Decorators

Introduction

Decorators are a way to modify or extend the behavior of functions or methods, without modifying the original function or method. This sounds confusing, but it’ll make more sense after you’ve seen a few examples of how decorators work.

Decorator syntax

In Python, decorators are implemented using the @ symbol, followed by the name of the decorator function. They always come before the definition of a function, and are applied to the function below. The syntax looks like this:

@some_decorator
def some_function():
    print("This is the inside of some_function.")

but even though the syntax is a bit strange, it is just a shorthand (i.e., "syntactic sugar") for the following:

def some_function():
    print("This is the inside of some_function.")

some_function = some_decorator(some_function)

In other words, the @some_decorator syntax is just a function that takes another function as input, and returns a new function, modified, version of the input function.

Decorator example

Python libraries are often full of decorators, and you’ve probably used them without even realizing it. Let's make a simple example of a decorator that prints the time it takes to run a function. We can use the time module to get the current time, and then subtract the start time from the end time to get the time it took to run the function.

import time

# The timer function (which we will use as a decorator) takes a function as input, 
# and returns a new function (the "wrapper" function) that does what the previous function did,
# plus a little extra (in this case, it prints the time it took to run the function).
def timer(func):
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        end = time.time()
        print(f"Elapsed time: {end - start} seconds")
        return result
    return wrapper

# Now we can use the `@timer` decorator to print the time it takes to run a function.
@timer
def some_function():
    print("This is the inside of some_function.")
    time.sleep(1)

some_function()

# Output:
This is the inside of some_function.
Elapsed time: 1.000000238418579 seconds