Boolean Logic

The eligibility rule sounded simple when the coach said it: a player can travel if they are on the roster and the form is in. Written with nested if statements it took six lines and still had a case nobody had thought about. Conditions that combine deserve an operator, not a staircase.

if on_roster and form_returned:
    print("Cleared to travel.")

The three operators

• and — True only when both sides are True.
• or — True when at least one side is True. This is not the everyday “or” that means one or the other but not both.
• not — flips the value.

A	B	A and B	A or B	not A
True	True	True	True	False
True	False	False	True	False
False	True	False	True	True
False	False	False	False	True

Two lines of that table earn their keep constantly. Row 2 is why an and with one missing form clears nobody. Row 3 is why an or written where you meant and lets everybody through, and looks fine doing it.

A Boolean is a value like any other

True and False are values with a type — bool — and comparisons produce them. You can store one:

days_late = 4
is_overdue = days_late > 0
print(is_overdue)

True

That is often the clearest thing you can do to a complicated condition: give it a name. if is_overdue and not renewed_recently: reads like the sentence the librarian actually said, and the names are checkable by the person who said it.

Two more things worth knowing:

• = assigns, == compares. if mark = 80: is a syntax error, which is Python doing you a favour.
• and stops as soon as it meets a False. That is why if sessions > 0 and minutes / sessions > 30: is safe: when sessions is 0, Python never evaluates the division, and the ZeroDivisionError never happens.

The trap that reads perfectly

day = "Mon"
 
if day == "Sat" or "Sun":
    print("Weekend!")

That prints Weekend! on a Monday. Python reads it as (day == "Sat") or ("Sun"), and a non-empty string counts as True, so the whole condition is always True. English collapses the repeated “day is”; Python does not. Write it as:

if day == "Sat" or day == "Sun":

Every comparison needs both of its sides, every time. This bug is a regular guest in Spot the Bug because it survives a casual reading so well.

Untangling a negative

not (on_roster and form_returned) and (not on_roster) or (not form_returned) are the same condition — “one of the two is missing”. Given the choice, write the version a human can say out loud, and use brackets whenever the reader might have to think. Nobody has ever complained about a condition being too clear.

Drill the combinations in Boolean Logic Practice, and try defending one of these operators in Which One Doesn’t Belong. The third “change it” in Branching Programs is the first place a compound condition does real work for somebody.

Curriculum connection

A1.1

use constants and variables, including integers, floating points, strings, and Boolean values, correctly in computer programs;

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A1.4

demonstrate the ability to use Boolean operators (e.g., AND, OR, NOT), comparison operators (i.e., equal to, not equal to, greater than, less than, greater than or equal to, less than or equal to), arithmetic operators (e.g., addition, subtraction, multiplication, division, exponentiation, parentheses), and order of operations correctly in computer programs;

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A2.2

use sequence, selection, and repetition control structures to create programming solutions;

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