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Returning Boolean Values

00:00 In this lesson, we’ll look at best practices when writing functions to return True or False. A Boolean-valued function, also known as a predicate function, is a function that returns a value of either True or False. It’s good programming to name this function beginning with the word is. It’s like you’re asking a question, “Is something True?” and the function then returns True, “Yes it is,” or False, “No, it isn’t.” Python has a set of rules to determine the truth value of any Python object.

00:33 Objects considered False, which we call falsy, are constants like None and False, numeric types with a value of 0, collection types that are empty, and objects with a .__bool__() method, which evaluates to False, or a .__len__() method, which evaluates to 0.

00:58 Any other Python object is considered truthy.

01:04 Here’s an example to determine if one number is divisible by another. It will return True to tell us it is and False to tell us it isn’t.

01:15 This function has two parameters, a and b, and we want to know if the argument provided for a is divisible by the argument provided for b. If a is divisible by b, then a divided by b will have a remainder of 0, and that’s what the modulus operator (%) gives us.

01:33 So, we’ll check if the operation a % b is 0. If it is, we return True. If it’s not, we return False.

01:42 Now, we could have written the condition with a comparison to 0, like I have in this commented version, but let’s be more Pythonic about it.

01:51 If a % b is 0, then it’s considered a falsy value. Putting a not in front of the falsy value makes the expression evaluate to True, so the condition not a % b will be True when a % b is 0. If that’s the case, we want to return True.

02:17 Since there’s no other condition to check for, we can have a return False outside the if block for the case when our condition is False. Again, if a % b isn’t 0, then it has a truthy value and taking the not of that truthy value evaluates to False.

02:39 The if condition will fail. We skip over the if block and execute what follows it, which is the return False statement. Let’s see this work.

02:59 We’ll see if 4 is divisible by 2, and it is. We’ll ask if 7 is divisible by 4, and it’s not.

03:13 Predicate functions often use the following: comparison operators, the membership operator, the identity operator, and the Boolean operator not.

03:26 In these cases, you can usually write the Boolean expression in the return statement. Here, we’re just returning the value of not a % b.

03:37 It will return True or False for us based on how not a % b is evaluated.

03:46 Let’s modify our version to look like that. We delete those two return statements. We are now going to return the value of this expression directly.

04:00 And since this is no longer an if statement, we’ll be sure to remove the colon that was there before. Let’s save this and try this out.

04:11 We’ll restart our interpreter,

04:18 import our new version, check that 4 still is divisible by 2,

04:27 and that 7 still is not divisible by 4. These evaluations work as before. A word of caution: you might not be able to return the expression directly if your condition involves or or and, and we’ll discuss that in the next lesson.

torrepreciado on Dec. 30, 2021

return not a % b

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