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Using Functions as Decorators

Метки: python

The only constraint on the result of a decorator is that it be callable, so it can properly replace the decorated function. In the above examples, I’ve replaced the original function with an object of a class that has a __call__() method. But a function object is also callable, so we can rewrite the previous example using a function instead of a class, like this:

# PythonDecorators/entry_exit_function.py
def entry_exit(f):
    def new_f():
        print("Entering", f.__name__)
        f()
        print("Exited", f.__name__)
    return new_f

@entry_exit
def func1():
    print("inside func1()")

@entry_exit
def func2():
    print("inside func2()")

func1()
func2()
print(func1.__name__)

new_f() is defined within the body of entry_exit(), so it is created and returned when entry_exit() is called. Note that new_f() is a closure, because it captures the actual value of f.

Once new_f() has been defined, it is returned from entry_exit() so that the decorator mechanism can assign the result as the decorated function.

The output of the line print(func1.__name__) is new_f, because the new_f function has been substituted for the original function during decoration. If this is a problem you can change the name of the decorator function before you return it:

def entry_exit(f):
    def new_f():
        print("Entering", f.__name__)
        f()
        print("Exited", f.__name__)
    new_f.__name__ = f.__name__
    return new_f

The information you can dynamically get about functions, and the modifications you can make to those functions, are quite powerful in Python.