converts from one unit to another

converter.py

conversions = [
  # length
	('inches','feet',lambda x: x/12.0),
	('feet','inches',lambda x: x*12),
	('meters','feet',lambda x: x*3.2808399),
	('feet','meters',lambda x: x*0.3048),
	('inches','centimeters',lambda x: x*2.54),
	('meters','centimeters',lambda x: x*100),
	('centimeters','meters',lambda x: x/100),
	# stoichiometry
	('moles','formula units',lambda x: x/6.022e23),
	('formula units','moles',lambda x: x*6.022e23),
	# pressure
	('atmospheres','millimeters of mercury',lambda x: x*760),
	('millimeters of mercury','atmospheres',lambda x: x/760),
	('atmospheres','pascals',lambda x: x*101.3),
	('pascals','atmospheres',lambda x: x/101.3),
	('pascals','kilopascals',lambda x: x/1000),
	('kilopascals','pascals',lambda x: x*1000),
	('torr','millimeters of mercury',lambda x: x),
	('millimeters of mercury','torr',lambda x: x),
]

abbreviations = {
	'inches': ['in'],
	'centimeters': ['cm'],
	'feet': ['ft'],
	'meters': ['m'],
	'moles': ['mol'],
	'formula units': ['molecules'],
	'atmospheres': ['atm'],
	'millimeters of mercury': ['mm hg'],
	'pascals': ['Pa'],
	'kilopascals': ['kPa'],
	'torr': ['torr']
}

def get_unit(abbreviation):
  if abbreviation in abbreviations.keys():
    return abbreviation
  else:
  	for abbrev in abbreviations.keys():
  		if abbreviation in abbreviations[abbrev]:
  			return abbrev
	raise NoAbbreviationException(abbreviation)

def get_abbreviation(unit):
  try:
    return abbreviations[unit][0]
  except KeyError:
    raise NoUnitException(unit)

class NoAbbreviationException(Exception):
	pass

class NoUnitException(Exception):
  pass

def conversions_from(unit):
  return [conversion for conversion in conversions if conversion[0] == unit]

def convert(amount, from_unit, to_unit, history=[]):
  if len(history) == 0: history = [from_unit]
  for conversion in conversions_from(from_unit):
    if conversion[1] == to_unit:
      return conversion[2](amount), history
    else:
      if conversion[1] not in history:
        answer, history = convert(conversion[2](amount),conversion[1],to_unit,history)
        if answer is not False:
          history.append(conversion[1])
          return answer, history
  return False, history

if __name__ == '__main__':
  try:
    while 1:      
      amt = float(raw_input('amount: '))
      try:
        from_unit = get_unit(raw_input('from unit: '))
      except NoAbbreviationException, e:
        print 'dont\'t know what', e, 'is an abbreviation for'
        continue
      try:
        to_unit = get_unit(raw_input('to unit: '))
      except NoAbbreviationException, e:
        print 'dont\'t know what', e, 'is an abbreviation for'
        continue
      answer, history = convert(amt,from_unit,to_unit)
      if answer is not False:
        if len(history) == 1:
          print answer
        else:
          print '%s > %s > %f' % (' > '.join(history),to_unit,answer)
      else:
        print 'sorry, don\'t know enough conversion factors to do that.'
  except (KeyboardInterrupt, EOFError):
    print