Nineteen, python precipitation road -- decorator

1. Preliminary knowledge for implementing decorators

Decorator = higher order function + cold element nesting + closure

1. Definition of high price function:

    1. The parameter received by the function is a function name
    2. The return value of the function is a function name
    3. Any one of the above conditions can be called a higher-order function

Example 1: Foreshadowing

1  import time
 2  #Example 1 
3  def fn():
 4      print ( ' This is the called function ' )
 5      time.sleep(2 )
 6  
7  def test(func):
 8      print ( ' The higher-order function is about to start running ' )
 9      start_time = time.time()
 10      func()
 11      end_time = time.time()
 12      print ( ' The running time of the called function %s ' %(end_time - start_time))
 13 
14 test(fn)

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Example 2

1  #Example 2 
2  import time
 3  def fn():
 4      print ( ' This is the called function ' )
 5      time.sleep(2 )
 6  def test(func):
 7      print ( ' Executing a higher-order function ' )
 8      return func
 9  #The first call 
10 f = test(fn)
 11  print (f)
 12  f()
 13  
14  # # The second call, upgraded version: assign the return value of the calling function test(func) to a The variable fn with the same name as the called function fn(), the final effect is 
15                       # (1) Do not change the source code of the called function fn() 
16                        # (2) Do not change the calling method of the called function fn() 
17 fn = test(fn)
 18  print (fn)
 19 fn()

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1  is executing a higher order function
 2 <function fn at 0x0000017065C999D8>
 3  this is the called function
 4  is executing a higher order function
 5 <function fn at 0x0000017065C999D8>
 6 this is the called function

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Upgraded version: assign the return value of the calling function test(func) to a variable fn with the same name as the called function fn(), the final effect is 
                      ( 1) does not change the source code of the called function fn() 
                      ( 2) Does not change how the called function fn() is called

 Example 3  The result of this situation is that one more line is run

1  #Example 3 
2  import time
 3  def fn():
 4      print ( ' This is the called function ' )
 5      time.sleep(2 )
 6  
7  def test(func):
 8      print ( ' Start executing the calling function ' )
 9      start_time = time.time()
 10      func()
 11      end_time = time.time()
 12      return func       #The result of this situation is that one more line is run 
13  
14 fn = test(fn)
15 fn()

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1  start executing the calling function
 2  this is the called function
 3 this is the called function

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Example 4

1  #Example 4 
2  def fn():
 3      print ( ' This is the called function ' )
 4  def test(func):
 5      print ( ' The higher-order function is being executed ' )
 6      res = func()
 7      print ( ' The The calling function is executed ' )
 8      return res
 9  
10 fn = test(fn)
 11  # fn() # Error: TypeError: 'NoneType' object is not callable

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1 The  higher-order function is being executed
 2  This is the called function
 3 The called function has finished executing

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Even if a value is assigned, it still cannot solve the result of printing one more line. So a single layer function can't solve this problem

2. Nested functions