class method ,static method ,bounded method

Occasionally when writing a class you want to include a function that is called from the class, not the instance. Perhaps this method creates new instances, or perhaps it is independent of any properties of any individual instance. Python actually gives you two ways to do this, depending if your method needs to (or should) know about which class called it. Both involve applying decorators to your methods.

A 'class method' receives the class as the first argument, just as a regular instance method receives the instance as the first argument. So, the method is aware if it is being called from its own class or from a subclass.

A 'static method' receives no information about where it is called; it is essentially a regular function, just in a different scope.

Class and static methods can be called straight from the class, as Class.method(), or from an instance as Class().method(). The instance is ignored except for its class. Here's an example of each, along with a regular instance method:

# 's

 1class Class:
 2    @classmethod
 3    def a_class_method(cls):
 4        print 'I was called from class %s' % cls
 5    #
 6    @staticmethod
 7    def a_static_method():
 8        print 'I have no idea where I was called from'
 9    #
10    def an_instance_method(self):
11        print 'I was called from the instance %s' % self
12
13instance = Class()
14
15Class.a_class_method()
16instance.a_class_method()
17# both print 'I was called from class __main__.Class'
18
19Class.a_static_method()
20instance.a_static_method()
21# both print 'I have no idea where I was called from'
22
23Class.an_instance_method()
24
25# raises TypeError
26instance.an_instance_method()
27# prints something like 'I was called from the instance <__main__.Class instance at 0x2e80d0>'

How to use *args and **kwargs in Python

How to use *args and **kwargs in Python

Date: 2008-01-03  |   Modified: 2010-03-15  |   Tags: core, python   |   83 Comments

Or, How to use variable length argument lists in Python.

The special syntax, *args and **kwargs in function definitions is used to pass a variable number of arguments to a function. The single asterisk form (*args) is used to pass a non-keyworded, variable-length argument list, and the double asterisk form is used to pass a keyworded, variable-length argument list. Here is an example of how to use the non-keyworded form. This example passes one formal (positional) argument, and two more variable length arguments.

def test_var_args(farg, *args):
    print "formal arg:", farg
    for arg in args:
        print "another arg:", arg

test_var_args(1, "two", 3)

Results:

formal arg: 1
another arg: two
another arg: 3

Here is an example of how to use the keyworded form. Again, one formal argument and two keyworded variable arguments are passed.

def test_var_kwargs(farg, **kwargs):
    print "formal arg:", farg
    for key in kwargs:
        print "another keyword arg: %s: %s" % (key, kwargs[key])

test_var_kwargs(farg=1, myarg2="two", myarg3=3)

Results:

formal arg: 1
another keyword arg: myarg2: two
another keyword arg: myarg3: 3

Using *args and **kwargs when calling a function

This special syntax can be used, not only in function definitions, but also when calling a function.

def test_var_args_call(arg1, arg2, arg3):
    print "arg1:", arg1
    print "arg2:", arg2
    print "arg3:", arg3

args = ("two", 3)
test_var_args_call(1, *args)

Results:

arg1: 1
arg2: two
arg3: 3

Here is an example using the keyworded form when calling a function:

def test_var_args_call(arg1, arg2, arg3):
    print "arg1:", arg1
    print "arg2:", arg2
    print "arg3:", arg3

kwargs = {"arg3": 3, "arg2": "two"}
test_var_args_call(1, **kwargs)

Results:

arg1: 1
arg2: two
arg3: 3

See also (updated 2009-04-12):

• Section 4.7 More on Defining Functions in the Official Python Tutorial
• PEP 3102 Keyword-Only Arguments
• Section 5.3.4 Calls in the Python Reference Manual