Python decorator without passing a function as argument

Question

I am learning to write the python decorators. Following are two examples.

Example 1:

def dec():
    def wrapper(func):
        print(func)
        return func

    return wrapper

@dec
def hello():
    print('hello')

Example 2:

def dec(name):
    def wrapper(fn):
        print(fn)
        return fn
    return wrapper

@dec('a')
def hello(x):
    print(x)

So my question is why the example 1 throws an exception that dec() takes 0 positional arguments but 1 was given while the example 2 works well.

Answer 1

Bot examples are incorrect. the outer function of a decorator (dec in this case) must take an argument which is the function it decorates.

So example 1 should look like this:

def dec(func):
    def wrapper():
        print(func)
        return func

    return wrapper

@dec
def hello():
    print('hello')

hello()

Example two while doesn't crash immediately, would if you tried to call hello (the decorated function). For a decorator to take an argument you need another level of nesting. So example 2 should look like this:

def dec_args(name):
    def dec(func):
        def wrapper():
            print(func)
            print(name)
            return func

        return wrapper
    return dec

@dec_args('a')
def hello():
    print('hello')

hello()

The inner decorator function ( wrapper ) should take the argument that are passed to the decorated function ( hello ). In these cases you don't have any Also note that the wrapper function doesn't really need to return func . wrapper is essentially a replacement of func . That's what decorators do. They replace the function you decorate with the inner function of the decorator (by calling the outer function with the decorated function as an argument).