Variable scopes in Python classes

by

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.

More Related Contents:

  1. Explaining the ‘self’ variable to a beginner [duplicate]
  2. The scope of names defined in class block doesn’t extend to the methods’ blocks. Why is that?
  3. What are metaclasses in Python?
  4. What is the purpose of the word ‘self’?
  5. What is the meaning of single and double underscore before an object name?
  6. Usage of __slots__?
  7. Using property() on classmethods
  8. Why do you need explicitly have the “self” argument in a Python method?
  9. How can I create a copy of an object in Python?
  10. Why does Python code use len() function instead of a length method?
  11. Why does assigning to my global variables not work in Python?
  12. Python – Classes and OOP Basics
  13. What is the difference between __init__ and __call__?
  14. Difference between methods and functions, in Python compared to C++
  15. Python scope: “UnboundLocalError: local variable ‘c’ referenced before assignment” [duplicate]
  16. How do I pass variables across functions?
  17. How to create module-wide variables in Python? [duplicate]
  18. How to avoid infinite recursion with super()?
  19. List all base classes in a hierarchy of given class?
  20. Difference between methods and attributes in python
  21. What is the purpose of the `self` parameter? Why is it needed?
  22. difference between variables inside and outside of __init__() (class and instance attributes)
  23. Global dictionaries don’t need keyword global to modify them? [duplicate]
  24. What is the purpose of static methods? How do I know when to use one? [duplicate]
  25. Is there a benefit to defining a class inside another class in Python?
  26. UnboundLocalError trying to use a variable (supposed to be global) that is (re)assigned (even after first use)
  27. How to assign a new class attribute via __dict__?
  28. Python global variable scoping
  29. In Python, when are two objects the same?
  30. Type Hinting for objects of type that’s being defined [duplicate]

Leave a Comment