Summary - Property and decorator

Combine decorators and @property to write a validated attribute.

Summary - Property and decorator

In this group we covered decorator basics, function decorators, @property with @x.setter, caching with @cached_property and metadata protection with @functools.wraps.

A decorator is written as @name and wraps the target function. @property intercepts reads, @x.setter writes. @cached_property computes once; @functools.wraps keeps the target's name and docstring. Now we'll write an example that combines these.

Exercise Combined summary example

Start with from functools import cached_property. Define a Square class; def __init__(self, side): and in the body write self.side = side. Add a method def perimeter(self): with @property; body return 4 * self.side. Add a method def area(self): with @cached_property; body return self.side ** 2. After the class create s = Square(5) and print with print(s.perimeter, s.area).

Show example solution

# Write the Square class with @property and @cached_property
from functools import cached_property

class Square:
    def __init__(self, side):
        self.side = side

    @property
    def perimeter(self):
        return 4 * self.side

    @cached_property
    def area(self):
        return self.side ** 2

s = Square(5)
print(s.perimeter, s.area)

Test yourself

Which combination is wrong?
- @property + @name.setter + @name.deleter
- @property + @name.getter a second time
- @functools.wraps inside a decorator
- @cached_property alone
Show answer

@cached_property alone — @cached_property already caches reads; no setter is added to it.
Match the decorator and its purpose

@property

Intercepts reads
@cached_property

Caching
@functools.wraps

Preserves metadata
@name.setter

Intercepts writes

Import cached_property and ...

from functools import cached_property, ____

def call(fn):
    @functools.wraps(fn)
    def wrapper(*a, **k):
        return fn(*a, **k)
    return wrapper

Show answer

from functools import cached_property, wraps

def call(fn):
    @functools.wraps(fn)
    def wrapper(*a, **k):
        return fn(*a, **k)
    return wrapper

Match the class parts

__init__

Instance setup
@property

Read attribute
@cached_property

Cached computation
self.side

Instance attribute

Next lesson: Quiz model

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