Why does “self” outside a function's parameters give a “not defined” error?

Question

Look at this code:

class MyClass():

    # Why does this give me \"NameError: name \'self\' is not defined\":
    mySelf = self

    # But this does not?
            


        

Answer 1

You can't refer to the class itself within the class body because the class doesn't exist at the time that the class body is executed. (If the previous sentence is confusing, reading up about metaclasses will either clear this up or make you more confused.)

Within an instance method, you can refer to the class of the instance with self.__class__, but be careful here. This will be the instance's actual class, which through the power of inheritance might not be the class in which the method was defined.

Within a class method, the class is passed in as the first argument, much like instances are the first argument to instance methods:

class MyClass(object):
    @classmethod
    def foo(cls):
        print cls.__name__

MyClass.foo() # Should print "MyClass"

As with instance methods, the actual class might differ due to inheritance.

class OtherClass(MyClass): 
    pass

OtherClass.foo() # Should print "OtherClass"

If you really need to refer to MyClass within a method of MyClass, you're pretty much going to have to refer to it as MyClass unless you use magic. This sort of magic is more trouble than it is worth.