Introduction to several special uses of asterisk variables in Python

In addition to being used for multiplication numerical operations and exponentiation operations, asterisks in Python also have a special usage of "adding a single asterisk or two asterisks before variables " to realize the incoming of multiple parameters or the disassembly of variables. This article will introduce the usage of "asterisk parameter" in detail.

1. What is an asterisk variable?

Initially, the asterisk variable is used to pass the parameters of the function. In the following example, a single asterisk represents This position receives any number of non-keyword parameters and converts them into tuples at the *b position of the function. The double asterisk indicates that this position receives any number of keyword parameters and converts them into a dictionary at the **b position. ：

#!/usr/bin/env python
#coding=utf-8
#--------
def one(a,*b):
  """a是一个普通传入参数，*b是一个非关键字星号参数"""
  print(b)
one(1,2,3,4,5,6)
#--------
def two(a=1,**b):
  """a是一个普通关键字参数，**b是一个关键字双星号参数"""
  print(b)
two(a=1,b=2,c=3,d=4,e=5,f=6)

#Program output

(2, 3, 4, 5, 6)
{'b': 2, 'c': 3, 'e': 5, 'f': 6, 'd': 4}

#As you can see from the output, in the first function, any number of parameters without keywords can be passed to the *b position, and *b will Convert these incoming parameters into a tuple, and the following call

one(1,2,3,4,5,6)

#pass in one(a,*b) Finally, it is equivalent to

one(1,(2,3,4,5,6))

#In the second function, the position of **b can receive any number of Keyword parameters, the following call

two(a=1,b=2,c=3,d=4,e=5,f=6)

#pass in one( a,*b), it is equivalent to

two(a=1,{'b': 2, 'c': 3, 'e': 5, 'f': 6, 'd' : 4})

After understanding the basic usage of single asterisk and double asterisk, let’s take a look at their extended usage.

2. Examples of single asterisk variables

Single asterisk variables can not only be used in parameter transfer of functions. In fact, using a single asterisk prefix for an ordinary variable can make this variable Split into single elements, please see the following example:

#!/usr/bin/env python
#coding=utf-8
#--------
def one(*x):
  """输出传入的第一个参数"""
  print(x[0])
#--------
lst=["a","b","c","d"]
stri="www.pythontab.com"
one(stri,lst)
one(*lst)
one(*stri)

#Program output

www.pythontab.com
a
w

#The first call of one(stri,lst) is equivalent to one(*x) after substitution With

one((["a","b","c","d"],"www.pythontab.com"))

#The second call to one( *lst), after substituting one(*x), it is equivalent to

one(("a","b","c","d"))

#the third time Calling one(*stri) and substituting one(*x) is equivalent to

one(("w","w","w",".","q","i", "n","g","s","w","o","r","d",".","c","o","m"))

#If you use a single asterisk in front of a variable, it is actually a disassembly operation of the variable. The individual elements in the variable are disassembled and then passed into the one() function in turn. After passing in the one() function, The one() function will save these incoming single elements as a tuple, which is why we print(x[0]) can extract the first element

In order to verify this, we modify Take a look at the one() function, as follows:

#!/usr/bin/env python
#coding=utf-8
#--------
def one(*x):
  """一个错误的实例，尝试修改传入的第一个参数值，引发异常"""
  print(x[0])
  x[0]="pythontab"
lst=["a","b","c","d"]
one(*lst)

#We know that the list can be changed, we split the list and pass it into the one() function, and try to change the value of the first element inside the function. The result The "TypeError" exception is triggered. You can try it yourself. The reason for this result has been explained above. No matter what the original type of the incoming parameters is, one(*x) receives these incoming parameters at the position of *x. Afterwards, it will be saved as a "tuple", and the tuple cannot be changed

Let's look at a few examples:

