[Python-Dev] inconsistent __abstractmethods__ behavior; lack of documentation

[Christoph Groth]

Hi,

while playing with abstract base classes and looking at their
implementation, I've stumbled across the following issue.  With Python
3.2, the script

class Foo(object):
    __abstractmethods__ = ['boo']
class Bar(object):
    pass
Bar.__abstractmethods__ = ['boo']
f = Foo()
b = Bar()

produces the following output

Traceback (most recent call last):
  File "/home/cwg/test2.py", line 9, in <module>
    b = Bar()
TypeError: Can't instantiate abstract class Bar with abstract methods buzz

This seems to violate PEP 3119: it is not mentioned there that setting
the __abstractmethods__ attribute already during class definition (as in
"Foo") should have no effect.

I think this happens because CPython uses the Py_TPFLAGS_IS_ABSTRACT
flag to check whether a class is abstract.  Apparently, this flag is not
set when the dictionary of the class contains __abstractmethods__
already upon creation.

As a second issue, the special __abstractmethods__ attribute (which is a
feature of the interpreter) is not mentioned anywhere in the
documentation.

If these are confirmed to be bugs, I can enter them into the issue
tracker.

Christoph