Python *args, **kwargs 的用法和用处

在 stackoverflow 上看到了对 *args和**kargs 比较好的解释，分享一下，顺便补充一些例子

def func(required_arg, *args, **kwargs):
    # required_arg is a positional-only parameter.
    print(required_arg)

    # args is a tuple of positional arguments,
    # because the parameter name has * prepended.
    if args: # If args is not empty.
        print(args)

    # kwargs is a dictionary of keyword arguments,
    # because the parameter name has ** prepended.
    if kwargs: # If kwargs is not empty.
        print(kwargs)

>>> func()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: func() takes at least 1 argument (0 given)

>>> func("required argument")
required argument

>>> func("required argument", 1, 2, "3")
required argument
(1, 2, "3")

>>> func("required argument", 1, 2, "3", keyword1=4, keyword2="foo")
required argument
(1, 2, "3")
{"keyword2": "foo", "keyword1": 4}

>>> func("required argument", 1, keyword1=4, 2, "3",keyword2="foo")
  File "<ipython-input-24-92187cb5dc7e>", line 1
    func("required argument", 1, keyword1=4, 2, "3",keyword2="foo")
                                            ^
SyntaxError: positional argument follows keyword argument
# 说明参数的顺序不能乱


>>> func(required_arg="required argument", 1, 2, "3", keyword1=4 ,keyword2="foo")
  File "<ipython-input-30-de2b4309d741>", line 1
    func(required_arg="required argument", 1, 2, "3", keyword1=4 ,keyword2="foo")
                                          ^
SyntaxError: positional argument follows keyword argument
# 说明第一个参数只能是positional arg, 不能指定名称

>>> func(1, 2, "3", required_arg="required argument", keyword1=4 ,keyword2="foo")
Traceback (most recent call last):

  File "<ipython-input-28-12444e9d53db>", line 1, in <module>
    func(1, 2, "3", required_arg="required argument", keyword1=4 ,keyword2="foo")

TypeError: func() got multiple values for argument "required_arg"
# 如果把已经定义的参数放在后面，就会出现一个参数有两个值的情况

---

下面举个例子说明 *args， **kwargs 的用处

class Foo(object):
    def __init__(self, value1, value2):
        # do something with the values
        print value1, value2

class MyFoo(Foo):
    def __init__(self, *args, **kwargs):
        # do something else, don"t care about the args
        print "myfoo"
        super(MyFoo, self).__init__(*args, **kwargs)

当一个类需要继承另一个类的时候，例如MyFoo需要继承Foo，假设你不知道Foo有哪些参数，而只关注子类MyFoo的扩展（另一种情况是将来Foo可能会有变化），这时你就可以利用*args, **kwargs 来代表Foo的所有参数，传入到MyFoo

星号 *， 涉及到解包， unpacking， 这又是另一个话题了