Truth Tables

Truth tables help us understand how logical operators work by showing all of the possible return values. Let’s look at the truth table for and, which assumes we have two boolean expressions, A and B.

Truth Table for `and`

A	B	A `and` B
`True`	`True`	`True`
`True`	`False`	`False`
`False`	`True`	`False`
`False`	`False`	`False`

Consider the first row of the table. This row states that if A is true and B is true, then A and B is true. The two middle rows show that if either A or B is false, then A and B is false. Finally, if both A and B are false, then A and B is false.

Truth Table for `or`

A	B	A `or` B
`True`	`True`	`True`
`True`	`False`	`True`
`False`	`True`	`True`
`False`	`False`	`False`

Similar to the and table, if A or B are both true, then A or B is true.
However, the middle two rows show that if either A or B is false, then A or B still remains true. This is very different from the and table. The last row shows that if both options are false, then the entire statement is false.

Order of Operations

We now have a lot of operators in our toolkit, so it is important to understand how they relate to each other. Which operators get done first?

Python always performs operations in a specific order:

It does all math calculations first.
Next, it evaluates all comparisons as True or False.
Next, it applies all not operators.
Finally, it evaluates and and or operations.

Example

The expression x * 5 >= 10 and y - 6 <= 20 will be completed in this order:

x * 5 is calculated, then y - 6.
The >= comparison is evaluated as True or False.
The <= comparison is evaluated as True or False.
The and operator is evaluated last.

Let’s say x = 2 and y = 46. Here we step through each stage of the evaluation:

Action	Result
Plug in the values into the expression	`2 * 5 >= 10 and 46 - 6 <= 20`
`x * 5` is calculated, then `y - 6`	`10 >= 10 and 40 <= 20`
The `>=` comparison is evaluated as `True` or `False`	`True and 40 <= 20`
The `<=` comparison is evaluated as `True` or `False`	`True and False`
The `and` operator is evaluated last	`False`

Table of Operator Order

The following table lists operators in order of importance, from highest (applied first) to lowest (applied last).

Level	Category	Operators
(Highest)	Parentheses	`()`
	Exponent	`` (For example: `23`)
	Multiplication and Division	`* / // %`
	Addition and subtraction	`+ -`
	Comparison	`== != <= >= > <`
	Logical	`not`
	Logical	`and`
(Lowest)	Logical	`or`

Tip

Using parentheses is not always necessary, but they make a BIG difference when someone else reads your code. As a best practice, use parentheses to make your code easier to read:

x * 5 >= 10 and y - 6 <= 20

vs.

(x * 5 >= 10) and (y - 6 <= 20)

Check Your Understanding

Question

Assume we have 3 boolean expressions (A, B, and C). Which combinations of values (A/B/C) will make the expression A or B and C evaluate to True?

True / True / True
False / True / True
True / False / True
True / True / False
False / False / True
False / True / False
True / False / False
False / False / False