decorator function syntax python

Question

I am learning decorator functions in python and I am wrapping my head around the @ syntax.

Here is a simple example of a decorator function which calls the relevant function twice.

def duplicator(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        func(*args, **kwargs)
        func(*args, **kwargs)
    return wrapper

If I understand correctly, it appears that:

@duplicator
def print_hi():
    print('We will do this twice')

Is equivalent to:

print_hi = duplicator(print_hi)
print_hi()

However, let's consider if I move to a more complex example. E.g. instead of calling the function twice, I want to call it a user-defined amount of times.

Using an example from here: https://realpython.com/primer-on-python-decorators/

def repeat(num_times):
    def decorator_repeat(func):
        @functools.wraps(func)
        def wrapper_repeat(*args, **kwargs):
            for _ in range(num_times):
                value = func(*args, **kwargs)
            return value
        return wrapper_repeat
    return decorator_repeat

I can call this via:

@repeat(num_times=4)
def print_hi(num_times):
    print(f"We will do this {num_times} times")

However, this is most certainly not equivalent to:

print_hi = repeat(print_hi)

Because we have the extra argument of num_times.

What am I misunderstanding? Is it equivalent to:

print_hi = repeat(print_hi, num_times=4)

repeat(num_times) returns a function and that function is used to decorate print_hi. — khelwood
– khelwood, Commented Oct 2, 2018 at 10:04
@deco def foo is equivalent to foo = deco(foo), and @deco(args) def foo is equivalent to foo = deco(args)(foo). Actually it makes perfect sense. — tobias_k
– tobias_k, Commented Oct 2, 2018 at 10:07

bruno desthuilliers · Accepted Answer · 2018-10-02 11:41:57Z

6

For the case of the repeat decorator, the equivalent is:

print_hi = repeat(num_times=4)(print_hi)

Here, repeat takes a num_times argument and returns the decorator_repeat closure, which itself takes a func arguments and returns the wrapper_repeat closure.

edited Oct 2, 2018 at 11:41

answered Oct 2, 2018 at 10:06

bruno desthuilliers

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Comments

khelwood · Accepted Answer · 2018-10-02 10:06:34Z

4

repeat(num_times) returns a function and that function is used to decorate print_hi.

@repeat(num_times=4)
def print_hi(num_times):
    ...

amounts to

f = repeat(num_times)
print_hi = f(print_hi)

The function that repeat returns is decorator_repeat, which decorates print_hi.

answered Oct 2, 2018 at 10:06

khelwood

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