Python：带有包装器的装饰器行为

Question

I don´t understand why the line "cache = {}" is executed only the first time i call the function multiply(x, y). After that, that line is ignored. This makes the program work well, but i don´t understand that behaviour.

def memoize(func):

     cache = {}
     print("cache")

     @functools.wraps(func)
     def wrapper(*args):
         if args in cache:
             return cache[args]

         result = func(*args)
         cache[args] = result

         return result

     return wrapper

 @memoize
 def multiply(x, y):
     return x * y

 print(multiply(2, 3))
 print(multiply(2, 3))

The result is:

 cache
 6
 6

So the lines "cache = {}" and "print("cache")" were only executed the frist time. Thanks

Answer 1

The part of the decorator code which is "executed everytime" is the wrapper function. The code outside that wrapper function is executed just when the operator is applied -

This nothing magic or "new" - just see the order things are and what is called when - the only thing out of normal execution order which cold be called just a bit magic is the decoration itself- just remember that:

 @memoize
 def multiply(x, y):
     return x * y

is just the same thing as:

def multiply(x, y):
     return x * y

multiply = memoize(multiply)

Also, if you are trying to understand decorators, leave the functools.wraps call out of it for now. It is good for production code, and for filling in small details: it disguises your wrapper function as the inner function it decorates (for example, the function wrapper __name__ attribute is set to multiply in this example), but is an unnecessary complication when trying to understand decorators.