Decorators with parameters in Python

Prerequisite: Decorators in Python, Function Decorators

We know Decorators are a very powerful and useful tool in Python since it allows programmers to modify the behavior of function or class. In this article, we will learn about the Decorators with Parameters with help of multiple examples. 
Python functions are First Class citizens which means that functions can be treated similarly to objects. 

• Function can be assigned to a variable i.e they can be referenced.
• Function can be passed as an argument to another function.
• Function can be returned from a function.

Decorators with parameters is similar to normal decorators.

The syntax for decorators with parameters :

@decorator(params)
def func_name():
    ''' Function implementation'''

The above code is equivalent to 

def func_name():
    ''' Function implementation'''

func_name = (decorator(params))(func_name)
"""

As the execution starts from left to right decorator(params) is called which returns a function object fun_obj. Using the fun_obj the call fun_obj(fun_name) is made. Inside the inner function, required operations are performed and the actual function reference is returned which will be assigned to func_name. Now, func_name() can be used to call the function with decorator applied on it.

How Decorator with parameters is implemented

Python3

def decorators(*args, **kwargs):
    def inner(func):
        '''
           do operations with func
        '''
        return func
    return inner #this is the fun_obj mentioned in the above content

@decorators(params)
def func():
    """
         function implementation
    """

Here params can also be empty.

Observe these first :

Python3

# Python code to illustrate
# Decorators basic in Python
def decorator_fun(func):
    print("Inside decorator")
    def inner(*args,**kwargs):
        print("Inside inner function")
        print("Decorated the function")
        # do operations with func
        func()
    return inner()
@decorator_fun
def func_to():
    print("Inside actual function")
func_to

Another Way:

Python3

# Python code to illustrate
# Decorators with parameters in Python

def decorator_fun(func):
  print("Inside decorator")

  def inner(*args, **kwargs):
    print("Inside inner function")
    print("Decorated the function")

    func()

  return inner


def func_to():
    print("Inside actual function")

# another way of using decorators
decorator_fun(func_to)()

Output:

Inside decorator
Inside inner function
Decorated the function
Inside actual function

Let’s move to another example:

Example #1: 

Python3

# Python code to illustrate
# Decorators with parameters in Python

def decorator(*args, **kwargs):
    print("Inside decorator")

    def inner(func):

        # code functionality here
        print("Inside inner function")
        print("I like", kwargs['like'])

        func()

    # returning inner function
    return inner

@decorator(like = "geeksforgeeks")
def my_func():
    print("Inside actual function")

Output:

Inside decorator
Inside inner function
I like geeksforgeeks
Inside actual function

Example #2: 

Python3

# Python code to illustrate
# Decorators with parameters in Python

def decorator_func(x, y):

    def Inner(func):

        def wrapper(*args, **kwargs):
            print("I like Geeksforgeeks")
            print("Summation of values - {}".format(x+y) )

            func(*args, **kwargs)

        return wrapper
    return Inner


# Not using decorator
def my_fun(*args):
    for ele in args:
        print(ele)

# another way of using decorators
decorator_func(12, 15)(my_fun)('Geeks', 'for', 'Geeks')

Output:

I like Geeksforgeeks
Summation of values - 27
Geeks
for
Geeks

This example also tells us that Outer function parameters can be accessed by the enclosed inner function. 

Example #3:  

Python3

# Python code to illustrate
# Decorators with parameters in Python  (Multi-level Decorators)


def decodecorator(dataType, message1, message2):
    def decorator(fun):
        print(message1)
        def wrapper(*args, **kwargs):
            print(message2)
            if all([type(arg) == dataType for arg in args]):
                return fun(*args, **kwargs)
            return "Invalid Input"
        return wrapper
    return decorator


@decodecorator(str, "Decorator for 'stringJoin'", "stringJoin started ...")
def stringJoin(*args):
    st = ''
    for i in args:
        st += i
    return st


@decodecorator(int, "Decorator for 'summation'\n", "summation started ...")
def summation(*args):
    summ = 0
    for arg in args:
        summ += arg
    return summ


print(stringJoin("I ", 'like ', "Geeks", 'for', "geeks"))
print()
print(summation(19, 2, 8, 533, 67, 981, 119))

Output:  

Decorator for 'stringJoin'
Decorator for 'summation'

stringJoin started ...
I like Geeksforgeeks

summation started ...
1729

1. Inside the Decorator

2. Inside the function

Note: Image snapshots are taken using PythonTutor.