CODEWITHSUNDEEP
python
Tenac
Tenac

Reputation: 1

How can a python base class tell whether a sub class has overridden its methods?

Here is my guess, which doesn't work:

class BaseClass(object):
    def foo(self):
        return 'foo'
    def bar(self):
        return 'bar'
    def methods_implemented(self):
        """This doesn't work..."""
        overriden = []
        for method in ('foo', 'bar'):
            this_method = getattr(self, method)
            base_method = getattr(BaseClass, method)
            if this_method is not base_method:
                overriden.append(method)
        return overriden

class SubClass(BaseClass):
    def foo(self):
        return 'override foo'

o = SubClass()
o.methods_implemented()

Ideally, methods_implemented() would return ['foo'].

How?

(Why would I want to do this? My base class is an HTTP Resource class which has methods GET, POST etc. By default they return 405 Method Not Implemented. It also has a method OPTIONS which should return a 200 response with the header Allow set to the methods which any subclass implements.)

Upvotes: 8

Views: 2877

Answers (2)

Ned Deily
Ned Deily

Reputation: 85045

Perhaps this?

>>> class BaseClass(object):
...     def foo(self):
...         return 'foo'
...     def bar(self):
...         return 'bar'
...     def methods_implemented(self):
...         """This does work."""
...         overriden = []
...         for method in ('foo', 'bar'):
...             this_method = getattr(self, method)
...             base_method = getattr(BaseClass, method)
...             if this_method.__func__ is not base_method.__func__:
...                 overriden.append(method)
...         return overriden
... 
>>> class SubClass(BaseClass):
...     def foo(self):
...         return 'override foo'
... 
>>> o = SubClass()
>>> o.methods_implemented()
['foo']

This checks whether the function objects behind the bound methods are the same.

Note, prior to Python 2.6, the __func__ attribute was named im_func.

Upvotes: 11

Mike Boers
Mike Boers

Reputation: 6735

The methods, even though calling the same object, are NOT the same object. You must test to see if the functions wrapped in the unbound method are the same object.

I'm using 2.6 over here, so I also changed the class to inherit from object.

>>> class BaseClass(object):
...     def foo(self):
...         return 'foo'
...     def bar(self):
...         return 'bar'
...     def methods_implemented(self):
...         """This doesn't work..."""
...         overriden = []
...         for method in ('foo', 'bar'):
...             this_method = getattr(self, method).__func__
...             base_method = getattr(BaseClass, method).__func__
...             if this_method is base_method:
...                 overriden.append(method)
...         return overriden
... 
>>> class SubClass(BaseClass):
...     def foo(self):
...         return 'override foo'
... 
>>> o = SubClass()
>>> o.methods_implemented()
['bar']

Upvotes: 0

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