Python decorators theory

Question

I have question about clean thory in Python. When:

@decorator_func
def func(bla, alba):
    pass

Is equivalent to:

def func(bla, alba):
    pass
func = decorator_func(func)

So:

@decorator_func(aaa, bar)
def func(bla, alba):
    pass

Is equvalent to...?

Answer 1

It's equivalent to:

def func(bla, alba):
    pass
func = decorator_func(aaa, bar)(func)

Or:

def func(bla, alba):
    pass
decorator = decorator_func(aaa, bar)
func = decorator(func)

So in your second example, decorator_func should be a callable that returns a callable.

Here's an example of such a construction:

class prepend_two_arguments:
    def __init__(self, a, b):
        self.a = a
        self.b = b

    def __call__(self, f):
        def wrapped_function(*args, **kwargs):
            return f(self.a, self.b, *args, **kwargs)
        return wrapped_function

@prepend_two_arguments(1,2)
def f(a, b, c):
    return a+b+c
print(f(3)) # 6

And another one, using only functions:

def add_to_result(x):
    def decorator(fn):
        def wrapped_function(*args, **kwargs):
            return fn(*args, **kwargs)+x
        return wrapped_function
    return decorator

@add_to_result(3)
def my_func(a, b):
    return a+b
print(my_func(1,2)) # 6

Answer 2

Here's an example of a decorator function that works using closures:

def print_string_before(string):
    def decorator_fn(fn):
        def wrapped_fn(*args, **kwargs):
            print string
            return fn(*args, **kwargs)
        return wrapped_fn
    return decorator_fn

Note that decorators can equally return the decorated function (or class), having modified it in some way (e.g. setting an attribute).