python

Decorators

David Albone

Oct 5, 2012 • 1 min read

To request database connections, flaskr uses decorators. For example:

@app.before_request
def before_request():
    g.db = connect_db()

From http://stackoverflow.com/questions/739654/understanding-python-decorators:

Functions are objects in python and can be assigned to a variable.

Functions can be embedded inside other functions, but the inner function is in-scope only within the outer function.

A function can be passed as a parameter to another function.

A decorator is a function that expects ANOTHER function as parameter.

Decorators allow extra code to be run before and after the code that they decorate.

Multiple decorators can be added to a function.

Based on this explanation, here is an example of decorator usage:

# Code showing decorators
# Based on explanation of decorators in:
# http://stackoverflow.com/questions/739654/understanding-python-decorators

def decorator_function(function_to_decorate):
	def wrapper_for_passed_function():
		print("Before function.")
		function_to_decorate()
		print("After function.")

	return wrapper_for_passed_function

def outer_decorater_function(function_to_decorate):
	def wrapper_for_passed_function():
		print("Before outer function.")
		function_to_decorate()
		print("After outer function.")

	return wrapper_for_passed_function

def function_to_be_wrapped():
	print("Function to be wrapped.")

@decorator_function # Shortcut to wrapped_function = decorator_function(function_to_be_decorated
def function_to_be_decorated():
	print("Function to be decorated.")

@outer_decorater_function
@decorator_function
def function_to_be_multiply_decorated():
	print("Funciton to be multiply decorated.")

# Function without wrapper
print("*** Without wrapper ***")
function_to_be_wrapped()

# Funtion with wrapper
print("\n*** With wrapper ***")
wrapped_function = decorator_function(function_to_be_wrapped)
wrapped_function()

# Using a decorators
print("\n*** Using a decorator ***")
function_to_be_decorated()

# Using multiple decorators
print("\n*** Using multiple decorators ***")
function_to_be_multiply_decorated()

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