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

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Concepts in Practice: Using return statements

1.

What is returned by calc_mpg(miles, gallons)?

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

1. mpg

None

Error

2.

What is returned by calc_sqft()?

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

1. sqft
2. None
3. Error

3.

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

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

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

Concepts in Practice: Multiple return statements

4.

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"

"dk"

"super bulky"

5.

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
   

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

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Concepts in Practice: Using function values

6.

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

126.0

7.

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

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.

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Try It: Averaging lists

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

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