**(双星/星号)和 *(星/星号)对参数有什么作用? [英] What does ** (double star/asterisk) and * (star/asterisk) do for parameters?

问题描述

在下面的方法定义中，* 和 ** 对 param2 有什么作用?

In the following method definitions, what does the * and ** do for param2?

def foo(param1, *param2):
def bar(param1, **param2):

推荐答案

*args 和 **kwargs 是一个常见的习惯用法，它允许函数有任意数量的参数如更多关于定义函数部分所述在 Python 文档中.

The *args and **kwargs is a common idiom to allow arbitrary number of arguments to functions as described in the section more on defining functions in the Python documentation.

*args 将为您提供所有函数参数 作为元组:

The *args will give you all function parameters as a tuple:

def foo(*args):
    for a in args:
        print(a)        

foo(1)
# 1

foo(1,2,3)
# 1
# 2
# 3

**kwargs 会给你所有关键字参数，除了那些对应于作为字典的形式参数的参数.

The **kwargs will give you all keyword arguments except for those corresponding to a formal parameter as a dictionary.

def bar(**kwargs):
    for a in kwargs:
        print(a, kwargs[a])  

bar(name='one', age=27)
# name one
# age 27

这两种习语都可以与普通参数混合使用，以允许一组固定参数和一些可变参数:

Both idioms can be mixed with normal arguments to allow a set of fixed and some variable arguments:

def foo(kind, *args, **kwargs):
   pass

也可以反过来使用:

def foo(a, b, c):
    print(a, b, c)

obj = {'b':10, 'c':'lee'}

foo(100,**obj)
# 100 10 lee

*l 习语的另一种用法是在调用函数时解压参数列表.

Another usage of the *l idiom is to unpack argument lists when calling a function.

def foo(bar, lee):
    print(bar, lee)

l = [1,2]

foo(*l)
# 1 2

在 Python 3 中，可以在赋值的左侧使用 *l (Extended Iterable Unpacking)，尽管在这种情况下它给出了一个列表而不是一个元组:

In Python 3 it is possible to use *l on the left side of an assignment (Extended Iterable Unpacking), though it gives a list instead of a tuple in this context:

first, *rest = [1,2,3,4]
first, *l, last = [1,2,3,4]

Python 3 还添加了新语义(请参阅 PEP 3102):

Also Python 3 adds new semantic (refer PEP 3102):

def func(arg1, arg2, arg3, *, kwarg1, kwarg2):
    pass

此类函数只接受3个位置参数，*之后的所有内容只能作为关键字参数传递.

Such function accepts only 3 positional arguments, and everything after * can only be passed as keyword arguments.

• 一个 Python dict，语义上用于关键字参数传递，是任意排序的.但是，在 Python 3.6 中，关键字参数保证记住插入顺序.
• **kwargs 中元素的顺序现在对应于关键字参数传递给函数的顺序."- Python 3.6 的新增功能
• 事实上，CPython 3.6 中的所有 dict 都会记住插入顺序作为实现细节，这在 Python 3.7 中成为标准.

• A Python dict, semantically used for keyword argument passing, are arbitrarily ordered. However, in Python 3.6, keyword arguments are guaranteed to remember insertion order.
• "The order of elements in **kwargs now corresponds to the order in which keyword arguments were passed to the function." - What’s New In Python 3.6
• In fact, all dicts in CPython 3.6 will remember insertion order as an implementation detail, this becomes standard in Python 3.7.

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