Roman numeral calculator

roman-numeral-calculator.py

"""
Roman Numeral calculator
"""

def value(numeral):
  """
  numeral = string
  
  Returns base 10 value of roman numeral
  if found else returns None
  """
  values = {
    "I": 1,
    "V": 5,
    "X": 10,
    "L": 50,
    "C": 100,
    "D": 500,
    "M": 1000
  }
  
  return values.get(numeral.upper())
  

def substract(numerals):
  """
  numerals = list of roman numerals (string)
  
  Checks if last numeral in numerals is biggest number
  Meaning we need to substract from it
  
  Returns True or False
  """
  return value(numerals[0]) < value(numerals[len(numerals) - 1])


def validate(input_numerals):
  """
  input_numerals = string
  
  Returns list of numerals or False
  """
  try:
      numerals = list(input_numerals)
  except TypeError:
      return False
  
  if all(value(i) for i in numerals):
    return numerals
  
  return False
  
  
def calculate(numerals):
  """
  numerals = list of roman numerals (string)
  returns int total value of roman numerals
  """
  total = 0
  
  nums = validate(numerals)
  
  if nums:
    substraction = substract(nums)
  else:
    return "Please enter a valid roman numeral"

  if substraction:
    # take last value in list
    # to substract from
    total = value(nums.pop())
    
    for n in nums:
      total -= value(n)
    return total
  
  for n in nums:
    total += value(n)
  
  return total
  

print(calculate("miiv")) # Answer: 1007
print(calculate("iiv")) # Answer: 3
print(calculate("miiv3")) # Error!