Python Tip 2: Python *args **kwargs 用法

关于Python中 *args 和 **kwargs的用法,网上有很多介绍文章,还是先看一段官方的解释:

When a final formal parameter of the form **name is present, it receives a dictionary (see Mapping Types — dict) containing all keyword arguments except for those corresponding to a formal parameter. This may be combined with a formal parameter of the form *name (described in the next subsection) which receives a tuple containing the positional arguments beyond the formal parameter list. (*name must occur before **name.)

首先需要了解Python函数传递参数的方式有两种:

位置参数(positional argument)
关键词参数(keyword argument)

然后再来看*args(*name)与**kwargs(**name)的区别,两者都是python中的可变参数:

*args表示任何多个无名参数,它本质是一个tuple;

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>>> def args_one(narg, *args):
...     print("Normal arg:", narg)
...     for arg in args:
...         print("other args:", arg)
... 
>>> args_one(1, 2, 3, 4, 5)
Normal arg: 1
other args: 2
other args: 3
other args: 4
other args: 5
>>> args_one('a', 'b', 'c', 'd', 'e')
Normal arg: a
other args: b
other args: c
other args: d
other args: e
>>> tuple_args = (1, 2, 3, 4, 5)
>>> args_one(1, tuple_args)
Normal arg: 1
other args: (1, 2, 3, 4, 5)
>>> args_one(1, *tuple_args)
Normal arg: 1
other args: 1
other args: 2
other args: 3
other args: 4
other args: 5

**kwargs表示关键字参数,它本质上是一个dict:

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>>> def args_two(narg, **kwargs):
...     print("Normal arg:", narg)
...     for key, value in kwargs.items():
...         print("other keyword arg: %s: %s" % (key, value))
... 
>>> args_two(narg=1, karg1="one", karg2="two")
Normal arg: 1
other keyword arg: karg1: one
other keyword arg: karg2: two
>>> key_args = {"a": "A", "b": "B", "c": "C"}
# 以下两个是错误的调用示范
>>> args_two('a', key_args)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: args_two() takes 1 positional argument but 2 were given
>>> args_two('a', *key_args)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: args_two() takes 1 positional argument but 4 were given
# 正确做法
>>> args_two('a', **key_args)
Normal arg: a
other keyword arg: a: A
other keyword arg: b: B
other keyword arg: c: C

同时使用*args和**kwargs时,必须*args参数列要在**kwargs前, 让我们最后来看一个综合例子:

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>>> def args_fun(normal, *args, **kwargs):
...     print("narmal=", normal)
...     print("*args=", args)
...     print("**args=", kwargs)
... 
>>> args_fun(1)
narmal= 1
*args= ()
**args= {}
>>> args_fun(1, 2)
narmal= 1
*args= (2,)
**args= {}
>>> args_fun(1, 2, 3)
narmal= 1
*args= (2, 3)
**args= {}
>>> args_fun(1, a=1, b=2, c=3)
narmal= 1
*args= ()
**args= {'a': 1, 'b': 2, 'c': 3}
>>> args_fun(1, 2, 3, a=1, b=2, c=3)
narmal= 1
*args= (2, 3)
**args= {'a': 1, 'b': 2, 'c': 3}
>>> args_fun(1, a=1, b=2, c=3, 2, 3)
  File "<stdin>", line 1
SyntaxError: positional argument follows keyword argument

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