[Python-Dev] type.__subclasses__() doesn't work

[Peter Otten]

Steven D'Aprano wrote:

> On Wed, Oct 09, 2013 at 12:20:18PM +0200, Antoine Pitrou wrote:
>> 
>> Hello,
>> 
>> Just noticed the following quirk:
>> 
>> >>> type.__subclasses__()
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> TypeError: descriptor '__subclasses__' of 'type' object needs an argument
>> 
>> Yet it would be nice to know about the subclasses of type.
> 
> py> type.__subclasses__(type)
> [<class 'abc.ABCMeta'>, <class 'string._TemplateMetaclass'>]

The underlying problem seems to be that there is no helper function to 
bypass the instance attribute. Compare:

>>> class T(type):
...     def __len__(self): return 0
... 
>>> class A(metaclass=T):
...     def __len__(self): return 1
... 
>>> A.__len__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __len__() missing 1 required positional argument: 'self'
>>> len(A)
0

So should there be a subclasses() function, in the operator module perhaps?