函数调用中的星号和双星号运算符是什么意思?

2022-01-30 00:00:00 python parameter-passing syntax iterable-unpacking argument-unpacking

问题描述

* 运算符在 Python 中是什么意思，例如在 zip(*x) 或 f(**k) 之类的代码中?

What does the * operator mean in Python, such as in code like zip(*x) or f(**k)?

  1. 在解释器内部是如何处理的?
  2. 它会影响性能吗?是快还是慢?
  3. 什么时候有用，什么时候没用?
  4. 应该用在函数声明还是调用中?

解决方案

单星 * 将序列/集合解包为位置参数，因此您可以这样做:

The single star * unpacks the sequence/collection into positional arguments, so you can do this:

def sum(a, b):
    return a + b

values = (1, 2)

s = sum(*values)

这将解包元组，使其实际执行为:

This will unpack the tuple so that it actually executes as:

s = sum(1, 2)

双星 ** 的作用相同，只是使用字典并因此命名参数:

The double star ** does the same, only using a dictionary and thus named arguments:

values = { 'a': 1, 'b': 2 }
s = sum(**values)

你也可以组合:

def sum(a, b, c, d):
    return a + b + c + d

values1 = (1, 2)
values2 = { 'c': 10, 'd': 15 }
s = sum(*values1, **values2)

将执行为:

s = sum(1, 2, c=10, d=15)

另请参阅4.7.4 - 解包参数列表部分Python 文档.

Also see section 4.7.4 - Unpacking Argument Lists of the Python documentation.

此外，您可以定义函数来接受 *x 和 **y 参数，这允许函数接受任意数量的位置和/或命名参数t 在声明中特别指定.

Additionally you can define functions to take *x and **y arguments, this allows a function to accept any number of positional and/or named arguments that aren't specifically named in the declaration.

例子:

def sum(*values):
    s = 0
    for v in values:
        s = s + v
    return s

s = sum(1, 2, 3, 4, 5)

或使用 **:

def get_a(**values):
    return values['a']

s = get_a(a=1, b=2)      # returns 1

这可以让您指定大量可选参数，而无需声明它们.

this can allow you to specify a large number of optional parameters without having to declare them.

再一次，你可以组合:

def sum(*values, **options):
    s = 0
    for i in values:
        s = s + i
    if "neg" in options:
        if options["neg"]:
            s = -s
    return s

s = sum(1, 2, 3, 4, 5)            # returns 15
s = sum(1, 2, 3, 4, 5, neg=True)  # returns -15
s = sum(1, 2, 3, 4, 5, neg=False) # returns 15

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