In Python, how do I indicate I'm overriding a method?

Solution 1:

Based on this and fwc:s answer I created a pip installable package https://github.com/mkorpela/overrides

From time to time I end up here looking at this question. Mainly this happens after (again) seeing the same bug in our code base: Someone has forgotten some "interface" implementing class while renaming a method in the "interface"..

Well Python ain't Java but Python has power -- and explicit is better than implicit -- and there are real concrete cases in the real world where this thing would have helped me.

So here is a sketch of overrides decorator. This will check that the class given as a parameter has the same method (or something) name as the method being decorated.

If you can think of a better solution please post it here!

def overrides(interface_class):
    def overrider(method):
        assert(method.__name__ in dir(interface_class))
        return method
    return overrider

It works as follows:

class MySuperInterface(object):
    def my_method(self):
        print 'hello world!'


class ConcreteImplementer(MySuperInterface):
    @overrides(MySuperInterface)
    def my_method(self):
        print 'hello kitty!'

and if you do a faulty version it will raise an assertion error during class loading:

class ConcreteFaultyImplementer(MySuperInterface):
    @overrides(MySuperInterface)
    def your_method(self):
        print 'bye bye!'

>> AssertionError!!!!!!!

Solution 2:

Here's an implementation that doesn't require specification of the interface_class name.

import inspect
import re

def overrides(method):
    # actually can't do this because a method is really just a function while inside a class def'n  
    #assert(inspect.ismethod(method))

    stack = inspect.stack()
    base_classes = re.search(r'class.+\((.+)\)\s*\:', stack[2][4][0]).group(1)

    # handle multiple inheritance
    base_classes = [s.strip() for s in base_classes.split(',')]
    if not base_classes:
        raise ValueError('overrides decorator: unable to determine base class') 

    # stack[0]=overrides, stack[1]=inside class def'n, stack[2]=outside class def'n
    derived_class_locals = stack[2][0].f_locals

    # replace each class name in base_classes with the actual class type
    for i, base_class in enumerate(base_classes):

        if '.' not in base_class:
            base_classes[i] = derived_class_locals[base_class]

        else:
            components = base_class.split('.')

            # obj is either a module or a class
            obj = derived_class_locals[components[0]]

            for c in components[1:]:
                assert(inspect.ismodule(obj) or inspect.isclass(obj))
                obj = getattr(obj, c)

            base_classes[i] = obj


    assert( any( hasattr(cls, method.__name__) for cls in base_classes ) )
    return method

Solution 3:

If you want this for documentation purposes only, you can define your own override decorator:

def override(f):
    return f


class MyClass (BaseClass):

    @override
    def method(self):
        pass

This is really nothing but eye-candy, unless you create override(f) in such a way that is actually checks for an override.

But then, this is Python, why write it like it was Java?