#!/usr/bin/env python
# encoding: utf-8
"""
Consumer Price Index conversions
2010-02-21 by Ryan Greenberg

Convert nominal dollars to real dollars using consumer price index
from the Bureau of Labor Statistics.
ftp://ftp.bls.gov/pub/special.requests/cpi/cpiai.txt

Example:
Convert $1 in 1974 to 2006 dollars

    import cpi
    cpi_convert = cpi.CPI(open('cpi_rates.txt', 'r')) # path to cpi data file
    print cpi_convert.real_dollars(1, 1974, 2006)
    ==> 4.0892494929

The input text file, cpi_rates.txt, is a tab-separated list of years and
the average CPI for that year.
"""

import sys
import os
import re

class CPI():
    """Loads the specified year/average CPI table which
    can be used for conversions from nominal dollars to
    real dollars."""
    def __init__(self, file):
        self.rates = {}
        for year in file:
            year = year.split("\t")
            if re.match('\d{4}', year[0]):
                self.rates[int(year[0])] = float(year[1])
            else:
                pass
    
    def real_dollars(self, dollars, frm, to):
        """Returns dollars in to year using from year.
        from dollars = to dollars * to CPI / from CPI
        See http://www.polsci.wvu.edu/duval/ps300/Notes/ConvertNom_to_Real_dollars.htm"""
        try:
            frm = int(frm)
            to = int(to)
        except Exception, e:
            raise Exception("Conversion year must be a number")
            
        if isinstance(dollars, str):
            dollars = float(dollars.replace('$', '').replace(',', ''))
        if frm in self.rates and to in self.rates:
            return dollars * self.rates[to] / self.rates[frm]
        elif frm not in self.rates:
            raise Exception("Cannot convert to real dollars: %s not in CPI lookup table" % str(frm))
        else:
            raise Exception("Cannot convert to real dollars: %s not in CPI lookup table" % str(to))
        
def main():
	print __doc__
	
if __name__ == '__main__':
	main()