Hello,
Python 2.x automatically converts the return value of __contains__ into
bool. There was a thread here about changing this in py3k
(http://mail.python.org/pipermail/python-3000/2008-January/011780.html),
but I cannot find whether this change has been accepted. It isn't in
recent tarball. If it was, this could be used f.e. to make a unit
conversion library with a natural API:
#v+
>>> from units import *
>>> radius_of_earth = Kilometers(6600)
>>> radius_of_earth in Meters
6600000 Meters
#v-
I guess it is a bit late for making this change now. I don't know python
source code well enough to make a patch, but I hope maybe this will
motivate someone to do so. Here's example code:
#v+
class Unit(object):
class __metaclass__(type):
def __contains__(dest_unit, value):
if isinstance(value, Unit):
return dest_unit(unit_convert(value.__class__,Unit,value))
else:
return Unit(value)
def __init__(self, value):
self.value = value
def __repr__(self):
return "%d %s" % (self.value, self.__class__.__name__)
class Meters(Unit): pass
class Kilometers(Unit): pass
class Centimeters(Unit): pass
# read as "Kilometer is 1000 Meters"
conv = [
[Kilometers, 1000.0, Meters],
[Meters, 100.0, Centimeters]
]
class CannotConvert(Exception): pass
def unit_convert(funit, tunit, value):
for f, m, t in conv:
if (funit is f) and (tunit is t):
return m*value
elif (funit is t) and (tunit is f):
return value/m
raise CannotConvert
#v-
Greetings,
Tomasz Melcer
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