Question

What is the difference between creating a variable using the self.variable syntax and creating one without?

I was testing it out and I can still access both from an instance:

class TestClass(object):
    j = 10
    def __init__(self):
        self.i = 20

if __name__ == '__main__':
    testInstance = TestClass()
    print testInstance.i
    print testInstance.j

However, if I swap the location of the self, it results in an error.

class TestClass(object):
    self.j = 10
    def __init__(self):
        i = 20

if __name__ == '__main__':
    testInstance = TestClass()
    print testInstance.i
    print testInstance.j  

>>NameError: name 'self' is not defined

So I gather that self has a special role in initialization.. but, I just don’t quite get what it is.

Answer 1

self refers to the current instance of the class. If you declare a variable outside of a function body, you’re referring to the class itself, not an instance, and thus all instances of the class will share the same value for that attribute.

In addition, variables declared as part of the class (rather than part of an instance) can be accessed as part of the class itself:

class Foo(object):
    a = 1

one = Foo()
two = Foo()

Foo.a = 3

Since this value is class-wide, not only can you read it directly from the class:

print Foo.a # prints 3

But it will also change the value for every instance of the class:

print one.a # prints 3
print two.a # prints 3

Note, however, that this is only the case if you don’t override a class variable with an instance variable. For instance, if you created the following:

class Bar(object)
    a = 1
    def __init__(self):
        self.a = 2

and then did the following:

one = Bar()
two = Bar()
two.a = 3

Then you’d get the following results:

print Bar.a # prints "1"
print one.a # prints "2"
print two.a # prints "3"

As noted in the comments, assigning to two.a creates an instance-local entry on that instance, which overrides the a from Bar, hence why Bar.a is still 1 but two.a is 3.