Defining a method for a class somewhere else

Question

I want to implement a method for a class - the method will use the results of other methods in the class, but it will be 100s of lines long, so I would like to define the method in another file, if possible. How can I do this? Something like this:

ParentModule.py:

import function_defined_in_another_file
def method(self):
 return function_defined_in_another_file.function()

ParentModule is the main module which I don't want to define the function in.

function_defined_in_another_file.py:

import ParentModule
def function():
 a = ParentModule.some_method()
 b = ParentModule.some_other_method()
 return a + b 

The function that is defined in another file has to be able to use methods available from the ParentModule.

Is the way I've done it above appropriate, or is there a better way?

• jonrsharpe: Having a single methods 100s of lines long is a sign something is wrong - are there sections of it you could cut out into smaller methods or even standalone functions? Repetitions that could be simplified?
• Robᵩ: The title of your question says "method for class", but you haven't defined any classes. Are you really talking about methods or about stand-alone functions?

Answer 1

You could just assign the method to the class:

import function_defined_in_another_file
class SomeClass():
 method = function_defined_in_another_file.function

It'll be treated just like any other method; you can call method on instances of SomeClass(), other SomeClass() methods can call it with self.method(), and method() can call any SomeClass() method with self.method_name().

You do have to make sure that function() accepts a self argument.

• user1654183: But then I would have to define the function in the ParentModule file? I want to define it in another file/module
• Martijn Pieters: @user1654183: sorry, had your modules mixed up
• user1654183: Thank you! that makes more sense. One last thing: "You do have to make sure that function() accepts a self argument." How exactly can I do that when the function is defined in another file not as part of a class?
• Martijn Pieters: @user1654183: just by adding self as the first argument. :-) The self parameter name is only a convention. It is by using a function as an attribute of a class that it becomes a method, and self is passed in.