75 lines
1.9 KiB
Python
75 lines
1.9 KiB
Python
"""
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A line item for a bulk food order has description, weight and price fields::
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>>> raisins = LineItem('Golden raisins', 10, 6.95)
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>>> raisins.weight, raisins.description, raisins.price
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(10, 'Golden raisins', 6.95)
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A ``subtotal`` method gives the total price for that line item::
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>>> raisins.subtotal()
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69.5
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The weight of a ``LineItem`` must be greater than 0::
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>>> raisins.weight = -20
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Traceback (most recent call last):
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...
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ValueError: value must be > 0
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No change was made::
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>>> raisins.weight
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10
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The value of the attributes managed by the descriptors are stored in
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alternate attributes, created by the descriptors in each ``LineItem``
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instance::
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>>> raisins = LineItem('Golden raisins', 10, 6.95)
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>>> dir(raisins) # doctest: +ELLIPSIS +NORMALIZE_WHITESPACE
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['_Quantity#0', '_Quantity#1', '__class__', ...
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'description', 'price', 'subtotal', 'weight']
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>>> getattr(raisins, '_Quantity#0')
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10
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>>> getattr(raisins, '_Quantity#1')
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6.95
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"""
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# BEGIN LINEITEM_V4
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class Quantity:
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__counter = 0 # <1>
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def __init__(self):
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cls = self.__class__ # <2>
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prefix = cls.__name__
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index = cls.__counter
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self.storage_name = '_{}#{}'.format(prefix, index) # <3>
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cls.__counter += 1 # <4>
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def __get__(self, instance, owner): # <5>
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return getattr(instance, self.storage_name) # <6>
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def __set__(self, instance, value):
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if value > 0:
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setattr(instance, self.storage_name, value) # <7>
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else:
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raise ValueError('value must be > 0')
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class LineItem:
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weight = Quantity() # <8>
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price = Quantity()
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def __init__(self, description, weight, price):
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self.description = description
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self.weight = weight
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self.price = price
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def subtotal(self):
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return self.weight * self.price
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# END LINEITEM_V4
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