From the documentation:
Q. Once I have valid two place inputs, how do I maintain that invariant throughout an application?
A. Some operations like addition, subtraction, and multiplication by an integer will automatically preserve fixed point. Others operations, like division and non-integer multiplication, will change the number of decimal places and need to be followed-up with a quantize() step:
>>> TWOPLACES = Decimal(10) ** -2 # same as Decimal('0.01')
>>> a = Decimal('102.72') # Initial fixed-point values
>>> b = Decimal('3.17')
>>> a + b # Addition preserves fixed-point
Decimal('105.89')
>>> a - b
Decimal('99.55')
>>> a * 42 # So does integer multiplication
Decimal('4314.24')
>>> (a * b).quantize(TWOPLACES) # Must quantize non-integer multiplication
Decimal('325.62')
>>> (b / a).quantize(TWOPLACES) # And quantize division
Decimal('0.03')
In developing fixed-point applications, it is convenient to define functions to handle the quantize() step:
>>> def mul(x, y, fp=TWOPLACES):
... return (x * y).quantize(fp)
>>> def div(x, y, fp=TWOPLACES):
... return (x / y).quantize(fp)
>>> mul(a, b) # Automatically preserve fixed-point
Decimal('325.62')
>>> div(b, a)
Decimal('0.03')
Seems like the solution is to set precision to 5 and Emax to 2 and use those quantising functions.
con = decimal.getcontext()
con.prec = 5
con.Emax = 2
con.Emin = 0
try:
Decimal(1) * 1000
except decimal.Overflow as e:
print(e)
else:
assert False
assert Decimal("0.99") * 1000 == Decimal("990.00")
assert div(Decimal(1), 3) == Decimal("0.33")
Creating a Fixed Point Decimal Class
It seems like it's surprisingly easy to modify the decimal module to become fixed point (at the expense of losing floating point decimals). This is because the Decimal class is referenced by global name in the module decimal. We can pop in our down compatible class and things will work just fine. First you need to prevent python from importing the C _decimal module and make it use the pure-python implementation of the decimal module (so we can override a private method of Decimal). Once you've done that you only need to override one method -- _fix. It's called for every new Decimal that is created for which it is possible that it may not abide by the current decimal context.
module set up
# setup python to not import _decimal (c implementation of Decimal) if present
import sys
if "_decimal" in sys.modules or "decimal" in sys.modules:
raise ImportError("fixedpointdecimal and the original decimal module do not work"
" together")
import builtins
_original_import = __import__
def _import(name, *args, **kwargs):
if name == "_decimal":
raise ImportError
return _original_import(name, *args, **kwargs)
builtins.__import__ = _import
# import pure-python implementation of decimal
import decimal
# clean up
builtins.__import__ = _original_import # restore original __import__
del sys, builtins, _original_import, _import # clean up namespace
main Decimal class
from decimal import *
class FixedPointDecimal(Decimal):
def _fix(self, context):
# always fit to 2dp
return super()._fix(context)._rescale(-2, context.rounding)
# use context to find number of decimal places to use
# return super()._fix(context)._rescale(-context.decimal_places, context.rounding)
# setup decimal module to use FixedPointDecimal
decimal.Decimal = FixedPointDecimal
Decimal = FixedPointDecimal
tests
getcontext().prec = 5
getcontext().Emax = 2
a = Decimal("0.34")
b = Decimal("0.20")
assert a * b == Decimal("0.07")
Using a customisable context
The context class is used to keep track of variables used control how new decimals are created. This way each program or even thread will be able to set the number of decimal places it wants to use for its decimals. Modifying the Context class is bit more long-winded. Below is the full class to create a compatible Context.
class FixedPointContext(Context):
def __init__(self, prec=None, rounding=None, Emin=None, Emax=None,
capitals=None, clamp=None, flags=None, traps=None,
_ignored_flags=None, decimal_places=None):
super().__init__(prec, rounding, Emin, Emax, capitals, clamp, flags,
traps, _ignored_flags)
try:
dc = DefaultContext
except NameError:
pass
self.decimal_places = decimal_places if decimal_places is not None else dc.decimal_places
def __setattr__(self, name, value):
if name == "decimal_places":
object.__setattr__(self, name, value)
else:
super().__setattr__(name, value)
def __reduce__(self):
flags = [sig for sig, v in self.flags.items() if v]
traps = [sig for sig, v in self.traps.items() if v]
return (self.__class__,
(self.prec, self.rounding, self.Emin, self.Emax,
self.capitals, self.clamp, flags, traps, self._ignored_flags,
self.decimal_places))
def __repr__(self):
"""Show the current context."""
s = []
s.append('Context(prec=%(prec)d, rounding=%(rounding)s, '
'Emin=%(Emin)d, Emax=%(Emax)d, capitals=%(capitals)d, '
'clamp=%(clamp)d, decimal_places=%(decimal_places)d'
% vars(self))
names = [f.__name__ for f, v in self.flags.items() if v]
s.append('flags=[' + ', '.join(names) + ']')
names = [t.__name__ for t, v in self.traps.items() if v]
s.append('traps=[' + ', '.join(names) + ']')
return ', '.join(s) + ')'
def _shallow_copy(self):
"""Returns a shallow copy from self."""
nc = Context(self.prec, self.rounding, self.Emin, self.Emax,
self.capitals, self.clamp, self.flags, self.traps,
self._ignored_flags, self.decimal_places)
return nc
def copy(self):
"""Returns a deep copy from self."""
nc = Context(self.prec, self.rounding, self.Emin, self.Emax,
self.capitals, self.clamp,
self.flags.copy(), self.traps.copy(),
self._ignored_flags, self.decimal_places)
return nc
__copy__ = copy
# reinitialise default context
DefaultContext = FixedPointContext(decimal_places=2)
# copy changes over to decimal module
decimal.Context = FixedPointContext
decimal.DefaultContext = DefaultContext
Context = FixedPointContext
# test
decimal.getcontext().decimal_places = 1
decimal.getcontext().prec = 5
decimal.getcontext().Emax = 2
a = Decimal("0.34")
b = Decimal("0.20")
assert a * b == Decimal("0.1")
