Dimensional analysis

unit.py

#!/usr/bin/python

class Unit(object):
	def __init__(self, value=0.0, unit={}):
		self.value = value
		self.unit = unit

	@staticmethod
	def check(unit1, unit2):
		p1 = [ (name, unit1[name]) for name in sorted(unit1.keys()) ]
		p2 = [ (name, unit2[name]) for name in sorted(unit2.keys()) ]
		if len(p1) != len(p2):
			return True
		else:
			for i in range(len(p1)):
				if p1[i][0] != p2[i][0] or p1[i][1] != p2[i][1]:
					return False
		return True

	@staticmethod
	def union(u1, u2, increase=True):
		result = u1.copy()
		m = (-1, 1)[increase]
		for name, exponent in u2.items():
			if result.has_key(name):
				result[name] += m * exponent
			else:
				result[name] = m * exponent
			if result[name] == 0:
				result.pop(name)
		return result

	def unit_name(self, joiner=' ', html=False):
		return joiner.join([ "%s%s" % (name, (('^%r', '<sup>%r</sup>')[html] % exponent, '')[not not exponent == 1]) for name, exponent in sorted(self.unit.items(), lambda y, x : cmp(x[1], y[1])) ])

	def __eq__(self, other):
		if self.value != other.value:
			return False
		else:
			return Unit.check(self.unit, other.unit)

	def __add__(self, other):
		if not Unit.check(self.unit, other.unit):
			raise TypeError("Incompatible units.")
		return Unit(self.value + other.value, self.unit)

	def __sub__(self, other):
		if not Unit.check(self.unit, other.unit):
			raise TypeError("Incompatible units.")
		return Unit(self.value - other.value, self.unit)

	def __mul__(self, other):
		return Unit(self.value * other.value, Unit.union(self.unit, other.unit))

	def __div__(self, other):
		return Unit(self.value / other.value, Unit.union(self.unit, other.unit, False))

	def __repr__(self):
		return "%r %s" % (self.value, self.unit_name())

print "1kWh =", Unit(1000, { 'J': 1, 's': -1 }) * Unit(3600, { 's': 1 })