Variable scopes in Python classes

Question

Declaring a variable in a class (outside of a function): all class functions can access it (basically a public variable)

Declaring a variable inside a function inside a class: only that function can access it (it's in that function's scope)

Declaring a variable with self.(variable name) inside a function inside a class: all class functions can access it (how is this different from global (variable name)?)

And since there is no private/protected, everything is public, so everything accessible from inside a class is accessible from outside the class.

Are there any other nuances I should know, or have I pretty much got it?

Answer 1

Since the listing in your question is not 100% clear, I've decided to explain it with a simple example. It also includes some things like __something variables you did not mention in your list.

class Test:     a = None     b = None      def __init__(self, a):         print self.a         self.a = a         self._x = 123         self.__y = 123         b = 'meow' 

At the beginning, a and b are only variables defined for the class itself - accessible via Test.a and Test.b and not specific to any instance.

When creating an instance of that class (which results in __init__ being executed):

• print self.a doesn't find an instance variable and thus returns the class variable
• self.a = a: a new instance variable a is created. This shadows the class variable so self.a will now reference the instance variable; to access the class variable you now have to use Test.a
• The assignment to self._x creates a new instance variable. It's considered "not part of the public API" (aka protected) but technically it has no different behaviour.
• The assignment to self.__y creates a new instance variable named _Test__y, i.e. its name is mangled so unless you use the mangled name it cannot be accessed from outside the class. This could be used for "private" variables.
• The assignment to b creates a local variable. It is not available from anywhere but the __init__ function as it's not saved in the instance, class or global scope.