Can I access class variables using self?

Question

I have a Foo class with a remote class variable. Can I access a variable of the remote class using self.remote ?

 class Foo: remote = False def __init__(self): self.remote = True @classmethod def print_remote(cls): print(cls.remote) #prints False but why? 

Answer 1

Assigning remote to self in __init__ means that instance.remote detected first when you access it through self (in the absence of descriptors). To get both options, use either self or type(self) , that is, either from an instance or from a class:

 def print_remote(self): print(type(self).remote) # class remote print(self.remote) # instance remote 

_{type(self).remote is essentially equivalent to self.__class__.remote , but in general, you should avoid capturing dunder names ( __*__ ) when there is a built-in that does this for you ( type in this case) sub>}

They live in different dictionaries and are different variables. self.remote lives in the dict instance, and class.remote in the dict class.

 >>> Foo().__dict__['remote'] True >>> Foo.__dict__['remote'] False 

When you access through cls using classmethod (or type(self) in a regular method), you will get the first class, when you access through self , you will get an instance.

Answer 2

 In [1]: class Foo: ...: x = 0 ...: In [2]: f = Foo() In [4]: f.__dict__ # empty Out[4]: {} In [5]: Foo.__dict__ # have the variable x = 0 Out[5]: mappingproxy({'__dict__': <attribute '__dict__' of 'Foo' objects>, '__doc__': None, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'Foo' objects>, 'x': 0}) 

When you try to access a variable in an object, Python will first look in the object, if it does not exist, then it looks in the dict class.

 In [6]: Foo.x = 10 # changing class variable In [7]: f.__dict__ # still empty. Out[7]: {} In [8]: fx # gives you Foo x as object doesn't have that item. Out[8]: 10 In [9]: fx = 20 # this line creates a new variable in x. Now both class and object has their own variable x In [10]: f.__dict__ # f has its own x. Out[10]: {'x': 20} In [11]: Foo.__dict__ # Foo has its own x. Out[11]: mappingproxy({'__dict__': <attribute '__dict__' of 'Foo' objects>, ... 'x': 10}) In [12]: fx # always you will get the value from f.__dict__ Out[12]: 20 In [16]: fx = 50 # changing the value of object variable In [17]: Foo.x # above statement didn't affect class variable. Out[17]: 10 In [13]: del fx # delete object x In [14]: fx # now f doesn't have x, you get the value from class Foo. Out[14]: 10 

Answer 3

Yes, you can access the class variable with self. But, if you have an instance variable, you will access the instance variable when using self, as it obscures the class variable.