function

1. What is a function

In the development of the project, there are some code blocks that are encapsulated and encapsulated after repeated use. We call them functions.

2. Why define a function

Advantages: Convenience (improves code utilization)
Improve code readability
Easy to debug bugs:

3. Classification of functions

System library functions
custom function

4. Custom function

grammar:

def funcName():
    函数体
----------------
funcName()   //调用函数
基本格式:
  1.以def开始
  2.自定义函数名
  3.小括号（）函数的参数写在里边，后跟冒号
  4.函数体注意缩进

name:

遵循标识符:
1.   以数字字母下划线组成
2.不能以数字开始
3.不能是系统关键字
  func_name funcName

function call

在函数定义之前不能调用
可以重复定义函数 后边的覆盖前边的
形参无默认值时不传参会报错
形参有默认值时可以不传参
形参有默认值且有多个参数时默认值在最后
多个参数有默认值的情况 传参率高的房子前边

keyword arguments

//实参的值可以通过形参的名字来传值
def demo(age,name,height):
    print(age,name,height)

demo(name = 'zhang',age = 1,height = 180)
//所有实参传参如果使用了关键字方式，name所有的参数都要指定对应关键字

parameter
1. After the function name is defined, it is called a formal parameter
2. Arguments after calling a function
3. Parameter values can be changed to improve code reusability
4. Formal parameters and actual parameters are one-to-one correspondence
5. A formal parameter is a formal parameter with a default value that can have a default value
return value

函数里边没有return 返回None
return 后边的值会返回给函数的调用处
想要返回多个值时 可以使用元组 列表 字典等形式
return 返回值可选
return后边只执行依据 其余下边有再多都不执行

def demo(num):
    num=30             
    print(num)            //30
    print(id(num))

num1 = 20
print(id(num1))
demo(num1)                //30
print(num1)               //20

variable length parameter

1.形参前面的*会把实参当做列表形式传
def goodMan(name,*goodList):
    print(goodList)

goodMan('a',['b'])
goodMan('b','c','d',['a','b'])
2.形参前面两个*会把实参当做字典形式传
3.实参前边的*会把 列表 元组转成可变参数
#计算一组数字的平方和
def calc(*nums):
    res = 0
    for x in nums:
        res+=x*x
    print(res)
calc(2,3,4,5)
4.实参前面**会把字典变成可变参数

Variable scope:

Local variables: variables defined in the function body are not available outside the function
Global variable: A variable defined outside the function body is called a global variable. It can get the value inside the function body, but cannot modify the value. If it is modified, a new variable will be declared in the function body.
If you want to use a global variable to declare global, you can modify it

global uses the global variables outside the entire function
nonlocal uses the local variables of the outer function

recursive function

The function itself calls its own process:
the execution core
Termination conditions
If the problems that can be solved by loop and traversal in sub-projects, try not to use recursion? memory usage

Anonymous functions Define functions
without def and use the ==lambda== keyword

func = lambda a,b:a-b
print(func(2,3))  //-1

print((lambda a,b:a-b)(10,20))  //-1

Closure function A
function is nested within a function and the return value of the outer function is a function

Functions are strictly case sensitive

identity operator

is
not is

Higher order functions

Functions that pass functions as parameters are called higher-order functions
- map()

    第一个参数：函数 第二个参数是 列表
    def myMap(x):
        return x*2
    print(list(map(myMap,[1,2,3,4,5])))

reduce

The first argument is a function and
the second argument is a list
def mySum(x,y):
return x+y
print(reduce(mySum,[1,2,3,4])) //10
filter

Parameter: function list
General users filter the sequence
according to the function return value of the parameter to determine whether to filter out
according to True and False

def func(num):
if num%2 ==0:
return True
return False

Python Notes (3) Function Higher-Order Functions