为什么调用Python时不执行包装函数？

Question

I have this simple code.

def decor(func):
  def wrap():
    print("============")
    func()
    print("============")
  return wrap

def print_text():
  print("Hello world!")

decor(print_text())

Why does this only print "Hello world!" and not the two wrappers?

Answer 1

Here you are evaluating print_text (thus printing "Hello World!"), and passing the result to decor :

decor(print_text())

Here we are passing print_text to decor , and calling the resulting function which it returns, which is wrap ::

decor(print_text)()

Notice that the first case will NOT call the function returned from decor . Try to call it and see what happens:

decor(print_text())()

TypeError: 'NoneType' object is not callable

because func is now None .

Answer 2

That is because you only return the function wrap - It is not called. If you do call it, either by assigning the result to a variable, or by using decor(print_text)() directly. Note that you should use print_text and not print_text() , as the latter will give the result of the function to decor, rather than the function itself. The working version would be:

def decor(func):
  def wrap():
    print("============")
    func()
    print("============")
  return wrap

def print_text():
  print "Hello world!"

wrapped_function = decor(print_text)
wrapped_function()

Answer 3

You are calling decor with a single argument. That argument is whatever print_text() returns. Well, print_text() prints something and then returns None. So far, your output is just Hello world! . None is now passed to decor , and it returns the wrap() function. Nowhere is wrap() actually called, and your final output is just Hello world! .

You are misunderstanding the use of decorators. The correct syntax is this:

@decor
def print_text():
    print("Hello world!")

That is a shortcut for this:

def print_text():
    print("Hello world!")

print_text = decor(print_text)

Now you can see that it is the returns of decor(print_text) (the wrap() function) that is being called, and everything is printed.