6.5 Return values

Learning objectives

By the end of this section you should be able to

Identify a function's return value.
Employ return statements in functions to return values.

Returning from a function

When a function finishes, the function returns and provides a result to the calling code. A return statement finishes the function execution and can specify a value to return to the function's caller. Functions introduced so far have not had a return statement, which is the same as returning None, representing no value.

Using multiple return statements

Functions that have multiple execution paths may use multiple return statements. Ex: A function with an if-else statement may have two return statements for each branch. Return statements always end the function and return control flow to the calling code.

In the table below, calc_mpg takes in miles driven and gallons of gas used and calculates a car's miles per gallon. calc_mpg checks if gallons is 0 (to avoid division by 0), and if so, returns -1, a value often used to indicate a problem.

def calc_mpg(miles, gallons): if gallons > 0: mpg = miles/gallons return mpg else: print("Gallons can't be 0") return -1 car_1_mpg = calc_mpg(448, 16) print("Car 1's mpg is", car_1_mpg) car_2_mpg = calc_mpg(300, 0) print("Car 2's mpg is", car_2_mpg) Car 1's mpg is 28.0 Gallons can't be 0 Car 2's mpg is -1

Using functions as values

Functions are objects that evaluate to values, so function calls can be used in expressions. A function call can be combined with other function calls, variables, and literals as long as the return value is compatible with the operation.

Note

Estimated days alive

Write a function, days_alive, that takes in an age in years and outputs the estimated days the user has been alive as an integer. Assume each year has 365.24 days. Use round, which takes a number and returns the nearest whole number.

Then write a main program that reads in a user's age and outputs the result of days_alive.

Given input:

The output is:


    You have been alive about 7670 days.

Note

Averaging lists

Write a function, avg_list, that takes in a list and returns the average of the list values.

# Define avg_list()

list1 = [34, 29, 77, 30, 48, 92]
print(list1)
print(f"Avg is: {avg_list(list1):.1f}")
list2 = [4, 2, 12, 8, 8]
print(list2)
print(f"Avg is: {avg_list(list2):.1f}")

Adapted from Introduction to Python Programming by OpenStax (openstax.org), licensed under CC BY-NC-SA 4.0. Changes were made. License: CC-BY-NC-SA-4.0.