How to define method of class outside of class?

Question

In C++ we can easily define methods outside of class:

struct X {
   int a, b ;

   // member function declaration only
   int add();
};

// define member function outside its class declaration
int X::add() { return a + b; }

I want to do the same thing (or something like this in python) but I don’t know how

I saw some solutions to similar problems but they included creating some new functions like this:

def my_func(self_class, arg):
    # do something with arg
    return something

class MyClass:
    function = my_func

Is it possible to define method outside of class without creating unnecessary functions?

Asked By: AsKreY

||

Source

Answer 1

Python doesn’t allow you to define functions in a namespace for another. You can only assign functions to class after defining them. If you want to avoid manual assignment, you can define a simple decorator to help you:

def methodof(tp):
    def setter(func):
        setattr(tp, func.__name__, func)
        return func
    return setter

Test:

>>> class Foo:
...     pass
...
>>> @methodof(Foo)
... def hello(self):
...     print('hello')
...
>>> Foo().hello()
hello

Answered By: Mechanic Pig

Answer 2

Guess I am not sure what you’re trying to avoid unless you have functions you want to use in many classes. In that case I would make a base class and inherit it.

As for your second example, calling an external function means you need to explicitly pass "self" since the instance won’t do that for you. Another reason I prefer inheritance.

The example is very basic so I am not sure what you wish to achieve. For example add can be built into the class using an overload of __add__. I do that with classes for Vectors.

Answered By: John Smith

Answer 3

The lambda way

You example can be done with lambda and =:

class X:
    pass

X.add = lambda s: s.a + s.b

The def way

The def keyword won’t allow you to assign directly to a class, but you can make a function, assign it, and then delete it. While this isn’t what you were hoping for, it is likely the best you can do:

class X:
    pass

def add(s):
    return s.a + s.b

X.add = add
del add

Answered By: Raymond Hettinger

How to define method of class outside of class?

Question:

Answers:

The lambda way

The def way