Instance method vs. Static method vs. Class method in Python

In a python class, there are three kinds of methods:

• Instance method
• Static method
• Class method

Example explained

Look at the following code, have a test for yourself, what are the output:

class Foo(object):

    variable = 1

    def method(self, param):
        print "run instance method (%s,%s)" % (self, param)

    @classmethod
    def class_method1(cls, param):
        print "run class method (%s,%s)" % (cls, param)

    @classmethod
    def class_method2(cls):
        print cls.variable

    @staticmethod
    def static_method1(param):
        print "run static method (%s)" % param

    @staticmethod
    def static_method2():
        print variable

foo = Foo()

foo.method("instance method")
#Foo.method("instance method again")

foo.class_method1("class method 1")
foo.class_method2()
Foo.class_method1("class method 1 again")
Foo.class_method2()

foo.static_method1("static method 1")
foo.static_method2()
Foo.static_method1("static method 1 again")
Foo.static_method2()

https://gist.github.com/arkilis/341bf667a577b04b8b3a58bce78809a6

Results:

1: foo.method("instance method") — OK

2: Foo.method("instance method again") is wrong, as method is the instance method. To run an instance method, you have to use a class instance

3: foo.class_method1("class method 1") outputs run class method (<class '__main__.Foo'>,class method 1)

4: foo.class_method2() outputs 1

5: Foo.class_method1("class method 1 again") outpus run class method (<class '__main__.Foo'>,class method 1 again)

6: Foo.class_method2() outputs 1

7: foo.static_method1("static method 1") outputs run static method (static method 1)

8: foo.static_method2() is wrong, as static_method2 is static method, which cannot access the instance level variable

9: Foo.static_method1("static method 1 again") outputs run static method (static method 1 again)

10: Foo.static_method2() is wrong, which cannot access the class level variable

Conclusion

• instance method (i.e. method()) must be called by a class instance
• class method can be called either by a class instance or class itself
• static method is quite special in Python, as it is also can be called by a class instance or class itself
• the only difference between a static method and a class method, class method can access the class’s variable that static can’t