Write a program that computes the average of a collection of values entered by the user. The user will enter 0 as a sentinel value to indicate that no further values will be provided. Your program should display an appropriate error message if the first value entered by the user is 0.

Hint: Because the 0 marks the end of the input it should not be included in the average.

#!/usr/bin/env python3


# get initial input
n = float(input())


# raise ValueError if user initial input is zero
if n == 0:
    raise ValueError("First value cannot be zero")


# initialize a list called numbers with the initial input as the first element
numbers = [n]


# loop while sentinel value is not entered
while n != 0:
    # get user input
    n = float(input())
    
    # add input to numbers list
    numbers.append(n)
    
# calculate average by summing all the numbers and dividing by the
# number of elements in the list minus 1
# minus 1 because it includes the last zero
average = sum(numbers) / (len(numbers) - 1)


# print the average
print(average)

Step-by-step explanation

The code is already heavily commented and should be self explanatory.

#!/usr/bin/env python3


# get initial input
n = float(input())


# raise ValueError if user initial input is zero
if n == 0:
    raise ValueError("First value cannot be zero")


# initialize a list called numbers with the initial input as the first element
numbers = [n]


# loop while sentinel value is not entered
while n != 0:
    # get user input
    n = float(input())
    
    # add input to numbers list
    numbers.append(n)
    
# calculate average by summing all the numbers and dividing by the
# number of elements in the list minus 1
# minus 1 because it includes the last zero
average = sum(numbers) / (len(numbers) - 1)


# print the average
print(average)

You can also implement it as a function like this:

def get_average():
    # get initial input
    n = float(input())


    # raise ValueError if user initial input is zero
    if n == 0:
        raise ValueError("First value cannot be zero")


    # initialize a list called numbers with the initial input as the first element
    numbers = [n]


    # loop while sentinel value is not entered
    while n != 0:
        # get user input
        n = float(input())
        
        # add input to numbers list
        numbers.append(n)
        
        # calculate average by summing all the numbers and dividing by the
        # number of elements in the list minus 1
        # minus 1 because it includes the last zero
        average = sum(numbers) / (len(numbers) - 1)


    # return the average
    return average