roman numeral to int | Python Fiddle

numeral_map = {
  'M': 1000,
  'D': 500,
  'C': 100,
  'L': 50,
  'X': 10,
  'V': 5,
  'I': 1  
}

def RomanNumberToInteger(roman_numeral):
  int_version = 0
  count = 0 
  char_counts = {}
  while count < len(roman_numeral): 
    current_numeral = roman_numeral[count]
    current_value = numeral_map[current_numeral]
    next_numeral = roman_numeral[count+1]
    next_value = numeral_map[next_numeral]
    if current_value < next_value:
      if SubtractionIsValid(current_numeral, next_numeral):
        int_version += next_value - current_value
        count += 2
      else: 
        return -1
    else:
      int_version += current_value  
      count += 1
  return int_version

def SubtractionIsValid(current_numeral, next_numeral):
  are_valid = True
  
  if current_numeral in ['D', 'L', 'V']:
    are_valid = False
  elif current_numeral == 'I' and next_numeral not in ['V', 'X']:
    are_valid = False
  elif current_numeral == 'X' and next_numeral not in ['L', 'C']:
    are_valid = False
  elif current_numeral == 'C' and next_numeral not in ['D', 'M']:
    are_valid = False    
          
  return are_valid

print RomanNumberToInteger('MCMLXXXIV')
print RomanNumberToInteger('MXDCMIV')

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