Python 2: __qualname__ is wrong (when subclassed or on metaclass)

[jdemeyer]

When defining a Python class using Cython, the __qualname__ attribute is always set, even on Python 2. When this Cython-defined class is either subclassed or used as metaclass in pure Python code, that new class gets the wrong __qualname__ from Cython.

Examples:

# In Cython
class X(type): pass

# In Python
>>> class Y(X): pass
>>> Y.__qualname__
X

# In Python
>>> class Y(object): __metaclass__ = X
>>> Y.__qualname__
X

As you can see, we get a wrong __qualname__ in both cases. This confuses for example tools like Sphinx, where it produces incorrect output.

Since __qualname__ is useless on Python 2, I suggest to simply not set __qualname__ on Python 2. For Sphinx, not having a __qualname__ is fine, since it will simply use __name__ which is correct.

[scoder]

Personally, I consider __qualname__ a useful debugging feature all by itself, so I'd prefer keeping it if/where possible. But the problem here is that Python doesn't override it for its own subtypes, so I understand that it gets in the way for classes. Functions should be ok (although, see #2562).

Should we remove it only from classes in Py2?

[jdemeyer]

Should we remove it only from classes in Py2?

I don't really have an opinion for functions. Maybe it's fine to keep __qualname__.