Replace a function with a different one

def deco(func):
    def inner():
        print("running inner()")
    return inner

@deco
def target():
    print("running target()")

target()  # running inner()

Adding a decorator

@deco
def target():
    ...

has the same effect as writing this:

target = deco(target)

Add some function

We are going to create a decorator that handles two common logging scenarios.

import functools
import logging

logging.basicConfig(level = logging.DEBUG)
logger = logging.getLogger()

def log(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        try:
            result = func(*args, **kwargs)
            return result
        except Exception as e:
            logger.exception(f"Exception raised in {func.__name__}. exception: {str(e)}")
            raise e
    return wrapper

@log
def foo():
    raise Exception("Something went wrong")

foo()
# ERROR:root:Exception raised in foo. exception: Something went wrong
# ...

In addition to setting up the logger, we have also used @functools.wraps decorator. If using a decorator always meant losing this information about a function, it would be a serious problem. That's why we have functools.wraps. This takes a function used in a decorator and adds the functionality of copying over the function name, docstring, arguments list, etc.

A decorator with parameters

def return_default(value):
    def deco(func):
        def wrapped(*args, **kwargs):
            ret = func(*args, **kwargs)
            if ret is None:
                ret = value
            return ret
        return wrapped
    return deco


@return_default(10)
def at_least_10(x):
    if x >= 10:
        return x


@return_default("python")
def greeting(msg):
    return msg


print(at_least_10(3)) # 10
print(greeting(None)) # "python"