Introduction to decorators

In Functions III you saw that functions are values: you can pass them as arguments and return them from other functions. A decorator builds on that idea.

Take an existing function and a wrapper that adds extra behaviour around it:

def shout(func):
	def wrapper():
		return func().upper()
	return wrapper
 
def greet():
	return "hi there"
 
loud = shout(greet)
print(loud())

Python

Output

shout takes a function in, defines a small wrapper around it, and returns the wrapper. Calling loud() actually runs wrapper(), which calls greet() and uppercases the result.

What will be the output?

def shout(func):
	def wrapper():
		return func().upper()
	return wrapper
 
def hi():
	return "hello"
 
print(shout(hi)())

Python

Reassigning the result back to the original name is so common that Python has a shortcut. The @ syntax does exactly the same thing.

These two snippets are equivalent:

def shout(func):
	def wrapper():
		return func().upper()
	return wrapper
 
@shout
def greet():
	return "hi"
 
# is the same as:
# greet = shout(greet)
 
print(greet())

Python

Output

What will be the output?

def shout(func):
	def wrapper():
		return func().upper()
	return wrapper
 
@shout
def name():
	return "amy"
 
print(name())

Python

The wrapper can do anything: print before and after, change the result, or skip the call entirely. The original function isn't modified, it's replaced by the wrapper.

A wrapper that prints around the call:

def loud(func):
	def wrapper():
		print("start")
		result = func()
		print("end")
		return result
	return wrapper
 
@loud
def work():
	return 42
 
print(work())

Python

Output

What will be the output?

def loud(func):
	def wrapper():
		print("a")
		result = func()
		print("b")
		return result
	return wrapper
 
@loud
def go():
	return 1
 
print(go())

Python

Decorator structure to memorise:

def my_decorator(func):
	def wrapper():
		# extra code before
		result = func()
		# extra code after
		return result
	return wrapper

Python

What will be the output?

def deco(func):
	def wrapper():
		return func() + 1
	return wrapper
 
@deco
def n():
	return 5
 
print(n())

Python

What will be the output?

def deco(func):
	def wrapper():
		return "wrapped"
	return wrapper
 
@deco
def hello():
	return "hello"
 
print(hello())

Python

What will be the output?

def deco(func):
	return func
 
@deco
def f():
	return 10
 
print(f())

Python