class attribute considered abstract method in python 2.7 - abc module

Question

I am trying to implement an abstract superclass (Base) with an abstract method (addfeature), which the Child class will override.

from lxml.builder import ElementMaker
from abc import ABCMeta, abstractmethod

class Base(object):
    __metaclass__ = ABCMeta

    ns = "http://www.foo.com/bar"
    em = ElementMaker(namespace=ns, nsmap={'bar': ns})

    @abstractmethod
    def addfeature(self):
        pass

class Child(Base):
    def addfeature(self):
        pass

child_instance = Child()

This code fails however with

"TypeError: Can't instantiate abstract class Child with abstract methods em"

Why? em is supposed to be a class attribute, not a method (and surely not an abstract method)

Answer 1

ABCMeta check whether the method is abstract or not by using __isabstractmethod__ attribute. The lxml.builder.ElementMaker dynamically generate a method (using __getattr__); access to __isabstractmethod__ confuse the ABCMeta.

>>> ns = "http://www.foo.com/bar"
>>> em = ElementMaker(namespace=ns, nsmap={'bar': ns})
>>> em.child_element
<functools.partial object at 0x0000000002B55598>
>>> em.child_element()
<Element {http://www.foo.com/bar}child_element at 0x27a8828>
>>> em.__isabstractmethod__
<functools.partial object at 0x0000000002B55598>
>>> bool(em.__isabstractmethod__)
True

By assigning __isabstractmethod__ as a False, you can work around it.

class Base(object):
    __metaclass__ = ABCMeta

    ns = "http://www.foo.com/bar"
    em = ElementMaker(namespace=ns, nsmap={'bar': ns})
    em.__isabstractmethod__ = False  # <-----

    @abstractmethod
    def addfeature(self):
        pass