Class attribute considered abstract method in python 2.7 - abc module
I am trying to implement an abstract superclass (Base) with an abstract method (addfeature) that 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 doesn't work however
"TypeError: unable to instantiate abstract class Child with abstract methods em"
Why? em should be a class attribute, not a method (and certainly not an abstract method)
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ABCMeta
check if a method is abstract or not using an attribute __isabstractmethod__
. lxml.builder.ElementMaker
dynamically generates a method (using __getattr__
); access to __isabstractmethod__
confused 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__
how False
, you can get 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
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