Question

I have a class in the main project I don’t want to change.

class A():
    def __init__(self, firstname, lastname):
        self.firstname = firstname
        self.lastname = lastname

    def name(self):
        # this method could be much more complex
        return self.lastname.upper()

I’m trying to build a plugin mechansim. So far so good, I have an extension point like this:

if __name__ == '__main__':
    ''' The main project has an extension point that allows me to do'''
    # for each class extension such as AExtended:
    A.name = AExtended.name
    ''' After the extensions are loaded, some behaviours may be changed'''
    a = A("John", "Doe")
    print(a.name())

A plugin can be written like this:

class AExtended(A):
    ''' This is an extension I provide through a plugin mechanism
    '''
    def name(self):
        return self.firstname + ' ' + self.lastname.upper()

This all works very well. I now get “John DOE”.

My problem is that the original name() method can be quite complex. In other words, I can’t afford to call self.lastname.upper() in the AExtended. I’d like to call the “super” method, which does not exist any more, because it has been overwritten.

How can I change my code, in order to achieve something like this:

class AExtended(A):
    def name(self):
        # I'm looking for a way to call the base implementation that was in A.name()
        return self.firstname + ' ' + parent.name()

Thanks for your help!

Edit: Some explanations of what I try to do.

• I want the plugin to patch the behaviour of A. I can’t afford to change existing consumers of A
• There are many classes like A that could be changed, I’d like plugins to have full control and responsibility
• It’s true AExtended does not have to inherit from A, but it was an easy way to access self.firstname. I have no problem following a different design pattern if it can help.

I have a workaround, but it’s not very elegant and hard to generalize

class AExtended(A):
    def name(self):
        # I'm looking for a way to call the base implementation that was in A.name()
        return self.firstname + ' ' + self.parentname()
#in main
A.parentname = A.name
A.name = AExtended.name

Answer 1

Here’s a full example of what I was hinting at. Feel free to yell at me and have me merge my answers, or downvote one or whatever, just easier to provide an alternative as a new answer. I’ll let the code do the talking instead of poorly explaining it. 🙂

## Some shared class that is used all over the place and needs to be patched.

class A(object):
    def __init__(self):
        self.firstname = 'Bob'

    # Print my first name.
    def name(self):
        return self.firstname

    # Use this to allow patching arbitrary methods...
    @classmethod
    def patch(cls, func_name):
        def patch_by_name(new_func):
            old_func = getattr(cls, func_name)
            def patched_func(self):
                return new_func(self, old_func)
            setattr(cls, func_name, patched_func)
        return patch_by_name

## Some other area of the code where you want to throw in a patch

class PatchedA(A):  # doesn't need to subclass, but comes in handy sometimes
    @A.patch('name')
    def name(self, orig_func):
        return 'I am ' + orig_func(self) + 'McWizwaz'

print 'Who are you, A class?'
print A().name()  # prints 'I am Bob McWizwaz'