Python - Decorators

Question

I'm trying to learn Decorators . I understood the concept of it and now trying to implement it.

Here is the code that I've written The code is self-explanatory. It just checks whether the argument passed in int or not.

def wrapper(func):
    def inner():
        if issubclass(x,int): pass
        else: return 'invalid values'

    return inner()

@wrapper
def add(x,y):
    return x+y

print add('a',2)

It's throwing error saying global name 'x' is not defined. I understand that it is not defined under inner, but didnt know how to rectify this code? Where I'm going wrong?

Answer 1

Your decorator should look like this:

def wrapper(func):
    def inner(x, y): # inner function needs parameters
        if issubclass(type(x), int): # maybe you looked for isinstance?
            return func(x, y) # call the wrapped function
        else: 
            return 'invalid values'
    return inner # return the inner function (don't call it)

Some points to note:

• issubclass expects a class as first argument (you could replace it with a simple try/except TypeError).
• the wrapper should return a function, not the result of a called function
• you should actually call the wrapped function in the inner function
• your inner function didn't have parameters

You can find a good explanation of decorators here.