https://bugs.python.org/issue44530 introduced the co_qualname field to code objects. This could be used to, e.g. enrich the information provided by tracebacks. Consider this simple example

~~~ python
import traceback


class Bogus:
    def __init__(self):
        traceback.print_stack()
        raise RuntimeError("Oh no!")


class Foo:
    def __init__(self):
        Bogus()


Foo()
~~~

The current output is

~~~
❯ python3.10 test_tb_format.py                                                                                       
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 15, in <module>
    Foo()
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 12, in __init__
    Bogus()
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 6, in __init__
    traceback.print_stack()
Traceback (most recent call last):
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 15, in <module>
    Foo()
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 12, in __init__
    Bogus()
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 7, in __init__
    raise RuntimeError("Oh no!")
RuntimeError: Oh no!
~~~

The proposed change is to use the co_qualname field instead of co_name to provide more immediate information about the distinct functions __init__, viz.

~~~
❯ ./python test_tb_format.py   
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 15, in <module>
    Foo()
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 12, in Foo.__init__
    Bogus()
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 6, in Bogus.__init__
    traceback.print_stack()
Traceback (most recent call last):
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 15, in <module>
    Foo()
    ^^^^^
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 12, in Foo.__init__
    Bogus()
    ^^^^^^^
  File "/home/gabriele/Projects/cpython/test_tb_format.py", line 7, in Bogus.__init__
    raise RuntimeError("Oh no!")
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: Oh no!
~~~

This makes it clear that two distinct __init__ functions are involved, without having to look at sources.

Similar changes at bpo-40679 accidentally broke Cython when it was assumed that co_qualname was non-null, which was then fixed by defaulting to co_name in that case. I don't know if Cython still produces cases like that, but we should make sure not to needlessly break such cases if it does.

code.co_qualname was introduced in 2021 with bpo-44530 and shuold give the same guarantees as code.co_name. The __qualname__ attribute is derived from code.co_qualname, so perhaps Cython can benefit from accessing code.co_qualname directly instead?