Python different behaviour with abstractmethod

Question

I have two classes inheriting from the same parent P:

from abc import ABCMeta, abstractmethod

class P(object):

    __metaclass__ = ABCMeta

    @abstractmethod  
    def foo(self):
        pass

class C(P):
    pass

class D(tuple, P):
    pass

The only difference is that D inherited from tuple and P while C inherits from P only.

Now this is the behavior: c = C() got error, as expected:

TypeError: Can't instantiate abstract class C with abstract methods foo

but d = D() works without error!

I can even call d.foo(). How can I explain this behaviour?

Is that capital case Class working for you? Are you not getting any syntax error? — hlkstuv_23900, Jul 28 '14 at 07:52
@Keith is that preferred? why doesn't `abstractmethod` work? — wong2, Jul 28 '14 at 07:52
@Keith: that's because in Python 3 you need to adjust the metaclass syntax; `class P(metaclass=ABCMeta):`. Once you do so you'll see the same issue on Python 3, `C()` throws an exception, `D()` does not. — Martijn Pieters, Jul 28 '14 at 07:52

Martijn Pieters · Answer 1 · 2014-07-28T09:17:00.157

Abstract methods are tested for in the object.__new__ method; when you inherit from tuple, which has its own __new__ method, object.__new__ is not called and the test for abstract methods is not made.

In other words, mixing abstract methods with any of the built-in immutable types will cause this problem.

The only solution that works is to do your own test in __new__ and then only if you put your abstract class before tuple when mixing in the two bases in a subclass.

class P(object):
    __metaclass__ = ABCMeta

    def __new__(cls, *args, **kwargs):
        super_new = super(P, cls).__new__
        if super_new.__self__ is not object:
            # immutable mix-in used, test for abstract methods
            if getattr(cls, '__abstractmethods__'):
                raise TypeError(
                    "Can't instantiate abstract class %s "
                    "with abstract methods %s" % (
                        cls.__name__,
                        ', '.join(sorted(cls.__abstractmethods__))))

        return super_new(cls, *args, **kwargs)

    @abstractmethod  
    def foo(self):
        pass

class D(P, tuple):
    pass

Man, you're a legend :) – ElmoVanKielmo Jul 28 '14 at 07:55 — ElmoVanKielmo, Jul 28 '14 at 07:55

Python different behaviour with abstractmethod

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