo[i:j]: t = bar
  foo[i:j:k]: t = bar
  foo: t |= bar
  foo: t &= bar
  foo: t -= bar
  foo: t ^= bar

I'm not sure if all of the above are legal syntax, but the implementation will likely look very similar and thus would cover all those cases out of the box.

Further, everything of the above also applies to object attributes, ie, we'd also cover foo.bar = -esque assignments if foo.bar is the loop iterable.

Container-Specific Operations

#451 already implements a check for 2 container specific operations: list.sort() and dict.popitem() .
Further operations are missing:

• dict.setdefault
• dict.update
• set.update
• set.intersection_update
• set.difference_update
• set.symmetric_difference_update
• set.add
• set.discard
• object.__setrattr__
• object.__delrattr__

These all can be implemented similarly to the ones in #451 by adding them to the MUTATING_FUNCTIONS list of the B038Checker.

The nasty ones

These are functions that are not used often, or just don't fall into the scheme of the ones listed above:

• delattr
• setattr

These will need special handling in visit_Call of the B038Checker, but shouldn't be too complicated.

Discussion

I hope my proposal covers all the mutations that are doable with the python built-ins.
If not please let me know.

One special mention goes to del x - this is actually not mutating the iterable itself and only deletes the reference to the iterator.
The following snippet runs without problems

>>> a = [1,2,3]
>>> for i in a:
...   if i == 1:
...     del a
...   print(i)
... 
1
2
3

However, def x[y] does mutate x.
Should this case distinction be made? Currently both would be reported.

I also would like to refactor the code for B038 a bit to just make it far more readable, currently it's a bit messy 😅

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Please let me know what you think about the proposed changes and whether I should go ahead and implement them 🙂

Start with the B038Checker, especially its MUTATING_FUNCTIONS list and visit_Call method, and review the behavior introduced by #451 and #453. Define how assignment, annotated assignment, container-specific methods, setattr, and delattr should be detected, while resolving the open distinction between deleting an iterable reference and mutating it. Done requires an agreed scope before implementation.