2.2 Control

2.2.1 Boolean Operators

Python supports three boolean operators: and, or, and not:

>>> True and True
True
>>> True and False
False
>>> True or False
True
>>> not True
False
>>> a = 4
>>> a < 2 and a > 0
False
>>> a < 2 or a > 0
True
>>> not (a > 0)
False

• and evaluates to True if both of its operands evaluate to True. If at least one operand is False, then and evaluates to False.
• or evaluates to True if at least one operand evaluates to True. If both operands are False, then or evaluates to False.
• not evaluates to True if its operand evaluates to False. It evaluates to False if its operand evaluates to True.

What do you think the following expression evaluates to? Try it out in your Python interpreter.

>>> True and not False or not True and False

It is difficult to read complex expressions, like the one above, and understand how a program will behave.

What about hits one?

>>> 1 * -0 + -1 * 0

As is explained in lab00, using parentheses can make your code easier to understand. Python interprets that complex boolean expression in the following way:

>>> (True and (not False)) or ((not True) and False)

This is because boolean operators, like arithmetic operators, have precedence:

• not has the highest priority, similar to negation
• and is much like multiplication
• or has the lowest priority, similar to subtraction

Truthy and Falsey Values: It turns out and and or work on more than just booleans (True, False). Python values such as 0, None, '' (the empty string), and [] (the empty list) are considered false values. All other values are considered true values.

How to prove it?

2.2.2 Short Circuiting

What do you think will happen if we type the following into Python?

1 / 0

Try it out in Python! You should see a ZeroDivisionError. But what about this expression?

True or 1 / 0

It evaluates to True because and and or operators in Python short-circuit. That is, they don't necessarily evaluate every operand.

Short-circuiting happens when the evaluation of the expression reaches an operand that allows Python to make a conclusion about the expression. For example, and will short-circuit as soon as it reaches the first false value because it then knows the expression cannot be true since not all the operands are true.

If and and or do not short-circuit, they just return the last value; another way to remember this is that and and or always return the last thing they evaluate, whether they short circuit or not. Keep in mind that and and or don't always return booleans when using values other than True and False.

Can you explain what [] and True evaluates to and why?

2.2.3 If Statements

You can review the syntax of if statements in Section 1.5.4 of Composing Programs.

Tip: We sometimes see code that looks like this:

if x > 3:
    return True
else:
    return False

This can be written more concisely as return x > 3. If your code looks like the code above, see if you can rewrite it more clearly!

2.2.4 While Loops

You can review the syntax of while loops in Section 1.5.5 of Composing Programs.