Use qualname to avoid key collisions

[fbidu]

Hi there! This PR adds the possibility to use a function's full qualified name while building the cache key, an attribute that has been available since python 3.3 so it is safe to be relied upon here.

This PR comes from a bug hunting I did today whose root cause was key collision. We had a structure that looked roughly like this:

class A:
	
	@classmethod
	def active(cls):
		...

class B

	@classmethod
	def active(cls):
		...

When both active methods are cached, their keys end up colliding because:

In [3]: class B:
   ...:     @classmethod
   ...:     def active(cls):
   ...:         ...
   ...:

In [4]: B.active.__name__
Out[4]: 'active'

but:

In [5]: B.active.__qualname__
Out[5]: 'B.active'

Hope this is useful 🙌

[zzzeek]

this is nice, I wonder if we should just switch to using __qualname__ across the board by default. we'd want to do a bigger version bump for that. but let's start with this

[zzzeek]

how about also in:

• def function_multi_key_generator
• def kwarg_function_key_generator

[zzzeek]

I would break up each test case here and have them come in using pytest.mark.parametrize as a decorator on def test_function_key_generator_qualname. so one function, but the cases come in parameters. each parameter results in one test.

[zzzeek]

same here, then also for the multi key and kwarg versions