星号 * 在 Python 中是什么意思? [英] What does asterisk * mean in Python?

问题描述

* 在 Python 中是否与在 C 中一样具有特殊含义?我在 Python Cookbook 中看到了这样一个函数:

Does * have a special meaning in Python as it does in C? I saw a function like this in the Python Cookbook:

def get(self, *a, **kw)

能否请您向我解释一下或指出我在哪里可以找到答案(Google 将 * 解释为通配符，因此我找不到满意的答案).

Would you please explain it to me or point out where I can find an answer (Google interprets the * as wild card character and thus I cannot find a satisfactory answer).

推荐答案

参见 语言参考中的功能定义.

如果表单 *identifier 是目前，它被初始化为一个元组接收任何多余的位置参数，默认为空元组.如果表单 **identifier 是目前，它被初始化为一个新的字典收到任何多余的关键字参数，默认为新的空字典.

If the form *identifier is present, it is initialized to a tuple receiving any excess positional parameters, defaulting to the empty tuple. If the form **identifier is present, it is initialized to a new dictionary receiving any excess keyword arguments, defaulting to a new empty dictionary.

另请参阅函数调用.

假设你知道什么是位置参数和关键字参数，这里有一些例子:

Assuming that one knows what positional and keyword arguments are, here are some examples:

示例 1:

# Excess keyword argument (python 2) example:
def foo(a, b, c, **args):
    print "a = %s" % (a,)
    print "b = %s" % (b,)
    print "c = %s" % (c,)
    print args

foo(a="testa", d="excess", c="testc", b="testb", k="another_excess")

正如你在上面的例子中看到的，我们在foo函数的签名中只有参数a, b, c.由于 d 和 k 不存在，它们被放入 args 字典中.程序的输出为:

As you can see in the above example, we only have parameters a, b, c in the signature of the foo function. Since d and k are not present, they are put into the args dictionary. The output of the program is:

a = testa
b = testb
c = testc
{'k': 'another_excess', 'd': 'excess'}

示例 2:

# Excess positional argument (python 2) example:
def foo(a, b, c, *args):
    print "a = %s" % (a,)
    print "b = %s" % (b,)
    print "c = %s" % (c,)
    print args

foo("testa", "testb", "testc", "excess", "another_excess")

这里，由于我们正在测试位置参数，多余的必须放在最后，并且 *args 将它们打包成一个元组，所以这个程序的输出是:

Here, since we're testing positional arguments, the excess ones have to be on the end, and *args packs them into a tuple, so the output of this program is:

a = testa
b = testb
c = testc
('excess', 'another_excess')

您还可以将字典或元组解包为函数的参数:

You can also unpack a dictionary or a tuple into arguments of a function:

def foo(a,b,c,**args):
    print "a=%s" % (a,)
    print "b=%s" % (b,)
    print "c=%s" % (c,)
    print "args=%s" % (args,)

argdict = dict(a="testa", b="testb", c="testc", excessarg="string")
foo(**argdict)

打印:

a=testa
b=testb
c=testc
args={'excessarg': 'string'}

和

def foo(a,b,c,*args):
    print "a=%s" % (a,)
    print "b=%s" % (b,)
    print "c=%s" % (c,)
    print "args=%s" % (args,)

argtuple = ("testa","testb","testc","excess")
foo(*argtuple)

打印:

a=testa
b=testb
c=testc
args=('excess',)

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