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"""

*What is this pattern about?

Define a family of algorithms, encapsulate each one, and make them interchangeable.

Strategy lets the algorithm vary independently from clients that use it.

*TL;DR

Enables selecting an algorithm at runtime.

"""

class Order:

def __init__(self, price, discount_strategy=None):

self.price = price

self.discount_strategy = discount_strategy

def price_after_discount(self):

if self.discount_strategy:

discount = self.discount_strategy(self)

else:

discount = 0

return self.price - discount

def __repr__(self):

fmt = "<Price: {}, price after discount: {}>"

return fmt.format(self.price, self.price_after_discount())

def ten_percent_discount(order):

return order.price * 0.10

def on_sale_discount(order):

return order.price * 0.25 + 20

def main():

"""

>>> Order(100)

<Price: 100, price after discount: 100>

>>> Order(100, discount_strategy=ten_percent_discount)

<Price: 100, price after discount: 90.0>

>>> Order(1000, discount_strategy=on_sale_discount)

<Price: 1000, price after discount: 730.0>

"""

if __name__ == "__main__":

import doctest

doctest.testmod()