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.

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

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

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

def hw_2():
  print("Hello world!")

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

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"

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)

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.

def tax(total):
  return .06 * total

def auto_tip(total):
  return .2 * total

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

def sq(num):
  return num * num

def offset(num):
  return num - 2

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