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.

Checkpoint: Returning a value from a function

Concepts in Practice \(\PageIndex{1}\): Using return statements

What is returned by calc_mpg(miles, gallons)?

def calc_mpg(miles, gallons):
  mpg = miles/gallons
  return mpg

1. mpg
2. None
3. Error

Answer

a. calc_mpg() specifies mpg to be returned with the return statement.

Concepts in Practice \(\PageIndex{2}\): Using return statements

What is returned by calc_sqft()?

def calc_sqft(length, width):
  sqft = length * width
  return

1. sqft
2. None
3. Error

Answer

b. return is a valid statement to end the function and return to the calling code without a specified value. calc_sqft() should return sqft with the line return sqft.

Concepts in Practice \(\PageIndex{3}\): Using return statements

What is the difference between hw_1() and hw_2()?

def hw_1():
  print("Hello world!")
  return

def hw_2():
  print("Hello world!")

1. hw_1() returns a string, hw_2() does not
2. hw_1() returns None, hw_2() does not
3. no difference

Answer

c. If the return value or return statement is not specified, the function returns None .

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.

Table 6.1: Calculating miles-per-gallon and checking for division by zero.

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

Concepts in Practice \(\PageIndex{4}\): Multiple return statements

What does yarn_weight(3) return?

def yarn_weight(num):
  if num == 0:
    return "lace"
  elif num == 1:
    return "sock"
  elif num == 2:
    return "sport"
  elif num == 3:
    return "dk"
  elif num == 4:
    return "worsted"
  elif num == 5:
    return "bulky"
  else:
    return "super bulky"

1. "lace"
2. "dk"
3. "super bulky"

Answer

b. num == 3 is True , so the return "dk" statement executes.

Concepts in Practice \(\PageIndex{5}\): Multiple return statements

What is the output?

def inc_volume(level, max):
  if level < max:
    return level
    level += 1
  else:
    return level

vol1 = inc_volume(9, 10)
print(vol1)
vol2 = inc_volume(10, 10)
print(vol2)

1. 9
   10
   

2. 10
   10
   

3. 10
   11

Answer

a. For inc_volume(9,10), return level returns 9 , and level += 1 is never reached.

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.

Checkpoint: Using function calls in expressions

Concepts in Practice \(\PageIndex{6}\): Using function values

What is the updated value of bill?

def tax(total):
  return .06 * total

def auto_tip(total):
  return .2 * total

bill = 100.0
bill += tax(bill) + auto_tip(bill)

1. 26.0
2. 126.0

Answer

b. bill += tax(bill) + auto_tip(bill) evaluates to bill += 6.0 + 20.0 to bill = 100.0 + 6.0 + 20.0 to 126.0.

Concepts in Practice \(\PageIndex{7}\): Using function values

What is the value of val2?

def sq(num):
  return num * num

def offset(num):
  return num - 2

val = 5
val2 = sq(offset(val))

1. 9
2. 23

Answer

a. val2 = sq(offset(5)) evaluates to val2 = sq(3) to 9.

Try It: 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:

    21

The output is:

    You have been alive about 7670 days.

Interactive Code

Input

21

Answer

def days_alive(age):
return round(age * 365.24)

user_age = int(input())
days = days_alive(user_age)
print("You have been alive about", days, "days.")

Try It: Averaging lists

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

Interactive Code

# 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}")

Answer

def avg_list(val_list):
sum = 0
count = 0
for val in val_list:
sum += val
count += 1
return sum / count

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}")