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You are here: Home / Are static class variables possible in Python?

Are static class variables possible in Python?

October 8, 2021 by James Palmer

Variables declared inside the class definition, but not inside a method are class or static variables:
>>> class MyClass:
… i = 3
…
>>> MyClass.i
3

As @millerdev points out, this creates a class-level i variable, but this is distinct from any instance-level i variable, so you could have
>>> m = MyClass()
>>> m.i = 4
>>> MyClass.i, m.i
>>> (3, 4)

This is different from C++ and Java, but not so different from C#, where a static member can’t be accessed using a reference to an instance.
See what the Python tutorial has to say on the subject of classes and class objects.
@Steve Johnson has already answered regarding static methods, also documented under “Built-in Functions” in the Python Library Reference.
class C:
@staticmethod
def f(arg1, arg2, …): …

@beidy recommends classmethods over staticmethod, as the method then receives the class type as the first argument, but I’m still a little fuzzy on the advantages of this approach over staticmethod. If you are too, then it probably doesn’t matter.

@Blair Conrad said static variables declared inside the class definition, but not inside a method are class or “static” variables:
>>> class Test(object):
… i = 3
…
>>> Test.i
3

There are a few gotcha’s here. Carrying on from the example above:
>>> t = Test()
>>> t.i # “static” variable accessed via instance
3
>>> t.i = 5 # but if we assign to the instance …
>>> Test.i # we have not changed the “static” variable
3
>>> t.i # we have overwritten Test.i on t by creating a new attribute t.i
5
>>> Test.i = 6 # to change the “static” variable we do it by assigning to the class
>>> t.i
5
>>> Test.i
6
>>> u = Test()
>>> u.i
6 # changes to t do not affect new instances of Test

# Namespaces are one honking great idea — let’s do more of those!
>>> Test.__dict__
{‘i’: 6, …}
>>> t.__dict__
{‘i’: 5}
>>> u.__dict__
{}

Notice how the instance variable t.i got out of sync with the “static” class variable when the attribute i was set directly on t. This is because i was re-bound within the t namespace, which is distinct from the Test namespace. If you want to change the value of a “static” variable, you must change it within the scope (or object) where it was originally defined. I put “static” in quotes because Python does not really have static variables in the sense that C++ and Java do.
Although it doesn’t say anything specific about static variables or methods, the Python tutorial has some relevant information on classes and class objects.
@Steve Johnson also answered regarding static methods, also documented under “Built-in Functions” in the Python Library Reference.
class Test(object):
@staticmethod
def f(arg1, arg2, …):
…

@beid also mentioned classmethod, which is similar to staticmethod. A classmethod’s first argument is the class object. Example:
class Test(object):
i = 3 # class (or static) variable
@classmethod
def g(cls, arg):
# here we can use ‘cls’ instead of the class name (Test)
if arg > cls.i:
cls.i = arg # would be the same as Test.i = arg1

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