What’s the Return Value of a Bool Expression in Python?

We know that the boolean value of 0 is False, and the boolean value of 1 is True. What about the boolean value of a negative number, for example, -2 ? What‘s the result of the expression True and 3 in Python? Is it True or False, or other value? Let’s find it out.

1. Boolean Value

Here is a question I came across while I was doing a lab of Berkeley CS61A Course . What would Python print?

>>> positive = 28 

>>> while positive: 

...     print("positive?") 

...     positive -= 3

I thought that after printing “positive?” 10 times , the variable “positive” would become negative, so the program would stop. But my answer didn’t pass the test, so I ran it locally to see what happened. To my surprise, the program didn’t stop! It’s an infinite loop. Why?

Because the boolean value of a negative number is True. Only the boolean value of 0 is False, while the boolean value of a positive or negative number is True, so bool(-2)=True .

In Python, the boolean value of False (or 0, or ‘’, or None) is False. The boolean value of others is True.

Then how to evaluate the bool expression?

2. Bool Expression

A bool expression usually contains logic operators such as and, or, not.

(1) <exp1> and <exp2>

The result of False and True is False, so for the expression <exp1> and <exp2> , only when the value of the first element <exp1> is True do we need to check the second element <exp2> . For example, the value of True and 0 is 0.

That is, and stops evaluating at the first false value. For example:

>>> 0 and 2
0
>>> False and -2
False

If all values evaluate to be True, the last value is returned. So the value of expression True and 3 is 3, while the expression 3 and True returns True. The following is another example of and between two numbers:

>>> 2 and -3
-3
>>> -3 and 2
2

From the results above, you‘ll see that the return value of a bool expression like <exp1> and <exp2> is not always True or False . Take True and 3 as an example, it evaluates the boolean value of <exp1> /<exp2> ( bool(3)=True ), but returns the actual value ( 3 ), so it‘s a number.

(2) <exp1> or <exp2>

or expression uses an evaluating order like and expression. The result of True or False is True, so for the expression <exp1> or <exp2> , only when the value of the first element <exp1> is False do we need to check the second element <exp2> .

That is, or stops evaluating at the first true value. For example:

>>> 0 or -1
-1
>>> 2 or -1
2
>>>True or 2
True
>>>0 or False or -3 or 1 / 0
-3

Here the last element 1/0 in the last expression won’t be evaluated, otherwise it’ll produce an error.

If all values evaluate to be False, the last value is returned. For example:

>>> False or 0
0
>>> 0 or False
False

(3) not <exp>

Unlike and and or expression, not expression returns the opposite boolean value of <exp>, so the result is either True or False. For example:

>>> not 10
False

Reference:

Lab 1: Variables & Functions, Control | CS 61A Fall 2020

disc01| CS 61A Fall 2020