10.6. Boolean Functions

A function that returns a boolean value (True or False) is known as a boolean function. One simple example tests if an integer is even or odd.

Example

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def is_int_even(integer):
   if integer % 2 == 0:
      return True
   else:
      return False

print(is_int_even(4))
print(is_int_even(7))

Console Output

True
False

Tip

Start the names of boolean functions with either is or has, which helps make their purpose more clear. For example, reading is_int_even(4) tells us that the function should answer the question: “Is the integer 4 even?”

This convention is so widely used by programmers that it extends to nearly every language.

Let’s look at is_int_even again to see how we can use the power of return statements to make it even stronger.

Since return ends the function, we can leave out the else clause and have the same effect. If integer is even, the return statement in line 3 runs, and the function ends. If integer is odd, the if block gets skipped and the second return statement executes.

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def is_int_even(integer):
   if integer % 2 == 0:
      return True

   return False

print(is_int_even(4))
print(is_int_even(7))

The updated version returns the same results as the original and contains less code.

The function returns True when the expression integer % 2 == 0 is True, and the function returns False when integer % 2 == 0 is False. In this case, the return value is exactly the same as the value of integer % 2 == 0. This means that we can simplify the function even more by returning the result of the expression!

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def is_int_even(integer):
   return integer % 2 == 0

print(is_int_even(4))
print(is_int_even(7))

This version of is_int_even is better than the first two, not because it is shorter (shorter isn’t always better), but because it is simpler to read. We don’t have to break down the conditional logic to see what is being returned.

Most boolean functions can be written so that they return the result of a boolean expression, rather than directly returning True or False.

10.6.1. Try It!

Example

Complete the is_capitalized function to return True if a string begins with a capital letter and False otherwise.

Hints:

  1. How do you access the first character in a string?
  2. The in operator will be useful, or you can try the .isupper() method.

Example

Complete the is_answer_correct function to return True if a user’s answer matches the correct response. The check should be case-insensitive.

With the given answer pairs, the program should count 2 correct responses.