How to set Python (global?) variable value based on if statement

Question

I'm learning Python and writing a function that takes an array input of the number of months users subscribed to a service (months_subscribed) and calculates revenue based on this. Revenue is $7 per month, with a bundle discount of $18 for 3 months. Let's start with an input of [3,2], so the expected total revenue (months_value) should be $18 + $14 = $32. I want to use a loop so that this can calculate for months_subscribed even if it has more elements, such as [3,2,18,0].

1. Currently, when I run this months_value gives an output of 35, not 32. It is doing 21 + 14 = 35 and not taking into account the bundle discount. Why is this?
2. How can months_value be fixed so that it incorporates the bundle discount, and months_value = 35? (I wonder if this involves specifying months_value as a global variable?)
3. Why do the first values in Print statements 1 & 2 differ? (18, vs 21) I calculate the value of months_value in the if/else statement, so I don't understand why they vary between these two statements.

My code is as follows:

import numpy as np

months_subscribed = [3,2]

def subscription_summary(months_subscribed):

    # 3-month Bundle discount: 3 months for $18, otherwise $7 per month
    for j in range(0,len(months_subscribed)):
        if (months_subscribed[j] % 3 == 0):
            months_value = np.multiply(months_subscribed, 6)
        else:
            months_value = np.multiply(months_subscribed,7)
        print(months_value[j]) # Print statement 1: 18 then 14
    print(months_value)        # Print statement 2: [21 14]
    print(sum(months_value))   # Print statement 3: 35. Expected: 18 + 14 = 32

subscription_summary(months_subscribed)

exit()

Thank you!

Answer 1

My coworker helped me solve this.

The key is to make an empty list that holds the count of months subscribed, calculate revenue and use append to update the revenue values in that list, and return them. This is flexible as it works for any size list. It assumes the count of months_subscribed is non-negative.

This is what we did:

months_subscribed = [3,2,0] # Expected Revenue Result: [18, 14, 0]

def new_subscription_summary(months_subscribed):
# Make an empty list to hold each subscription's value
values = []

# Loop over each subscription
for this_sub in months_subscribed:
    # Start with a price of $7/mo by default
    price = 7
    # But, if the subscription was for a multiple of 3 months, use the promo price of $6
    if this_sub % 3 == 0: price = 6
    # Multiply the price per month by the number of months to find the value, and add that to our list
    values.append(price * this_sub)

print(values)
# Add up all the values and return
return sum(values)

new_subscription_summary(months_subscribed)

And this returns:

[18, 14, 0]