Editing Decorator pattern (section)

=== Python ===

The following Python example, taken from [https://wiki.python.org/moin/DecoratorPattern Python Wiki - DecoratorPattern], shows us how to pipeline decorators to dynamically add many behaviors in an object:

<syntaxhighlight lang="python">
"""
Demonstrated decorators in a world of a 10x10 grid of values 0-255. 
"""

import random


def s32_to_u16(x):
    if x < 0:
        sign = 0xF000
    else:
        sign = 0
    bottom = x & 0x00007FFF
    return bottom | sign


def seed_from_xy(x, y):
    return s32_to_u16(x) | (s32_to_u16(y) << 16)


class RandomSquare:
    def __init__(s, seed_modifier):
        s.seed_modifier = seed_modifier

    def get(s, x, y):
        seed = seed_from_xy(x, y) ^ s.seed_modifier
        random.seed(seed)
        return random.randint(0, 255)


class DataSquare:
    def __init__(s, initial_value=None):
        s.data = [initial_value] * 10 * 10

    def get(s, x, y):
        return s.data[(y * 10) + x]  # yes: these are all 10x10

    def set(s, x, y, u):
        s.data[(y * 10) + x] = u


class CacheDecorator:
    def __init__(s, decorated):
        s.decorated = decorated
        s.cache = DataSquare()

    def get(s, x, y):
        if s.cache.get(x, y) == None:
            s.cache.set(x, y, s.decorated.get(x, y))
        return s.cache.get(x, y)


class MaxDecorator:
    def __init__(s, decorated, max):
        s.decorated = decorated
        s.max = max

    def get(s, x, y):
        if s.decorated.get(x, y) > s.max:
            return s.max
        return s.decorated.get(x, y)


class MinDecorator:
    def __init__(s, decorated, min):
        s.decorated = decorated
        s.min = min

    def get(s, x, y):
        if s.decorated.get(x, y) < s.min:
            return s.min
        return s.decorated.get(x, y)


class VisibilityDecorator:
    def __init__(s, decorated):
        s.decorated = decorated

    def get(s, x, y):
        return s.decorated.get(x, y)

    def draw(s):
        for y in range(10):
            for x in range(10):
                print("%3d" % s.get(x, y), end=' ')
            print()


# Now, build up a pipeline of decorators:

random_square = RandomSquare(635)
random_cache = CacheDecorator(random_square)
max_filtered = MaxDecorator(random_cache, 200)
min_filtered = MinDecorator(max_filtered, 100)
final = VisibilityDecorator(min_filtered)

final.draw()
</syntaxhighlight>

'''Note:'''

The Decorator Pattern (or an implementation of this design pattern in Python - as the above example) should not be confused with [[Python syntax and semantics#Decorators|Python Decorators]], a language feature of Python. They are different things.

Second to the Python Wiki:

<blockquote>The Decorator Pattern is a pattern described in the Design Patterns Book. It is a way of apparently modifying an object's behavior, by enclosing it inside a decorating object with a similar interface.
This is not to be confused with Python Decorators, which is a language feature for dynamically modifying a function or class.<ref>{{cite web|url=https://wiki.python.org/moin/DecoratorPattern|title=DecoratorPattern - Python Wiki|website=wiki.python.org}}</ref></blockquote>