How do I create a decorator for an abstract class method in Python 2.7?
Yes, this is similar to this question, except I would like to combine abc.abstractmethod and classmethod, instead of staticmethod. Also, it looks like abc.abstractclassmethod was added in Python 3 (I think?), but I'm using Google App Engine, so I'm currently limited to Python 2.7
Thanks in advance.
Here's a working example derived from the source code in Python 3.3's abc module:
from abc import ABCMeta
class abstractclassmethod(classmethod):
__isabstractmethod__ = True
def __init__(self, callable):
callable.__isabstractmethod__ = True
super(abstractclassmethod, self).__init__(callable)
class DemoABC:
__metaclass__ = ABCMeta
@abstractclassmethod
def from_int(cls, n):
return cls()
class DemoConcrete(DemoABC):
@classmethod
def from_int(cls, n):
return cls(2*n)
def __init__(self, n):
print 'Initializing with', n
Here's what it looks like when running:
>>> d = DemoConcrete(5) # Succeeds by calling a concrete __init__()
Initializing with 5
>>> d = DemoConcrete.from_int(5) # Succeeds by calling a concrete from_int()
Initializing with 10
>>> DemoABC() # Fails because from_int() is abstract
Traceback (most recent call last):
...
TypeError: Can't instantiate abstract class DemoABC with abstract methods from_int
>>> DemoABC.from_int(5) # Fails because from_int() is not implemented
Traceback (most recent call last):
...
TypeError: Can't instantiate abstract class DemoABC with abstract methods from_int
Note that the final example fails because cls() won't instantiate. ABCMeta prevents premature instantiation of classes that haven't defined all of the required abstract methods.
Another way to trigger a failure when the from_int() abstract class method is called is to have it raise an exception:
class DemoABC:
__metaclass__ = ABCMeta
@abstractclassmethod
def from_int(cls, n):
raise NotImplementedError
The design ABCMeta makes no effort to prevent any abstract method from being called on an uninstantiated class, so it is up to you to trigger a failure by invoking cls() as classmethods usually do or by raising a NotImplementedError. Either way, you get a nice, clean failure.
It is probably tempting to write a descriptor to intercept a direct call to an abstract class method, but that would be at odds with the overall design of ABCMeta (which is all about checking for required methods prior to instantiation rather than when methods are called).
You could upgrade to Python 3.
Starting with Python 3.3, it is possible to combine @classmethod and @abstractmethod:
import abc
class Foo(abc.ABC):
@classmethod
@abc.abstractmethod
def my_abstract_classmethod(...):
pass
Thanks to @gerrit for pointing this out to me.
Another possible workaround:
class A:
__metaclass__ = abc.ABCMeta
@abc.abstractmethod
def some_classmethod(cls):
"""IMPORTANT: this is a class method, override it with @classmethod!"""
class B(A):
@classmethod
def some_classmethod(cls):
print cls
Now, one still can't instantiate from A until some_classmethod is implemented, and it works if you implement it with a @classmethod.
I recently encountered the same problem. That is, I needed abstract classmethods but was unable to use Python 3 because of other project constraints. The solution I came up with is the following.
abc-extend.py:
import abc
class instancemethodwrapper(object):
def __init__(self, callable):
self.callable = callable
self.__dontcall__ = False
def __getattr__(self, key):
return getattr(self.callable, key)
def __call__(self, *args, **kwargs):
if self.__dontcall__:
raise TypeError('Attempted to call abstract method.')
return self.callable(*args,**kwargs)
class newclassmethod(classmethod):
def __init__(self, func):
super(newclassmethod, self).__init__(func)
isabstractmethod = getattr(func,'__isabstractmethod__',False)
if isabstractmethod:
self.__isabstractmethod__ = isabstractmethod
def __get__(self, instance, owner):
result = instancemethodwrapper(super(newclassmethod, self).__get__(instance, owner))
isabstractmethod = getattr(self,'__isabstractmethod__',False)
if isabstractmethod:
result.__isabstractmethod__ = isabstractmethod
abstractmethods = getattr(owner,'__abstractmethods__',None)
if abstractmethods and result.__name__ in abstractmethods:
result.__dontcall__ = True
return result
class abstractclassmethod(newclassmethod):
def __init__(self, func):
func = abc.abstractmethod(func)
super(abstractclassmethod,self).__init__(func)
Usage:
from abc-extend import abstractclassmethod
class A(object):
__metaclass__ = abc.ABCMeta
@abstractclassmethod
def foo(cls):
return 6
class B(A):
pass
class C(B):
@classmethod
def foo(cls):
return super(C,cls).foo() + 1
try:
a = A()
except TypeError:
print 'Instantiating A raises a TypeError.'
try:
A.foo()
except TypeError:
print 'Calling A.foo raises a TypeError.'
try:
b = B()
except TypeError:
print 'Instantiating B also raises a TypeError because foo was not overridden.'
try:
B.foo()
except TypeError:
print 'As does calling B.foo.'
#But C can be instantiated because C overrides foo
c = C()
#And C.foo can be called
print C.foo()
And here are some pyunit tests which give a more exhaustive demonstration.
test-abc-extend.py:
import unittest
import abc
oldclassmethod = classmethod
from abc-extend import newclassmethod as classmethod, abstractclassmethod
class Test(unittest.TestCase):
def setUp(self):
pass
def tearDown(self):
pass
def testClassmethod(self):
class A(object):
__metaclass__ = abc.ABCMeta
@classmethod
@abc.abstractmethod
def foo(cls):
return 6
class B(A):
@classmethod
def bar(cls):
return 5
class C(B):
@classmethod
def foo(cls):
return super(C,cls).foo() + 1
self.assertRaises(TypeError,A.foo)
self.assertRaises(TypeError,A)
self.assertRaises(TypeError,B)
self.assertRaises(TypeError,B.foo)
self.assertEqual(B.bar(),5)
self.assertEqual(C.bar(),5)
self.assertEqual(C.foo(),7)
def testAbstractclassmethod(self):
class A(object):
__metaclass__ = abc.ABCMeta
@abstractclassmethod
def foo(cls):
return 6
class B(A):
pass
class C(B):
@oldclassmethod
def foo(cls):
return super(C,cls).foo() + 1
self.assertRaises(TypeError,A.foo)
self.assertRaises(TypeError,A)
self.assertRaises(TypeError,B)
self.assertRaises(TypeError,B.foo)
self.assertEqual(C.foo(),7)
c = C()
self.assertEqual(c.foo(),7)
if __name__ == "__main__":
#import sys;sys.argv = ['', 'Test.testName']
unittest.main()
I haven't evaluated the performance cost of this solution, but it has worked for my purposes so far.
