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@justanr /money.py
Last active Jul 20, 2016

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Example of a small value object representing money.
import decimal
from functools import total_ordering
from numbers import Real
class Context(object):
def __init__(self, **kwargs):
self.context = decimal.Context(**kwargs)
def __enter__(self):
with decimal.localcontext(self.context) as c:
return c
def __exit__(self, *args):
# we'll rely on decimal.localcontext for error handling
# this makes our implementation much simpler
pass
def __getattr__(self, attr):
return getattr(self.context, attr)
MONEY_CONTEXT = Context(prec=3, rounding=decimal.ROUND_HALF_UP)
TWO_PLACES = decimal.Decimal('0.01')
@total_ordering
class Money(object):
def __init__(self, symbol, amount):
self.symbol = symbol
self.amount = amount
@classmethod
def fromstr(cls, money):
symbol, amount = money[0], money[1:]
return cls(
symbol,
MONEY_CONTEXT.quantize(
decimal.Decimal(amount),
TWO_PLACES
)
)
def _raise_if_separate_types(self, other):
if not isinstance(other, Money):
raise TypeError("Can't use money and non-money together")
if not self.symbol == other.symbol:
# not true, but conversions are another matter
raise TypeError("Can't compare across monetary types")
def __eq__(self, other):
self._raise_if_separate_types(other)
return self.amount == other.amount
def __lt__(self, other):
self._raise_if_separate_types(other)
return self.amount < other.amount
def __add__(self, other):
self._raise_if_separate_types(other)
with MONEY_CONTEXT:
amount = self.amount + other.amount
return self.__class__(self.symbol, amount)
def __neg__(self):
amount = (-self.amount)
return self.__class__(self.symbol, amount)
def __sub__(self, other):
self._raise_if_separate_types(other)
return self + (-other)
def __mul__(self, other):
if not isinstance(other, Real):
raise TypeError("Can only multiple money by real numbers")
if isinstance(other, float):
other = MONEY_CONTEXT.create_decimal_from_float(other)
with MONEY_CONTEXT:
amount = MONEY_CONTEXT.quantize(self.amount * other, TWO_PLACES)
return self.__class__(self.symbol, amount)
def __div__(self, other):
if not isinstance(other, Real):
raise TypeError("Can only divide money by real numbers")
if isinstance(other, float):
other = MONEY_CONTEXT.create_decimal_from_float(other)
with MONEY_CONTEXT:
amount = MONEY_CONTEXT.quantize(self.amount / other, TWO_PLACES)
return self.__class__(self.symbol, amount)
def __str__(self):
return '{}{!s}'.format(self.symbol, self.amount)
def __repr__(self):
return 'Money(symbol={!r}, amount={!r})'.format(self.symbol, self.amount)
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