gh-96663: Add a better error message for __dict__-less classes setattr

[Gobot1234]

I think this addresses all the issues I have with the current message. Thanks to Eryk for the pointer as to where I should be editing.

• Issue: Better error message for assignment to non-existent __slots__  #96663

[Gobot1234]

I can't find the tests for this bit of code, are there any?

[thatbirdguythatuknownot]

There's an example that raises the error message when a __dict__ is available, so there has to be a check in the second path.

[Gobot1234]

I'm not entirely sure why the tests are failing

>>> class B:
...     y = 0
...     __slots__ = ('z', 'foo')
... 
>>> B().fod = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'B' object has no attribute 'fod' and no __dict__ for setting new attributes. Did you mean: 'foo'?

AFAICT this should be the error that you get? Unless it's failing due to not being printed.

[thatbirdguythatuknownot]

I'm not entirely sure why the tests are failing

>>> class B:
...     y = 0
...     __slots__ = ('z', 'foo')
... 
>>> B().fod = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'B' object has no attribute 'fod' and no __dict__ for setting new attributes. Did you mean: 'foo'?

AFAICT this should be the error that you get? Unless it's failing due to not being printed.

@Gobot1234 The suggestion (Did you mean: 'foo'?) does not seem to be part of the error.

[ionite34]

Should this perhaps check for __setattr__ overrides? There can be cases where the class has no __dict__ but some non slot attribute setting can still happen. In such a case this error might be misleading.

class Foo:
  __slots__ = ("x", "y")
	
  def __setattr__(self, name, val):
    if name == "all":
      self.x = val
      self.y = val
    else:
      super().__setattr__(name, val)
	
f = Foo()
f.all = 5

[Gobot1234]

@pablogsal since this is an error message change would you be willing to review?