Chapter 1: “Multi-Currency Money”

Iterations

First test. There is no other code! Fails with “NameError: name ‘Dollar’ is not defined”.

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def test_multiplication():
    five = Dollar(5)
    five.times(2)
    assert five.amount == 10

First iteration: add class. Fails with “TypeError: Dollar() takes no arguments”.

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class Dollar:
    pass

Second iteration: add stub constructor. Fails with “TypeError: init() takes 1 positional argument but 2 were given”.

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class Dollar:
    def __init__(self):
        pass

Third iteration: add constructor parameter. Fails with “AttributeError: ‘Dollar’ object has no attribute ‘times’”.

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class Dollar:
    def __init__(self, amount: float):
        pass

Fourth iteration: add stub method. Fails with “AttributeError: ‘Dollar’ object has no attribute ‘amount’”.

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class Dollar:
    def __init__(self, amount: float):
        pass
    
    def times(self, multiplier):
        pass

Fifth iteration: add class attribute amount. Fails with AssertionError: assert 5 == 10.

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class Dollar:
    def __init__(self, amount: float):
        self.amount = amount
    
    def times(self, multiplier):
        pass

Sixth iteration: hard-code a value that will pass the test. Passes.

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class Dollar:
    def __init__(self, amount: float):
        self.amount = 10
    
    def times(self, multiplier):
        pass

Seventh iteration: refactor to remove the duplication of the value 10 (once in the test, once in the code). Passes.

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class Dollar:
    def __init__(self, amount: float):
        self.amount = 5 * 2
    
    def times(self, multiplier):
        pass

Eight iteration: refactor to move the computation from the class attribute to the method. Passes.

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class Dollar:
    def __init__(self, amount: float):
        self.amount = 0
    
    def times(self, multiplier):
        self.amount = 5 * 2

Ninth iteration: refactor to get the value 5 from the constructor argument. Passes.

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class Dollar:
    def __init__(self, amount: float):
        self.amount = amount
    
    def times(self, multiplier):
        self.amount *= 2

Tenth iteration: refactor to get the value 2 from the method argument. Passes.

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class Dollar:
    def __init__(self, amount: float):
        self.amount = amount
    
    def times(self, multiplier):
        self.amount *= multiplier