Text Ratio Calculator | Python Fiddle

import argparse

# Parse arguments.
parser = argparse.ArgumentParser(description='Calculate the missing number in a ratio equation.')

parser.add_argument('numbers',
                    metavar='N',
                    type=str,
                    nargs='+',
                    help='An integer in the ratio equation. There must be three numbers and either an x or a ? in the set to calculate the missing value. E.g.: 2 4 ? 10')

parser.add_argument('-r',
                    '--round',
                    metavar='N',
                    type=int,
                    default=2,
                    help='How many decimal places to which the result will be rounded. Default: 2')

parser.add_argument('-v',
                    '--verbose',
                    action='store_true',
                    default=False,
                    help='Returns ratio equation with missing ratio highlighted, instead of just missing number. E.g.: 1/2 [2]/4')

arguments = parser.parse_args()

# Extract numbers from arguments.
numbers = { 'a' : arguments.numbers[0],
            'b' : arguments.numbers[1],
            'c' : arguments.numbers[2],
            'd' : arguments.numbers[3] }

# Shorter variables for readability in equations.
a = numbers['a']
b = numbers['b']
c = numbers['c']
d = numbers['d']

# Find which of the four numbers is missing.
for key, value in numbers.items():
    if value == '?' or value == 'x':
        missing_number = key

# Perform the appropriate calculation.
if missing_number == 'a':
    result = float(b) * (float(c)/float(d))
    result = round(result, arguments.round)
    verbose_result = '[' + str(result) + ']' + '/' + b + ' = ' + c + '/' + d

if missing_number == 'b':
    result = float(a) / (float(c)/float(d))
    result = round(result, arguments.round)
    verbose_result = a + '/' + '[' + str(result) + ']' + ' = ' + c + '/' + d

if missing_number == 'c':
    result = float(d) * (float(a)/float(b))
    result = round(result, arguments.round)
    verbose_result = a + '/' + b + ' = ' + '[' + str(result) + ']' + '/' + d

if missing_number == 'd':
    result = float(c) / (float(a)/float(b))
    result = round(result, arguments.round)
    verbose_result = a + '/' + b + ' = ' + c + '/' + '[' + str(result) + ']'

# If verbose output was requested, give it. If not, just give the number.
if arguments.verbose:
    print verbose_result
else:
    print result

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