Python methods --- instance method, classmethod, staticmethod

Do you know what are the differences?

1. instance method, must be called by an instance: d.instance_method(), demo.instance_method(d)
2. class method(the first arg must be the class itself), can be called by class or instance: d.class_method(), demo.class_method()
3. static method, can be called by class or instance: d.static_method(), demo.static_method()

class demo:

    def __init__(self):

        self.i=0 # i, instance variable

    def instance_method(self):

        self.i +=1

        print self.i

    @classmethod

    def class_method(cls): # cls is an instance of demo

        cls.v +=1

        print cls.v

    @staticmethod

    def static_method(s): # s, just a parameter

        s+=1

        print s

    v=0  # class variable

Result:

>>> import sys

>>> sys.path.append('/home/mengyg/Documents/pythondemo')

>>> from methods import *

>>> demo.instance_method() # instance method cann't be called by class

Traceback (most recent call last):

  File "<pyshell#49>", line 1, in <module>

    demo.instance_method()

TypeError: unbound method instance_method() must be called with demo instance as first argument (got nothing instead)

>>> demo.v

0

>>> demo.i # instance variable, not class variable

Traceback (most recent call last):

  File "<pyshell#51>", line 1, in <module>

    demo.i

AttributeError: class demo has no attribute 'i'

>>> demo.class_method()

1

>>> demo.static_method() # must have a parameter

Traceback (most recent call last):

  File "<pyshell#53>", line 1, in <module>

    demo.static_method()

TypeError: static_method() takes exactly 1 argument (0 given)

>>> demo.static_method(1)

2

>>> demo.v

1

>>> d=demo()

>>> d.i

0

>>> d.v # the value of v is 1

1

>>> d.instance_method()

1

>>> d.class_method()

2

>>> d.v

2

>>> d.static_method(4)

5

>>> d.v

2

>>> d.v=3 # if the class variable is changed external, the value will be different with class itself, i don't know the reason

>>> demo.v

2

>>> demo.v=5

>>> demo.v

5

>>> d.v

3

>>> 

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Kind Regards,
Yonggang Meng