Define a method outside of class definition?

Question

class MyClass:
    def myFunc(self):
        pass

Can I create MyFunc() outside of the class definition, maybe even in another module?

Answer 1

You can define a function outside of a class and then add it. However, there is a subtle difference in assigning the function to the class or to the instance object. Here is an example:

class MyClass1(object):
    def __init__(self, bar):
        self.foo = 'up'
        MyClass1.foobar = bar

class MyClass2(object):
    def __init__(self, bar):
        self.foo = 'up'
        self.foobar = bar

def bar(self):
    return "What's " + self.foo

Let's first look at what is happening in MyClass1. foobar in this class is similar to a normal method as though it was defined inside the class definition (i.e. it is a method bound to the instance of this class). Let's take a look at what this looks like...

In [2]: x = MyClass1(bar)

In [3]: x.foobar
Out[3]: >

In [4]: x.foobar()
Out[4]: "What's up"

How does this differ from MyClass2? In MyClass2, foobar is simply a reference to the bar function and is NOT a bound method. Because of this we must pass the instance in for this function to work properly. e.g.

In [5]: y = MyClass2(bar)

In [6]: y.foobar
Out[6]: 

In [7]: y.foobar()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
 in ()
----> 1 y.foobar()

TypeError: bar() takes exactly 1 argument (0 given)

In [8]: y.foobar(y)
Out[8]: "What's up"

Although I'm not sure if this is ever good practice to be doing it this way...