Compound Boolean Expressions

Note

Source: Logical operator descriptions adapted from the C# edition and the SE4ML Python chapter (chapter_python.rst, lines 920–948). Truth tables and common patterns are original additions.

Simple comparisons test one thing at a time. Often we need to combine conditions: “the temperature is above 70 and it is not raining.” Python uses the keywords and, or, and not — not symbols like &&, ||, and ! as in C#.

and

a and b is True only when both a and b are true:

>>> x = 5
>>> x > 0 and x < 10
True
>>> x > 0 and x > 10
False

or

a or b is True when at least one of a or b is true:

>>> x = 15
>>> x < 0 or x > 10
True
>>> x < 0 or x > 20
False

not

not a flips the truth value:

>>> not True
False
>>> not False
True
>>> x = 5
>>> not (x > 10)
True

Truth Tables

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

Operator Precedence

Among Boolean operators, not has the highest precedence, then and, then or. Comparison operators (<, ==, etc.) have higher precedence than all three. So this expression:

x > 0 and x < 10 or y == 5

is parsed as:

(x > 0 and x < 10) or (y == 5)

Use parentheses when in doubt — they make intent clear.

Short-Circuit Evaluation

Python evaluates and and or lazily: it stops as soon as the result is determined.

• a and b: if a is false, b is never evaluated.

• a or b: if a is true, b is never evaluated.

This is called short-circuit evaluation. It lets you write safe checks like:

if s and s[0] == 'A':
    print("starts with A")

If s is an empty string, s is falsy, so s[0] is never evaluated — avoiding an IndexError.

Common Patterns

Check if a value is in a numeric range:

if 0 <= score <= 100:
    print("valid score")

Test for two possible values:

if answer == "yes" or answer == "y":
    print("confirmed")

Negate a condition:

if not name.startswith("Dr."):
    name = "Dr. " + name