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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.

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torrepreciadoon Dec. 30, 2021`return not a % b`