Question

Could I code differently to slim down the point of this Python source code? The point of the program is too get the users total amount and add it too the shipping cost. The shipping cost is determined by both country (Canada or USA) and price of product:
The shipping of a product that is $125.00 in Canada is $12.00.

---

input ('Please press "Enter" to begin')

while True:
print(‘This will calculate shipping cost and your grand total.’)

totalAmount = int(float(input('Enter your total amount: ').replace(',', '').replace('$', '')))
Country = str(input('Type "Canada" for Canada and "USA" for USA: '))

usa = "USA"
canada = "Canada"
lessFifty = totalAmount <= 50
fiftyHundred = totalAmount >= 50.01 and totalAmount <= 100
hundredFifty = totalAmount >= 100.01 and totalAmount <= 150
twoHundred = totalAmount

if Country == "USA":
    if lessFifty:
        print('Your shipping is: $6.00')
        print('Your grand total is: $',totalAmount + 6)
    elif fiftyHundred:
        print('Your shipping is: $8.00')
        print('Your grand total is: $',totalAmount + 8)
    elif hundredFifty:
        print('Your shipping is: $10.00')
        print('Your grand total is: $',totalAmount + 10)
    elif twoHundred:
        print('Your shipping is free!')
        print('Your grand total is: $',totalAmount)

if Country == "Canada":
    if lessFifty:
        print('Your shipping is: $8.00')
        print('Your grand total is: $',totalAmount + 8)
    elif fiftyHundred:
        print('Your shipping is: $10.00')
        print('Your grand total is: $',totalAmount + 10)
    elif hundredFifty:
        print('Your shipping is: $12.00')
        print('Your grand total is: $',totalAmount + 12)
    elif twoHundred:
        print('Your shipping is free!')
        print('Your grand total is: $',totalAmount)

endProgram = input ('Do you want to restart the program?')
if endProgram in ('no', 'No', 'NO', 'false', 'False', 'FALSE'):
    break

Answer 1

Here’s a basic strategy for the cost calculation:

import math

amount = int(totalAmount)
assert amount >= 0
shipping = { 
  'USA': [6, 8, 10],
  'Canada': [8, 10, 12]
}
try:
    surcharge = shipping[country][amount and (math.ceil(amount / 50.0) - 1)]
except IndexError:
    surcharge = 0
total = amount + surcharge

The key notion here is that the the shipping cost ranges follow a fairly linear progression: [0-50], (50-100], (100-150], (150, inf)

Note that the first group is a little funny, as it includes the lower bound of 0 where the other groups don’t include the lower bound (they are an open interval at the bottom). So going forward we’ll consider the first group as the following: 0 or (0-50]

We want to transform the amount the user supplies into an index into the shipping cost lists [6, 8, 10] and [8, 10, 12]. The lists are of length 3, so the indexes are 0, 1 and 2. Notice that if we divide any number in the range (0, 150] by 50.0 (we add the .0 to 50 so we a get a real number back — 1 / 50 == 0 but 1 / 50.0 == 0.02 — for the next step) we get a number in the range (0 and 3].

Now realize that math.ceil will round a real number to the nearest whole number that is greater than or equal to itself — ex. math.ceil(.001) == 1.0, math.ceil(1.5) == 2.0, math.ceil(2) == 2.0. So applying math.ceil to numbers in the range (0, 3] we will supply either 1.0, 2.0 or 3.0. Cast those numbers to int (int(2.0) == 2) and we get the values 1, 2 and 3. Subtract 1 from those values and we get the 0, 1, 2. Voila! Those numbers match the indexes into our shipping array.

You can express this transformation with the Python: int(math.ceil(amount / 50.0) - 1)

We are almost there. We’ve handled any amount in the range (0, 150]. But what if amount is 0. math.ceil(0) == 0.0 and 0.0 - 1 == -1.0 This will not index properly into our array. So we handle 0 separately by checking first if amount is equal to 0, and if it is, using 0 as our shipping array index instead of the applying our transformation to amount to determine the index. This can be accomplished using Python’s short circuit and operator (the web should have plenty of information on this) in the following expression: amount and int(math.ceil(amount / 50.0) - 1)

Note that any value above 150 will reduce to an index greater than 2 and applying that index against the shipping array will result in an IndexError. But any value greater than 150 has free shipping so we can catch the IndexError and apply a surcharge of 0:

try:
    surcharge = shipping[country][amount and int(math.ceil(amount / 50.0) - 1)]
except IndexError:
    surcharge = 0

And we’re done!