python decorate function call

Question

I recently learned about decorators and wondered if it's possible to use them not in a function definition but in a function call, as some kind of general wrapper.

The reason for that is, that I want to call functions from a module through a user-defined interface that does repeatable things to a function and I don't want to implement a wrapper for every single function.

In principle I would like to have something like

def a(num):
    return num

@double
a(2)

returning 4 without the need of having access to the implementation of a. Or would in this case a global wrapper like

def mutiply(factor,function,*args,**kwargs):
    return factor*function(*args,*kwargs)

be the better choice?

Solution 1

There is a very good detailed section on decorators in Marty Alchin's book Pro Python from Apress.

While the new style @decorator syntax is only available to be used at function/class definition, you can use the older syntax, which does the same thing this way:

from module import myfunc

myfunc = double_decorator(myfunc)

x = myfunc(2) # returns 4

Solution 2

You could do something like that:

def a(num):
    return num * 1

def double(f):
    def wrapped(*args, **kwargs):
        return f(*args, **kwargs)
    return wrapped

print(double(a)(2))

It's because we can decorate functions and run functions using a decorator function explicit as in the example above. So in this one:

print(double(a)(2))

In the place of a you can put any function and in place of the 2, args and kwargs.

Comments

[Dschoni]

How would I use that now together with the @ syntax?

[turkus]

In your case you can't. You can use @ syntax only on function declaration.

[Dschoni]

This doesn't work for me. If I change the definition of a to return num+1, the last line, still prints 4.

[ruohola]

Kinda bad naming of the double function since it doesn't actually double anything.

[InvaderZim]

Came looking for this comment! In general terms: decorated_func_return_val = some_decorator(some_funk)(*some_args, **some_kwargs)

[InvaderZim]

@turkus: perhaps you should change it so that the wrapper actually returns double the value? return 2 * f(*args, **kwargs). Or call the decorator something else, like do_nothing.