Decorators in Python

Question

Say you have the following code:

def addTags(functionHere):
    def wrapper():
        return "NumberTag" + functionHere() + "NumberTagOver"
    return wrapper

@addTags
def numbers(firstValue, secondValue):
    return firstValue + secondValue

Then putting numbers(4, 5) into the interpreter spits back a trace saying that wrapper takes no arguments. Okay fine, then putting numbers() into the interpreter spits back that numbers wants two arguments! Confused.

Answer 1

Your wrapper function replaces the wrapped function, and needs to match the number of arguments it takes. Your wrapped function takes two arguments (firstValue, secondValue) , but your wrapper takes none at all.

You could add these two to your decorator wrapper too:

def addTags(functionHere):
    def wrapper(firstValue, secondValue):
        return "NumberTag" + functionHere(firstValue, secondValue) + "NumberTagOver"
    return wrapper

but that ties your decorator to that specific function.

There is a trick you can use however:

def addTags(functionHere):
    def wrapper(*args):
        return "NumberTag" + functionHere(*args) + "NumberTagOver"
    return wrapper

The *args positional argument acts as a catch-all, all positional arguments to the function are now passed on to the wrapped function.

There is an equivalent for keyword arguments too:

def addTags(functionHere):
    def wrapper(*args, **kw):
        return "NumberTag" + functionHere(*args, **kw) + "NumberTagOver"
    return wrapper