"""

A line item for a bulk food order has description, weight and price fields::

>>> raisins = LineItem('Golden raisins', 10, 6.95)

>>> raisins.weight, raisins.description, raisins.price

(10, 'Golden raisins', 6.95)

A ``subtotal`` method gives the total price for that line item::

>>> raisins.subtotal()

69.5

The weight of a ``LineItem`` must be greater than 0::

>>> raisins.weight = -20

Traceback (most recent call last):

...

ValueError: weight must be > 0

Negative or 0 price is not acceptable either::

>>> truffle = LineItem('White truffle', 100, 0)

Traceback (most recent call last):

...

ValueError: price must be > 0

No change was made::

>>> raisins.weight

10

"""

# tag::LINEITEM_QUANTITY_V3[]

class Quantity: # <1>

def __init__(self, storage_name):

self.storage_name = storage_name # <2>

def __set__(self, instance, value): # <3>

if value > 0:

instance.__dict__[self.storage_name] = value # <4>

else:

msg = f'{self.storage_name} must be > 0'

raise ValueError(msg)

def __get__(self, instance, owner): # <5>

return instance.__dict__[self.storage_name]

# end::LINEITEM_QUANTITY_V3[]

# tag::LINEITEM_V3[]

class LineItem:

weight = Quantity('weight') # <1>

price = Quantity('price') # <2>

def __init__(self, description, weight, price): # <3>

self.description = description

self.weight = weight

self.price = price

def subtotal(self):

return self.weight * self.price

# end::LINEITEM_V3[]