Boolean Expressions

Learning Goals

• Use Equality Comparison
• Use Inequality Comparison
• Use Greater-Than Comparison >
• Use Less-Than Comparison <
• Use Greater-Than-or-Equal-To Comparison >=
• Use Less-Than-or-Equal-To Comparison <=
• Invert Truth Value with "Bang" (!)
• Invert Truth Value with "Double-Bang" (!)
• Identify Truthy and Falsey Values in Python
• Join Boolean Expressions with AND
• Join Boolean Expressions with OR

Introduction

As we saw in the ternary expression, sometimes we need to get a Boolean value (True or False) from an expression in order to use it in another expression. We showed that we can use greater than (>) and less than (<) to perform comparisons that produce True or False. Let's learn more!

Arithmetic Comparisons

Use Equality Comparison

To check whether two values are equal, we use the equality operator represented with == ("double-equal-sign"). If two values are equal, then the statement will return True. If they are not equal, then it will return False. For example:

1 == 1 #=> True

1 == 7 #=> False

IMPORTANT: The comparison operator == is distinct from the assignment operator, =, that is used to set a variable equal to a value. Mistaking these for each other is a common cause of unexpected behavior.

Now, this might feel a bit weird, because you're used to thinking about = only being around numbers like Integer and Float. But you can also compare Strings:

"Razz" == "Matazz" #=> False

"Poodle" == "Poodle" #=> True

"Poodle" == "poodle" #=> False

Use Inequality Comparison

To check whether two values are not equal, we use the inequality operator represented with != ("bang-equal-sign;" more on "bang" below). If two values are not equal, then the statement will return True. If they are equal, then it will return False. For example:

1 != 1 #=> False

1 != 7 #=> True

"Poodle" != "Lord of the Manor" #=> True

Quantity Comparisons

You might recall these from middle-school: comparisons of greater-than / greater-than-or-equal-to.

Use Greater-Than Comparison >

If the value on the left of the operator is greater than the value on the right, then the evaluation is True; False otherwise.

Use Less-Than Comparison <

If the value on the left of the operator is less than the value on the right, then the evaluation is True; False otherwise.

Use Greater-Than-or-Equal-To Comparison >=

If the value on the left of the operator is greater than or equal to the value on the right, then the evaluation is True; False otherwise.

Use Less-Than-or-Equal-To Comparison <=

If the value on the left of the operator is less than or equal to the value on the right, then the evaluation is True; False otherwise.

Invert Truth Value with not

The not keyword inverts a truth value. Here's the most simple version:

not True #=> False
not False #=> True

We can also invert the truth value of an expression:

( 1 + 1 == 2 ) #=> True
not ( 1 + 1 == 2 ) #=> False

Since 1 + 1 evaluates to 2; and since 2 == 2 the return value is True.

Invert Truth Value Twice with not not

Using not not inverts a truth value twice. Here's the most simple version:

not not True #=> True
not not False #=> False

We can also do this to an expression:

not not ( 1 + 1 == 2 ) #=> True

Now why would this ever be useful? Great question. It turns out Python will treat a whole bunch of values as True that aren't the literal True. We call those values "truthy." Similarly, there are values that, even if they aren't the literal False, Python treats as false. We call those values "falsey."

This next statement is very important:

IMPORTANT: Python will treat anything that is False, 0 or None as falsey

So:

True if False else False  #=> False
True if 0 else False  #=> False
True if None else False  #=> False
True if 6.7 else False  #=> True
True if 1 + 1 else False  #=> True
True if "hello" else False #=> True

In each of the examples above, we wanted to return whether the truthy or falsey value was a real-deal True or False. What a lot of code to type. But here's where our friend not not comes in:

not not False #=> False
not not 0   #=> False
not not None #=> False
not not 6.7   #=> True
not not 1 + 1 #=> True
not not "hello" #=> True

Programmers often use not not to show other programmers "Hey, I'm being clever here and am using a truthy (or falsey) value.

Identify Truthy and Falsey Values in Python

This concept is so important we're going to repeat it again here:

IMPORTANT: Python will treat anything that is False, 0 or None as falsey

Join Boolean Expressions with AND

In Python and is used to join two Boolean expressions, returning True or False. For an and to evaluate to True, both values of either side of and must evaluate to True. For example:

True and True #=> True

True and False #=> False

It's common to say things like:

IF it's Thursday AND my Mom is not home THEN I will play scary video games all night on the living room TV.

In Python we would express this "double-conditional" like so.

day_is_thursday = True
mom_is_not_home = True

# Ternary
# Position 1                # Position 2                             # Position 3
"play scary video games" if day_is_thursday and mom_is_not_home else "do homework"

Join Boolean Expressions with OR

In Python or is also used to join two Boolean expressions. For an or to evaluate to True, only one value on either side of or must evaluate to True. For example:

False or True #=> True

Of course, keep in mind, these Boolean values can, themselves, be Boolean expressions! Instead of False and True it could be another expression that results in True or False like (poodle_count > 12) and (owner == "Lorlei Gilmore")

Longer Expressions

Because of the ability to use () and and and or, we can create surprisingly rich programs:

chance_of_precipitation = 1000
temperature = -1000
it_is_wet = ( chance_of_precipitation > 0.5 )
it_is_cold = ( temperature <= 5 )
"snow-suit" if it_is_wet and it_is_cold else "something less bulky" #=> "snow-suit"
"umbrella" if it_is_wet and not it_is_cold else "light fabric" #=> "light fabric"

Try changing some of the values or expressions to make sure you understand how to express your ideas using variables and Boolean conjunctions!

Conclusion

While it might seem strange that these simple little conditional expressions are so tiny, stacked together, they can have a big impact!

Most social media sites have a bit of conditional logic just like this one.

top_corner_image = retrieve_profile_pic if (user_logged_in and profile_pic_uploaded) else default_avatar

With this collection of comparison operators you're able to express a surprisingly complex series of desires to Python! Your programming conversational level is nearing the pre-teen stage!