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

问题:

In Java, for example, the @Override annotation not only provides compile-time checking of an override but makes for excellent self-documenting code.

I'm just looking for documentation (although if it's an indicator to some checker like pylint, that's a bonus). I can add a comment or docstring somewhere, but what is the idiomatic way to indicate an override in Python?

回答1:

UPDATE (23.05.2015): 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!!!!!!!

回答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

回答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?

回答4:

Python ain't Java. There's of course no such thing really as compile-time checking.

I think a comment in the docstring is plenty. This allows any user of your method to type help(obj.method) and see that the method is an override.

You can also explicitly extend an interface with class Foo(Interface), which will allow users to type help(Interface.method) to get an idea about the functionality your method is intended to provide.

回答5:

As far as I know, there is no special way to indicate an override in Python. You just define the method and include a docstring, like always.

回答6:

Like others have said unlike Java there is not @Overide tag however above you can create your own using decorators however I would suggest using the getattrib() global method instead of using the internal dict so you get something like the following:

def Override(superClass):
    def method(func)
        getattr(superClass,method.__name__)
    return method

If you wanted to you could catch getattr() in your own try catch raise your own error but I think getattr method is better in this case.

Also this catches all items bound to a class including class methods and vairables

回答7:

Hear is simplest and working under Jython with Java classes:

class MyClass(SomeJavaClass):
     def __init__(self):
         setattr(self, "name_of_method_to_override", __method_override__)

     def __method_override__(self, some_args):
         some_thing_to_do()