Python decorator principle and usage analysis

This article mainly introduces the principle and usage of Python decorators, and analyzes the concepts, principles, usage methods and related operation precautions of Python decorators in the form of examples. Friends in need can refer to this article

Examples describe the principles and usage of Python decorators. Share it with everyone for your reference, the details are as follows:

1. The essence of the decorator is the function , which is mainly used to decorate other functions, that is, Add additional functions to other functions

2. Principles of decorators:

(1) Decorators cannot modify the source code of the decorated function

(2) Decorators cannot modify the calling method of the decorated function

3. Knowledge reserve for implementing decorators

(1) Functions in Python are 'variables'

a. Storage of variables in Python

x='Tomwenxing'
y=x

[Explanation]:

##When the Python interpreter encounters the statement x='Tomwenxing' , it mainly completes two tasks:

## 1. Creates a space in the memory to store the string 'Tomwenxing'

　2. Create a variable named x in the memory and use it to point to the memory space occupied by the string 'Tomwenxing' (can be understood as the room and room number Relationship)

And the statement y=x means to assign the reference of variable x to the string to variable y,

That is, create a variable y in the memory and point it to the memory space pointed to by the variable x

b. The function is in Storage in Python

def test():
  pass

[Explanation]:

In Python, the storage of functions is similar to variables. Take the above function as an example, Python explanation It mainly does two things:

1. Open up a memory space in the memory to store the string of the function code (in this example, the code has only one sentence: pass)

#　2. Create a variable test in the memory to point to the memory space where the function code string is stored(Equivalent to test='function body')

So in Python, functions are variables

(2) Higher-order function (any one of the following two conditions is met to be a higher-order function)

a. Pass a function name as an actual parameter to another function

[Impact on decorators]: To achieve the effect of "adding functions to the decorated function without modifying its source code"

import time
def bar():
  time.sleep(2)
  print('in the bar')
def test(func):
  start_time=time.time()
  func()
  stop_time=time.time()
  print('函数的运行时间为:',stop_time-start_time)
test(bar)

Running results:

in the bar

The running time of the function is: 2.0021145343780518

b. The return value contains the function name

[Impact on decorators]: Achieve the effect of "not changing the calling method of the function"

import time
def bar():
  time.sleep(3)
  print('in the bar')
def test2(func):
  print('新添加的功能')
  return func
bar=test2(bar)
bar()

Running results:

Newly added functions

in the bar

(3) Nested function: Use def in a function body to declare a new function (not a call)

def foo():
  print('in the foo')
  def bar(): #声明一个新的函数，而不是调用函数
    print('in the bar')
  bar()
foo()

Running result:

in the foo

in the bar

4. Decorator syntax: High-order function nested function => Decorator (the following example can use the pycharm debugger Debug it and see the running sequence of the code)

import time
def timer(func):
  def deco(*args,**kwargs):#使用了不定参数
    start_time=time.time()
    res=func(*args,**kwargs) #运行函数
    stop_time=time.time()
    print('运行时间:',stop_time-start_time)
    return res # 若无返回值，则返回None
  return deco
@timer #等价于test1=timer(test1)=deco,即test1()=deco()
def test1():
  time.sleep(3)
  print('in the test1')
@timer #等价于test2=timer(test2)=deco,即test2(name)=deco(name)
def test2(name):
  time.sleep(3)
  print('in the test2',name)
test1()
print('-------------分界线------------------------')
test2('Tomwenxing')

Running result:

in the test1

Running time: 3.0001718997955322

-------- -----Boundary line---------------------
in the test2 Tomwenxing
Running time: 3.000171422958374

5. Decorator with parameters

# -*- coding:utf-8 -*-
user,passwd='Tomwenxing','123'
#如装饰器带参数，一般是三层嵌套
def auth(auth_type): #第一层的参数是装饰器的参数
  def outer_wrapper(func):#第二层的参数是装饰器要装饰的目标函数
    def wrapper(*args,**kwargs):#第三次的参数是目标函数的参数
      if auth_type=='local':
        username = input('Username:').strip()
        password = input('Password:').strip()
        if user == username and passwd == password:
          print('用户Tomwenxing已经成功登录！')
          res = func(*args, **kwargs) #运行目标函数
          return res
        else:
          exit('用户名或密码有错误')
      elif auth_type=='ldap':
        print('暂不支持这种登录方式！')
    return wrapper
  return outer_wrapper
def index():
  print('欢迎来到index页面')
@auth(auth_type='local') #home=wrapper()
def home(name):
  print('%s,欢迎来到home页面' %name)
  return 'This is home page'
@auth(auth_type='ldap')
def bbs():
  print('欢迎来到bbs页面 ')
index()
print('----------------------分界线-------------------')
print('函数的返回值为:',home('wenxing'))
print('----------------------分界线-------------------')
bbs()

Running result:

Welcome to the index page

------------ ----------Boundary line------------------

Username:Tomwenxing
Password:123
User Tomwenxing has succeeded Log in!
wenxing, welcome to the home page
The return value of the function is: This is home page
----------------------Boundary line ------------------
This login method is not supported yet!

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