calc_functions.py

class ConvertError(ValueError):
	message = 'Whoops - one of the values you entered cannot be converted to something that will work for these calculations. Try this again.'


class BadStuff(BaseException):
	message = 'Something went wrong with this calculation. Maybe try passing in new values.'


class Zero(ZeroDivisionError):
	message = 'Yeah, dividing with a 0 in the mix does not work.'


def try_division(value_list):
	try:
		return dividing(value_list)
	except Zero as a:
		zero_error = a.message
		return zero_error


def adding(value_list):
	new_value = 0
	for value in value_list:
		new_value += value
	return new_value


def subtracting(value_list):
	new_value = 0
	for value in value_list:
		new_value -= value
	return new_value


def multiplying(value_list):
	new_value = 1
	for value in value_list:
		new_value *= value
	return new_value


def dividing(value_list):
	new_value = 1
	for value in value_list:
		new_value /= value
	return new_value

views.py

from django.http import HttpResponse
from django.shortcuts import render
import calc_functions

def call_calc_functions(value_list):
    calculations = [
        (calc_functions.adding, 'Adding all of the values:'),
        (calc_functions.subtracting, 'Subtracting all of the values:'),
        (calc_functions.multiplying, 'Multiplying all of the values:'),
        (calc_functions.try_division, 'Dividing all of the values:'),
        ]
    results = [((method(value_list), header)) for method, header in calculations]
    return results


def do_math(request):
    context = {
        'heading': 'Here are some calculations based '
                    'on the information provided in '
                    'the URL for this page:',
    }

    try:
        value_list = [(float(value)) for key, value in request.GET.iteritems()]
    except calc_functions.ConvertError as x:
        convert_error = x.message
        context['convert_error'].append(convert_error)
    else:
        context['params'] = value_list

    try:
        result = call_calc_functions(value_list)
    except calc_functions.BadStuff as e:
        error = e.message
        context['error'] = error
    else:
        context['results'] = result

    return render(request, 'calculations.html', context)