A brief analysis of the parameters of Python functions

1. Introduction

Python’s function definition is very simple, but its flexibility is very large. In addition to the normally defined required parameters, you can also use default parameters, variable parameters and keyword parameters, so that the interface defined by the function can not only handle complex parameters, but also simplify the caller's code.

2. Variable parameters

In Python functions, there are also Variable parameters can be defined. As the name suggests, variable parameters mean that the number of parameters passed in is variable, it can be 1, 2, or any number, or it can be 0.

Case: Please calculate a2 b 2 c2….

Take a math problem as an example, given a set of numbers a, b, c...,

To define this function, the input parameters must be determined. Since the number of parameters is uncertain, I first thought that a, b, c... could be passed in as a list or tuple.

The function can be defined as follows:

def calc(numbers):   
  sum = 0    
  for n in numbers:       
    sum = sum + n * n   
  return sum

But when calling, you need to assemble a list or tuple first:

print(calc([1, 2, 3]))
#结果：14
print(calc((1, 3, 5, 7))
#结果：84

##Code Simplification

If you use variable parameters, the way to call the function can be simplified to this:

So, change the parameters of the function to variable parameters:

def calc(*numbers):    
  sum = 0    
  for n in numbers:      
    sum = sum + n * n   
    return sum

Compared with defining a list or tuple parameter, defining a variable parameter only adds an * sign in front of the parameter.

Inside the function, the parameter numbers receives a tuple, so the function code remains completely unchanged. However, when calling this function, you can pass in any number of parameters, including 0 parameters:

print(calc(1, 2))
#结果：5
print(calc())
#结果：0

If there are already What should I do if I want to call a list or tuple with a variable parameter?

例：

nums = [1, 2, 3]


print(calc(nums[0], nums[1], nums[2]))

这种写法当然是可行的，问题是太繁琐。

所以Python允许在list或tuple前面加一个*号，把list或tuple的元素变成可变参数传进去：

nums = [1, 2, 3]


calc(*nums)

*nums表示把nums这个list的所有元素作为可变参数传进去。这种写法相当有用，而且很常见。

三、关键字参数

可变参数允许传入0个或任意个参数，这些可变参数在函数调用时自动组装为一个tuple。

而关键字参数允许传入0个或任意个含参数名的参数，这些关键字参数在函数内部自动组装为一个dict。请看示例：

def person(name, age, **kw):    
  print('name:', name, 'age:', age, 'other:', kw)

函数person除了必选参数name和age外，还接受关键字参数kw。在调用该函数时，可以只传入必选参数：

person('Michael', 30)

运行结果：

也可以传入任意个数的关键字参数：

extra = {'city': 'Beijing', 'job': 'Engineer'}
person('Jack', 24, **extra)

关键字参数有什么用？

它可以扩展函数的功能。

比如，在person函数里，保证能接收到name和age这两个参数，但是，如果调用者愿意提供更多的参数，也能收到。

试想正在做一个用户注册的功能，除了用户名和年龄是必填项外，其他都是可选项，利用关键字参数来定义这个函数就能满足注册的需求。

extra表示把extra这个dict的所有key-value用关键字参数传入到函数的**kw参数，kw将获得一个dict，注意kw获得的dict是extra的一份拷贝，对kw的改动不会影响到函数外的extra。

4. Summary

This article is based on the basics of Python and introduces the parameters of Python functions. Python's functions have a very flexible parameter form, which can not only implement simple calls, but also pass in very complex parameters. Common variable parameters and keyword parameters are introduced.

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