Python static variable

Variables declared inside the class definition, but not inside a method are class or static variables:

>>> class MyClass: 
...     i = 3 
... 
>>> MyClass.i 
3  

As @ Daniel points out, this creates a class-level "I" variable, but this is distinct from any instance-level "I" variable, So you coshould 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 variable can't be accessed from an instance at all.

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 incluented 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 has es 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.

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@ 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 abve:

>>> 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 

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 incluented 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 the the same as  Test.i = arg1