#!/usr/bin/env python
#coding=utf-8
#--------
def one(*x):
  """打印出传入参数"""
  for a in x:
    print(a)
lst=["abc",123,"www.pythontab.com"]
stri="abcd"
dect={1:"one",2:"two",3:"three"}
one(*lst)
one(*stri)
one(*dect)

#Program output

abc
123
www.pythontab.com
a
b
c
d
1
2
3

#Previous Both calls are easy to understand. Finally, we passed in a dictionary element and found that only the key of the dictionary element was output and did not contain the value. In fact, a single asterisk cannot read the value in the dictionary. It will always only The keys in the dictionary will be read. If you want to read the values ​​in the dictionary, you need to use double asterisks

3. Double asterisk variable examples

At the end of section 2, we use A single asterisk splits a dictionary and passes it to the function, but only the keys of the dictionary can be obtained. The following demonstrates how to use double asterisks to obtain the values ​​of the dictionary:

#!/usr/bin/env python
#coding=utf-8
#--------
def one(**x):
  """将传入的关键字参数的值保存成元组输出"""
  print(x)
  b=()
  for a in x.keys():
    b+=(x[a],)
  print(b)
dect={"one":1,"two":2,"three":3}
one(**dect)

#Program output

{'three': 3, 'one': 1, 'two': 2}
(3, 1, 2)

# Using a double asterisk prefix on a dictionary is equivalent to splitting it into the form of keyword parameters. **dect is equivalent to splitting the dictionary into the following form

one=1,two=2, three=3

#将上面这些关键字参数传入one(**x)，就等价与（还记得前面说的，双星号将接收到的所有关键字参数都保存成一个字典吧）

one({"one":1,"two":2,"three":3})

#既然是字典，那么字典中的所有方法都能使用，使用for循环遍历这个字典的键，然后使用一个元组来添加这些键对应的值，最后打印出这个元组

Ps：注意，使用这种方法将字典传入函数的时候，字典的键的命名要符合python变量的命名规则，通过上面的分析也不难看出，双星号会将字典首先转换成关键字参数的形式，就相当于使用字典中的键作为变量名，如果键不符合变量命名规则，则会抛出一个"TypeError"异常，大家可以尝试着颠倒一下上面字典中的键和值，使用数字作为键，看看会出现什么问题。

在一个函数的接收参数中，同时出现"非关键字参数（位置参数）"和"关键字参数"时，可以使用一个单星号来分隔这两种参数，例如：

#!/usr/bin/env python
#coding=utf-8
#--------
def mix(a,b,*,x,y):
  """位置参数与关键字参数混合"""
  return a,b,x,y
#星号前面的a和b是位置参数，星号后面的x和y是关键字参数，调用mix()函数并传入参数时，关键字参数一定要使用"变量名=值"的形式传入数据，如果同位置参数一样传入数据，就会引发一个TypeError异常
print(mix(1,2,x=3,y=4))

#程序输出

(1, 2, 3, 4)

#在上面的mix函数中，如果位置参数与关键字参数之间已经存在了一个单星号位置参数，那么，这个参数后面的就都是关键字参数，也不需要再使用星号来分隔他们了，例如

#!/usr/bin/env python
#coding=utf-8
#--------
def mix(a,b,*c,x,y):
  """位置参数与关键字参数混合"""
  return a,b,c,x,y
#在*c的位置可以输入任意多个位置参数值
print(mix(1,2,3,4,5,x=6,y=7))

#程序输出

(1, 2, (3, 4, 5), 6, 7)

如果我们要在一个函数中包含多种参数的组合，必须遵守这样的顺序：位置参数（必选参数），默认参数，单星号参数或星号分隔符，关键字参数，双星号参数；

请看下面的实例：

#!/usr/bin/env python
#coding=utf-8
#--------
def mix(a,b=0,*c,x,**y):
  """位置参数与关键字参数混合"""
  return a,b,c,x,y
print(mix(1,2,3,4,5,x=6,y=7,z=8))

#程序输出

(1, 2, (3, 4, 5), 6, {'y': 7, 'z': 8})

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