COMP1531
🐶Software Engineering
7.3 - Design - Decorators
Author: Hayden Smith 2021
In this lecture
Why?
- Writing clean and well designed code has huge benefits we've discussed previously, so let's learn some more
What?
- Decorators
Decorators
Decorators allow us to add functionality to a function without altering the function itself, by "decorating" (wrapping) around it.
But first... some background
Function Arguments
def foo1(zid, name, age, suburb):
print(zid, name, age, suburb)
def foo2(zid=None, name=None, age=None, suburb=None):
print(zid, name, age, suburb)
if __name__ == '__main__':
foo1('z3418003', 'Hayden', '72', 'Kensington')
foo2('z3418003', 'Hayden')
foo2(name='Hayden', suburb='Kensington', age='72', zid='z3418003')
foo2(age='72', zid='z3418003')
foo2('z3418003', suburb='Kensington')
decor1.py
Arguments in python can either be keyword arguments (named) or non-keyword arguments.
Non-keyword arguments cannot appear after keyword arguments in the argument list
Function Arguments
def foo(zid=None, name=None, *args, **kwargs):
print(zid, name)
print(args) # A list
print(kwargs) # A dictionary
if __name__ == '__main__':
foo('z3418003', None, 'mercury', 'venus', planet1='earth', planet2='mars')decor2.py
def foo(*args, **kwargs):
print(args) # A list
print(kwargs) # A dictionary
if __name__ == '__main__':
foo('this', 'is', truly='dynamic')decor3.py
We can use a generalised method of capturing:
- *args: non-keyword arguments as a list
- *kwargs: keyword arguments as a dictionary
Decorators: First principles
def make_uppercase(input):
return input.upper()
def get_first_name():
return "Hayden"
def get_last_name():
return "Smith"
if __name__ == '__main__':
print(make_uppercase(get_first_name()))
print(make_uppercase(get_last_name()))decor4.py
Consider "make_uppercase" to be a decorator function. It allows you to add functionality to the get first name function without altering the function.
A proper decorator
def make_uppercase(function):
def wrapper(*args, **kwargs):
return function(*args, **kwargs).upper()
return wrapper
@make_uppercase
def get_first_name():
return "Hayden"
@make_uppercase
def get_last_name():
return "Smith"
if __name__ == '__main__':
print(get_first_name())
print(get_last_name())decor5.py
This code can be used as a template
Now let's generalise it with the proper python decorator syntax.
Decorator, run twice
def run_twice(function):
def wrapper(*args, **kwargs):
return function(*args, **kwargs) \
+ function(*args, **kwargs)
return wrapper
@run_twice
def get_first_name():
return "Hayden"
@run_twice
def get_last_name():
return "Smith"
if __name__ == '__main__':
print(get_first_name())
print(get_last_name())decor6.py
Decorator, more
class Message:
def __init__(self, id, text):
self.id = id
self.text = text
messages = [
Message(1, "Hello"),
Message(2, "How are you?"),
]
def get_message_by_id(id):
return [m for m in messages if m.id == id][0]
def message_id_to_obj(function):
def wrapper(*args, **kwargs):
argsList = list(args)
argsList[0] = get_message_by_id(argsList[0])
args = tuple(argsList)
return function(*args, **kwargs)
return wrapper
@message_id_to_obj
def printMessage(message):
print(message.text)
if __name__ == '__main__':
printMessage(1)
decor7.py
Feedback
