Python inheritance, metaclasses and type() function

Question

I can't understand why the following code behaves a particular way, which is described below:

from abc import ABCMeta   

class PackageClass(object):
    __metaclass__ = ABCMeta        

class MyClass1(PackageClass):
    pass

MyClass2 = type('MyClass2', (PackageClass, ), {})

print MyClass1
print MyClass2

>>> <class '__main__.MyClass1'> 
>>> <class 'abc.MyClass2'>

Why does repr(MyClass2) says abc.MyClass2 (which is by the way not true)? Thank you!

The project structure seems to be completely irrelevant here. I would advise removing that part and just having it all as 1 module (the described behavior is still there). — mgilson, Jan 07 '13 at 15:39
And, as a side note, this behavior still holds in python3.x (I just tested it) — mgilson, Jan 07 '13 at 15:42
@mgilson It does not on my system - I get ` .` with cpython 3.1, 3.2, and 3.3. — phihag, Jan 07 '13 at 15:50
@phihag -- Are you using python3 style metaclasses? `class PackageClass(metaclass = ABCMeta): pass`? — mgilson, Jan 07 '13 at 15:51
Simpler testcase: `type('bar', (ABCMeta('foo', (object,), {}),), {})` — nneonneo, Jan 07 '13 at 15:52

score 6 · Accepted Answer · edited May 23 '17 at 12:04

The problem stems from the fact that ABCMeta overrides __new__ and calls its superclass constructor (type()) there. type() derives the __module__ for the new class from its calling context¹; in this case, the type call appears to come from the abc module. Hence, the new class has __module__ set to abc (since type() has no way of knowing that the actual class construction took place in __main__).

The easy way around is to just set __module__ yourself after creating the type:

MyClass2 = type('MyClass2', (PackageClass, ), {})
MyClass2.__module__ = __name__

I would also recommend filing a bug report.

Related: Base metaclass overriding __new__ generates classes with a wrong __module__, Weird inheritance with metaclasses

1: type is a type object defined in C. Its new method uses the current global __name__ as the __module__, unless it calls a metaclass constructor.

Python inheritance, metaclasses and type() function

1 Answers1