Logic

Previously, we learned about Booleans. Booleans are variables that can take two values: True or False.

We can make a variable a Boolean by setting it equal to True or False, but as we discussed earlier we can also use comparison operators to generate

< less than

> greater than

<= less than/equal to

>= greater than or equal to

== equal to

!= not equal to

Let’s create a basic list to get started

# create a list called my_list with numbers 1,2,3 inside
my_list = [1, 2, 3]

Let’s say a list with more than 2 items is considered long. Using the len function, we can create a Boolean value that tells us if our list is long or short.

# create a boolean called long_list that is True if our list has more than 2 items
long_list = len(my_list) > 2

# print the value of long_list
print(long_list)

True

What if we wanted to create a Boolean that had the opposite value?

Certain pairs of operators are opposites of one another.

> and <= are opposites

< and >= are opposites

== and != are opposites

So, since we used > to figure out if our list was long, we can use <= to figure out if our list is short.

# create a boolean called short_list that is False if our list doesn't have more than 2 items
short_list = len(my_list) <= 2

# print the value of short_list
print(short_list)

False

One cool thing about Python is that we can use words in place of some of the operators we talked about earlier. The operator == is used to test whether two things are equal.

First, let’s use the == operator to test if the first item of our list is equal to 1.

# use == to see if the first item of my_list equals 1
my_list[0] == 1

True

Now, let’s do the same thing using the is operator

# use is to see if the first item of my_list equals 1
my_list[0] is 1

True

In most cases, it doesn’t matter whether you use is or ==. Using is might help you avoid accidentally using = instead of ==, which is a common mistake.

Another useful python keyword is not. Putting not in front of a boolean will flip the value to its oposite—remember that True and False are opposites.

# use the "not" keyword to calculate the opposite of True
not True

False

The not keyword can be used with logical operators. Recall that the operator != determines if two things are not equal to each other. We learned above that is can be used to compare two objects, too. Then, we can use is and not together to see if two values are different.

# use != to see if the first item of my_list is not equal to 5
my_list[0] != 5

True

# use "is not" to see if the first item of my_list is not equal to 5
my_list[0] is not 5

True

Just like comparison operators such as > and <= can work on things like strings and integers (and booleans, too!), there are operators just for booleans.

First, we’ll look at or.

or looks at the two booleans and returns True if at least one is True, and otherwise returns False.

# print the value of True or True
print(True or True)

# print the value of True or False
print(True or False)

# print the value of False or False
print(False or False)

True
True
False

Another really useful boolean operator is and.

and works similarly to or, except it only returns True if both of the booleans are True.

# print the value of True and True
print(True and True)

# print the value of True and False
print(True and False)

# print the value of False and False
print(False and False)

True
False
False

One kind of problem that or and and can be useful for solving is testing if a value is inside a certain range.

Say we wanted to see if the length of our list was greater than 5 or less than 4—so any length except 4 or 5.

# use "or" to see if my_list has less than 4 or more than 5 items
len(my_list) > 5 or len(my_list) < 4

True

Say we wanted to solve the opposite problem—we want to see if the length of our list is exactly 4 or 5.

# use "and" to see if my_list has 4 or 5 items
len(my_list) >=4 and len(my_list) <= 5

False

We see that the length of our list is less than or equal to 5, but it isn’t greater than or equal to 4, so the value is False.

This is a bit of a tricky example, so it may take a minute or two for it to sink in. If it is confusing, try making a list with two columns - one representing >= 4 and one representing <= 5. For different example values, place a check in each column if that condition is true, and an X if it is false - only values where all columns have checks will result in True using the and operator.

Challenge: What about when we use the or operator?

Great job! You just learned about logic in Python! You learned: - How to create a boolean using operators - That some operators (for example, > and <=) are “opposites” - That keywords like is and not can be used in place of symbol-based operators - How to use or and and to combine booleans