unit.py

#!/usr/bin/env python
#
#  unit.py
#  
#  Copyright September 2015 
#  
#   Documentation is like sex.
#   When it's good, it's very good.
#   When it's bad, it's better than nothing.
#   When it lies to you, it may be a while before you realize something's wrong.
#  
#  

'''
run without arguments to enter a loop.
first loop must have 3 arguments: number, from unit, to unit
further loops can have 1, 2, or 3 arguments:
  1: a new number to convert with previous from and to units
  2: a new number and new from unit, with previous to unit
  3: all new arguments
enter without arguments to exit the loop.
run with 2 arguments to set the from and to units
run with 3 arguments for a single conversion, then exit
'''

import subprocess
import sys
if sys.version_info < (3,):
	input = raw_input

def get_input(prompt):
	try:
		return input(prompt)
	except EOFError: #catch ctrl-D (Alt-F4) to quit
		print()
		quit()

def convert(number, from_unit, to_unit):
	subprocess.call([
		'units', '-v', 
		"%s %s"%(number, from_unit), 
		to_unit])

def looper(from_unit = None, to_unit = None):
	if from_unit is None and to_unit is None:
		while True:
			user_in = get_input("(? to ?) > ")
			user_in = user_in.split()
			if len(user_in) not in (0,2,3):
				print("Error: first run must have 2 or 3 arguments")
				print("Enter a value, a from unit, and a to unit")
				print('e.g. "9 m ft" converts 9 meters to feet')
				print('(which is the world record for a longjump)')
				print("Or enter an initializing from unit and to unit")
				print("Or enter nothing to quit")
				continue
			elif len(user_in) ==0:
				quit()
			elif len(user_in) == 2:
				from_unit, to_unit = user_in
			elif len(user_in) == 3:
				number, from_unit, to_unit = user_in
				convert(number, from_unit, to_unit)
			break
	
	while True:
		user_in = get_input("(%s to %s) > "%(from_unit, to_unit))
		user_in = user_in.split()
		if len(user_in) == 3:
			number, from_unit, to_unit = user_in
		elif len(user_in) == 2:
			number, from_unit = user_in
		elif len(user_in) == 1:
			number = user_in[0]
		else:
			quit()	
		convert(number, from_unit, to_unit)

def main():
	if len(sys.argv) in (1,3):
		looper(*sys.argv[1:])
	elif len(sys.argv) == 4:
		convert(*sys.argv[1:])
	else:
		print("Error: 0, 2 or 3 arguments expected; got %d"%(len(sys.argv)-1))
		print(__doc__)

if __name__ == '__main__':
	main()