How do I pass extra arguments to a Python decorator?

Question

I have a decorator like below.

def myDecorator(test_func):
    return callSomeWrapper(test_func)
def callSomeWrapper(test_func):
    return test_func
@myDecorator
def someFunc():
    print 'hello'

I want to enhance this decorator to accept another argument like below

def myDecorator(test_func,logIt):
    if logIt:
        print "Calling Function: " + test_func.__name__
    return callSomeWrapper(test_func)
@myDecorator(False)
def someFunc():
    print 'Hello'

But this code gives the error,

TypeError: myDecorator() takes exactly 2 arguments (1 given)

Why is the function not automatically passed? How do I explicitly pass the function to the decorator function?

Answer 1

Since you are calling the decorator like a function, it needs to return another function which is the actual decorator:

def my_decorator(param):
    def actual_decorator(func):
        print("Decorating function {}, with parameter {}".format(func.__name__, param))
        return function_wrapper(func)  # assume we defined a wrapper somewhere
    return actual_decorator

The outer function will be given any arguments you pass explicitly, and should return the inner function. The inner function will be passed the function to decorate, and return the modified function.

Usually you want the decorator to change the function behavior by wrapping it in a wrapper function. Here's an example that optionally adds logging when the function is called:

def log_decorator(log_enabled):
    def actual_decorator(func):
        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            if log_enabled:
                print("Calling Function: " + func.__name__)
            return func(*args, **kwargs)
        return wrapper
    return actual_decorator

The functools.wraps call copies things like the name and docstring to the wrapper function, to make it more similar to the original function.

Example usage:

>>> @log_decorator(True)
... def f(x):
...     return x+1
...
>>> f(4)
Calling Function: f
5

Answer 2

Just to provide a different viewpoint: the syntax

@expr
def func(...): #stuff

is equivalent to

def func(...): #stuff
func = expr(func)

In particular, expr can be anything you like, as long as it evaluates to a callable. In particular particular, expr can be a decorator factory: you give it some parameters and it gives you a decorator. So maybe a better way to understand your situation is as

dec = decorator_factory(*args)
@dec
def func(...):

which can then be shortened to

@decorator_factory(*args)
def func(...):

Of course, since it looks like decorator_factory is a decorator, people tend to name it to reflect that. Which can be confusing when you try to follow the levels of indirection.

Answer 3

Just want to add some usefull trick that will allow to make decorator arguments optional. It will also alows to reuse decorator and decrease nesting

import functools

def myDecorator(test_func=None,logIt=None):
    if test_func is None:
        return functools.partial(myDecorator, logIt=logIt)
    @functools.wraps(test_func)
    def f(*args, **kwargs):
        if logIt==1:
            print 'Logging level 1 for {}'.format(test_func.__name__)
        if logIt==2:
            print 'Logging level 2 for {}'.format(test_func.__name__)
        return test_func(*args, **kwargs)
    return f

#new decorator 
myDecorator_2 = myDecorator(logIt=2)

@myDecorator(logIt=2)
def pow2(i):
    return i**2

@myDecorator
def pow3(i):
    return i**3

@myDecorator_2
def pow4(i):
    return i**4

print pow2(2)
print pow3(2)
print pow4(2)

Answer 4

Just another way of doing decorators. I find this way the easiest to wrap my head around.

class NiceDecorator:
    def __init__(self, param_foo='a', param_bar='b'):
        self.param_foo = param_foo
        self.param_bar = param_bar

    def __call__(self, func):
        def my_logic(*args, **kwargs):
            # whatever logic your decorator is supposed to implement goes in here
            print('pre action baz')
            print(self.param_bar)
            # including the call to the decorated function (if you want to do that)
            result = func(*args, **kwargs)
            print('post action beep')
            return result

        return my_logic

# usage example from here on
@NiceDecorator(param_bar='baaar')
def example():
    print('example yay')


example()

Answer 5

Now if you want to call a function function1 with a decorator decorator_with_arg and in this case both the function and the decorator take arguments,

def function1(a, b):
    print (a, b)

decorator_with_arg(10)(function1)(1, 2)