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Chapter 14
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‎Ch14/cafe_queue_abc.py‎

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from collections.abc import Collection, Iterator
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from abc import abstractmethod
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class CustomerQueue(Collection):
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@abstractmethod
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def add_customer(self, customer): pass
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@property
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@abstractmethod
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def first(self): pass
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class CafeQueue(CustomerQueue):
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def __init__(self):
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self._queue = []
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self._orders = {}
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self._togo = {}
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def __iter__(self):
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return Iterator_CafeQueue(self)
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def __len__(self):
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return len(self._queue)
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def __contains__(self, customer):
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return (customer in self._queue)
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def add_customer(self, customer, *orders, to_go=True):
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self._queue.append(customer)
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self._orders[customer] = tuple(orders)
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self._togo[customer] = to_go
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@property
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def first(self):
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return self._queue[0]
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class Iterator_CafeQueue(Iterator):
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def __init__(self, iterable):
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self._iterable = iterable
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self._position = 0
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def __next__(self):
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if self._position >= len(self._iterable):
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raise StopIteration
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customer = self._iterable._queue[self._position]
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orders = self._iterable._orders[customer]
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togo = self._iterable._togo[customer]
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self._position += 1
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return (customer, orders, togo)
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def __iter__(self):
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return self
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def serve_customers(queue):
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if not isinstance(queue, CustomerQueue):
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raise TypeError("serve_next() requires a customer queue.")
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if not len(queue):
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print("Queue is empty.")
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return
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def brew(order):
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print(f"(Making {order}...)")
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for customer, orders, to_go in queue:
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for order in orders: brew(order)
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if to_go:
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print(f"Order for {customer}!")
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else:
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print(f"(Takes order to {customer})")
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queue = CafeQueue()
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queue.add_customer('Raquel', 'double macchiato', to_go=False)
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queue.add_customer('Naomi', 'large mocha, skim')
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queue.add_customer('Anmol', 'mango lassi')
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print(f"The first person in line is {queue.first}.")
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serve_customers(queue)

‎Ch14/classes_from_type.py‎

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Food = type('Food', (), {})
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def __init__(obj, toppings):
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obj.toppings = toppings
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Pizza = type('Pizza', (Food,), {'name': 'pizza', '__init__': __init__})
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print(Pizza.name) # 'name' is a class attribute
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pizza = Pizza(['sausage', 'garlic']) # instantiate like normal
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print(pizza.toppings) # prints "['sausage', 'garlic']"

‎Ch14/metaclass.py‎

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class Gadget(type):
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def __new__(self, name, bases, namespace):
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print(f"Creating a {name} gadget!")
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return super().__new__(self, name, bases, namespace)
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@classmethod
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def __prepare__(cls, name, bases):
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return {'color': 'white'}
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class Thingamajig(metaclass=Gadget):
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def __init__(self, widget):
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self.widget = widget
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def frob(self):
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print(f"Frobbing {self.widget}.")
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thing = Thingamajig("button") # also prints "Creating Thingamajig gadget!"
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thing.frob() # prints "Frobbing button"
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print(Thingamajig.color) # prints "white"
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print(thing.__class__) # prints "<class '__main__.Thingamajig'>"

‎Ch14/palindrome_check.py‎

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from abc import ABC, abstractmethod
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from collections.abc import Sequence
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class Palindromable(ABC):
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@abstractmethod
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def __reversed__(self): pass
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@abstractmethod
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def __iter__(self): pass
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@abstractmethod
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def __str__(self): pass
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class LetterPalindrome(Palindromable):
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def __init__(self, string):
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self._raw = string
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self._stripped = ''.join(filter(str.isalpha, string.lower()))
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def __str__(self):
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return self._raw
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def __iter__(self):
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return self._stripped.__iter__()
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def __reversed__(self):
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return reversed(self._stripped)
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@classmethod
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def __subclasshook__(cls, C):
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if issubclass(C, Sequence):
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return True
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return NotImplemented
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def check_palindrome(sequence):
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if not isinstance(sequence, Palindromable):
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raise TypeError("Cannot check for palindrome on that type.")
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for c, r in zip(sequence, reversed(sequence)):
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if c != r:
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print(f"NON-PALINDROME: {sequence}")
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return False
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print(f"PALINDROME: {sequence}")
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return True
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canal = LetterPalindrome("A man, a plan, a canal - Panama!")
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print(check_palindrome(canal)) # prints 'True'
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bolton = LetterPalindrome("Bolton")
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print(check_palindrome(bolton)) # prints 'False'
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print(check_palindrome([1, 2, 3, 2, 1])) # prints 'True'
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print(check_palindrome((1, 2, 3, 2, 1))) # prints 'True'
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print(check_palindrome('racecar')) # prints 'True'
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print(check_palindrome('race car')) # prints 'False'
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print(check_palindrome(LetterPalindrome('race car'))) # prints 'True'
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print(check_palindrome({1, 2, 3, 2, 1})) # raises TypeError

‎Ch14/product_of_thirds.py‎

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from collections.abc import Sequence
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from numbers import Complex
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def product_of_thirds(sequence):
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if not isinstance(sequence, Sequence):
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raise ValueError("Argument must be a sequence.")
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if not isinstance(sequence[0], Complex):
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raise TypeError("Sequence elements must support multiplication.")
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r = sequence[0]
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for i in sequence[1::3]:
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r *= i
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return r
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print(product_of_thirds(range(1, 50))) # prints '262134882788466688000'
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print(product_of_thirds("Foobarbaz")) # raises TypeError

‎Ch14/types.py‎

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print(type("Hello")) # prints "<class 'str'>"
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print(type(123)) # prints "<class 'int'>"
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class Thing: pass
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print(type(Thing)) # prints "<class 'type'>"
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something = Thing()
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print(type(something)) # prints "<class '__main__.Thing'>"
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print(type(type)) # prints "<class 'type'>"

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