Design Patterns in Python for the Untrained Eye

design_patterns.py

# Exercises from PyCon 2019 workshop 
# https://www.youtube.com/watch?v=o1FZ_Bd4DSM

from abc import ABC, abstractmethod


# Template method design pattern

class AverageCalculator(ABC):
    '''Design Patterns in Python Exercise B.''' 

    def average(self): 
        try:
            num_items = 0
            total_sum = 0
            while self.has_next():
                total_sum += self.next_item()
                num_items += 1
            if num_items == 0:
                raise RuntimeError("Can't compute the average of zero items.")
            return total_sum / num_items
        finally:
            self.dispose()

    @abstractmethod
    def has_next(self): 
        pass

    @abstractmethod
    def next_item(self): 
        pass

    def dispose(self): 
        pass


# Exercise solution:

class MemoryAverageCalculator(AverageCalculator):
    '''Exercise B solution.'''

    def __init__(self, input_list):
        self.memory = input_list

    def has_next(self):
        return not not self.memory

    def next_item(self):
        return self.memory.pop()


# Observer pattern

class Observable:

    def __init__(self):
        self.__observers = []

    def add_observer(self, observer):
        self.__observers.append(observer)

    def delete_observer(self, observer):
        self.__observers.remove(observer)

    def notify_observers(self, *args):
        for observer in self.__observers:
            observer.update(self, *args)


class Observer(ABC):

    @abstractmethod
    def update(self, observable, *args):
        pass


# Exercise solution:

class Twitter(Observer, Observable):
    '''Exercise D solution.'''
    
    def __init__(self, name: str):
        super().__init__()
        self.name = name
        self.tweets = list()

    def follow(self, other):
        other.add_observer(self)
        return self

    def tweet(self, twit):
        self.tweets.append(twit)
        self.notify_observers(twit)

    def update(self, changed_account, new_tweet):
        print(f'{self.name} received a tweet from {changed_account.name}: {new_tweet}')

        


if __name__ == "__main__":

    # Template method pattern
    mac = MemoryAverageCalculator([3, 1, 4, 1, 5, 9, 2, 6, 5, 3])
    print(mac.average())

    # Observer pattern
    a = Twitter('Alice')
    k = Twitter('King')
    q = Twitter('Queen')
    h = Twitter('Mad Hatter')
    c = Twitter('Cheshire Cat')

    a.follow(c).follow(h).follow(q) 
    k.follow(q)
    q.follow(q).follow(h)
    h.follow(a).follow(q).follow(c)

    print(f'==== {q.name} tweets ====')
    q.tweet('Off with their heads!')
    print(f'\n==== {a.name} tweets ====')
    a.tweet('What a strange world we live in.')
    print(f'\n==== {k.name} tweets ====')
    k.tweet('Begin at the beginning, and go on till you come to the end: then stop.')
    print(f'\n==== {c.name} tweets ====')
    c.tweet("We're all mad here.")
    print(f'\n==== {h.name} tweets ====')
    h.tweet('Why is a raven like a writing-desk?')