Before starting about decorators, first, understand that functions in python have below three properties.

1. Functions are objects. Which means we can assign the function to a variable.
2. A function can be defined inside another function
3. A function can return another function

See example below:

Assigning a function to another variable

# Assigning function to another variable
# a function which adds * around text
def beautify(txt="python"):
    print("*" + txt + "*")

# Assign above function to another variable
star = beautify

# call star
print(star())

# output
>>> *python*

Defining a function inside another function

def beautify():
    # define an inner function
    def inside_beauty(txt="python"):
        print("*" + txt + "*")

    # call inner function
    inside_beauty()

# call outer function
beautify()

# output
>>> *python*

Returning function from another function

def beautify():
    # define an inner function
    def inner_beauty(txt="python"):
        print("*" + txt + "*")

    # return this inner function
    return inner_beauty

# collect return type of beautify in a variable
f = beautify()

print(f)

# output
>>> <function inner_beauty at 0x7fe5c176b7d0>

# call f
f()

# output
>>> *python*

Now when we know that we can assign a function to a variable, can define a function inside another function and can return a function from another function, we can jump to next strep. We can pass a function as a parameter to another function.

def beautify(func):
    print("going to call a function passed as parameter to this function")
    func()
    print("function which was passed as parameter has been called")


def add_stars(txt="python"):
    print("*" + txt + "*")

beautify(add_stars)

# output
>>> going to call a function passed as parameter to this function
*python*
function which was passed as parameter has been called

Now coming to what a decorator is: Decorator is a wrapper function that executes some code before and after the function being decorated, just like the example above.

Another Example:
Let's say you want to separate the output of a function by surrounding it with horizontal dividers.

Original Function:

def say_something():
    print("I am the original function.")

Decorator function:

def separate_content(function_to_decorate):
    # a wrapper function
    def wrapper_func():
        print("---------------------------")
        function_to_decorate()
        print("---------------------------")

    return wrapper_func

Now if you call the original function, the output will be I am the original function.

You can decorate the original function by passing it to the decorator function as below.

say_something = separate_content(say_something)
say_something()

# output
---------------------------
I am the original function.
---------------------------

Shortcut to decorate a function is:

@separate_content
def say_something():
    print("I am the original function.")

which when called will give the same out as above.

Now you can just use @separate_content above any function definition and the behavior of that method will be changed without changing the content of that method