Decorator for python study notes (syntactic sugar)

• what is a decorator
• The knowledge points of decorators (functions are variables, higher-order functions, nested functions)
• Example of a decorator with no arguments
• Decorator example with parameters
• Operation

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1. What is a decorator

Essentially, decorators are higher-order functions that return a function. A decorator is a function

Principles of decorators:

• Do not modify the source code of the decorated object
• Do not modify how the decorated object is called

Second, the knowledge points involved in the decorator

• function as variable
• Higher order functions
• nested functions

Functions are variables:

In python, a variable is first defined, allocated memory space, and then used.

Take x=1, this simple assignment statement as an example. First allocate a space in memory, x points to this memory space, and "1" is stored in this memory space

The function name is also a variable in essence, so after the def test() statement is executed, a space is allocated in the memory, test points to the memory, and the function body is stored in the memory.

Code example:

#Author:Yueru Sun
#Functions are variables
#Example one:
def foo():
    print('in the foo')
foo()
#Example 2:
def bar():
    print('in the bar')
def foo():
    print('in the foo')
    bar()
foo()
#Demo three:
#When variables are used, they are defined first and then used
def foo():
    print('in the foo')
    bar()
def bar():
    print('in the bar')
foo()
#Demo four:
# def foo():
#     print('in the foo')
#     bar()
#foo()# reports an error because bar is not defined yet
# def bar():
#     print('in the bar')

In particular, it should be noted that example 3 can be executed successfully, "example 3" = "example 2"

Because foo() is executed, foo() and bar() are def first, so when foo() is executed, foo() and bar() can be found

Example 4 is not executable. Because bar is not yet defined in memory before foo() executes, it cannot be found

Higher order functions:

A higher-order function is a function that is passed as a parameter or whose return value is a function

Nested functions:

def f1():
    def f2():
        def f3():
            print('from f3')
        f3()
    f2()

f1 ()
f3() #Error, why? see next section

Or explain it from the perspective that a function is a variable, because f3() is defined inside f1(), which is equivalent to a "local variable", so f3() cannot be accessed outside the function.

3. Example of a decorator without parameters

import time
def timer(func): #timer(test1)  func=test1
    def deco(*args,**kwargs):
        start_time=time.time()
        func(*args,**kwargs)   #run test1()
        stop_time = time.time()
        print("the func run time  is %s" %(stop_time-start_time))
    return deco
@timer # test1 = timer (test1)
def test1():
    time.sleep(1)
    print('in the test1') 
test1() #Execution 
result: 
#in the test1 
#the func run time is 1.0033304691314697

The code above explains:

The most essential function of the function timer is to return a deco, which is the address space of a function

The next thing to understand is that @timer is equivalent to: test1=timer(test1), timer(test1) will return deco after execution, then executing test1 is equivalent to executing deco, that is, test1()=deco()

So the result of test1() is:

in the test1
the func run time  is 1.0033304691314697

Another example:

Author:Yueru Sun
import time
def timmer(func):
    def wrapper(*args,**kwargs):
        start_time=time.time()
        func(*args,**kwargs)
        end_time=time.time()
        print('The running time of func is: %s'%(end_time-start_time))
    return wrapper
@timmer
def test():
    time.sleep(2)
    print('test')
test()
@timmer
def test1(number):
    time.sleep(1)
    print('The %s of func is executed'%(number))
test1(1)

　

Or the above understanding process, sort out the implementation process:

First, the timer function is defined, and then executing @timer is equivalent to executing test1=timer(test1), and returning deco after execution, then test1(number)=deco(number)

Fourth, the decorator example with parameters

If the decorator needs to pass in parameters, then nest the function again on the basis of the above

def timmer1(text):
    def out_wrapper(func):
        def wrapper(*args,**kwargs):
            start_time=time.time()
            func(*args,**kwargs)
            end_time=time.time()
            print('%s func running time is %s'%(text,end_time-start_time))
        return wrapper
    return out_wrapper
@timmer1('sun')
def test2():
    print('test2')
test2 ()

The specific understanding will not be repeated, you can review this article:

http://www.cnblogs.com/alex3714/articles/5765046.html

5. Homework

Write a decorator to add authentication functions to multiple functions (the user's account password comes from a file), requiring a successful login, and subsequent functions do not need to enter the user name and password