Objects, attributes, functions, and methods

Sunday, April 07, 2013

I set some time out to open the box on some Python fundamentals this weekend. I have a few topics to write about but, for today, let's talk a little about objects.


Objects are a big deal in Python. And rightfully so, as most developers use Python in an object oriented manner, though you can certainly write imperatively or pseudo-functionally too. In fact, some people subscribe to the thought that everything in Python is an object. I'm not going to argue about whether that's true or not so we'll just tip-toe around it and state that there are an awful lot of objects.

So what is an object? Let's use Wikipedia's definition. Objects are characterized by these properties:

Identity (Objects are unique from each other)

class Trivial(object):

obj, diff_obj = Trivial(), Trivial()

id(obj) # 25851216
id(diff_obj) # 26133840

State (Can store data in object)

import os # modules are objects
os.my_data = 'arbitrary'
print os.my_data # arbitrary

Behavior (Can manipulate the object)

print ' '.join(['strings', 'are', 'objects', 'with', 'methods'])
# strings are objects with methods

Attributes and Methods

You're probably familiar with setting object attributes to static data or writing simple methods if you've worked with objects in the past.

class Lannister(object):
    def __init__(self, name):
        self.name = name

    def say_family_motto(self):
        print '{} says: "Hear Me Roar!"'.format(self.name)

tyrion = Lannister()

print tyrion.name
# Tyrion Lannister
# Tyrion Lannister says: "Hear Me Roar!"

But you're actually not limited to such trivial behavior. For instance, since functions are first class objects in Python, you could easily do the following too.

def say_common_motto():
    print 'A Lannister Always Pays His Debts.'

tyrion.say_common_motto = say_common_motto

# A Lannister Always Pays His Debts.

As you can see, we've set the .say_common_motto attribute to the function object say_common_motto, then invoked the attribute.

Before we continue, we should make a distinction between .say_common_motto and .say_family_motto.

# <type 'function'>
# <type 'instancemethod'>

Python resolves the type of .say_common_motto to a function object and .say_family_motto to an instance method object. What's the difference?

You can think of a method as a function with a unique difference. A method always takes the object it's associated with as it's first argument. You don't have a choice on the matter.

Under the hood, when Python sees tyrion.say_family_motto(), it first looks for a .say_family_motto attribute in tyrion (this means you can override an object method by writing to an object attribute with the same name). When it doesn't find that, the interpreter will jump up to the class level and look for say_family_motto function defined at the class level. Once found, it will invoke the function as a method - namely, by attaching the tyrion object to the head of the argument list. That's also why a method definition always includes a positional argument placeholder first (traditionally called self in Python).

Methods are really just a shortcut though. You can actually just define your own function with an argument placeholder for an object and attach the function to the object attribute. An example makes this clearer. Suppose we rewrote the say_common_motto function to make it equivalent to a method.

def say_common_motto(self):
    print '{} says: "A Lannister Always Pays His Debts."'.format(self.name)

tyrion.say_common_motto = say_common_motto

# Tyrion Lannister says: "A Lannister Always Pays His Debts."

# Tyrion Lannister says: "Hear Me Roar!"

As you can see, defining a method in the class is a lot cleaner than attaching a function to the instance object. But both styles get you to the same resolution eventually.

Anyways, hopefully you know a little more about Python objects now.


[objects] [python] [functions]