Question

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Variables inside and outside of a class __init__() function

I understand that when a class is called it will run the code in __init__ before anything. I still don’t see the difference between that, and writing the code directly under the class.

For example:

class main():
  x = 1

  def disp(self):
    print self.x

class main():
  def __init__(self):
    self.x = 1

  def disp(self):
    print self.x

To me, both have the same functionality. (Maybe I am missing out something.) I would like to know which is more (ahem) pythonic and why.

Answer 1

There are a couple key differences here, both between __init__ and writing it just under the class, as well as what you wrote.

Working with x = 1

First, you are right–these two items of code do effectively the same thing for your purposes (specifically because we’re working with int objects here, this would be different for mutable objects):

Note that they don’t actually do the same thing–please see the comments on this answer for clarification.

class main(object):
    x = 1

class main(object):
    def __init__(self):
        self.x = 1

This is why many non-standard Python libraries, like mongoengine and django models, have a standard where you create classes without using an __init__ statement so as not to override the built-in one, but still allowing you to create class attributes, e.g., a Django example:

class mymodel(models.model):
    name = models.CharField(max_length=20)
    url = models.UrlField()

However, as the other poster points out, there is a difference between the two in that when x=1 is outside of the __init__ function, it is part of the class itself even when not intialized–see Zagorulkin Dmitry‘s answer for more detail on that. In most cases, though, that distinction won’t be relevant for you.

Other considerations

There are more uses for __init__ beyond just setting variables. The most important one offhand is the ability to accept arguments during initialization. To my knowledge, there is not a way to do this without an __init__ function. I’ll show you what I mean by this in this example.

Let’s say we’re creating a Person class, and when we create a Person, we supply their age, and then their birth year is automatically calculated from that for us.

import datetime
class Person(object):
    def __init__(self, age):
        self.age = age
        self.birth_year = (datetime.date.today() - datetime.timedelta(days=age*365)).year

In use:

>>>joe = Person(23)
>>>joe.age
23
>>>joe.birth_year
1990

This wouldn’t be possible without __init__, since we couldn’t pass the initialization the age argument otherwise.