Logical Operators: and, or, not

Real decisions combine conditions: *"if you're 18 and have a ticket"* — *"if it's raining or cloudy"* — *"if not logged in."* Three operators handle all of it.

The truth table (the whole lesson on one screen)

code

print(True and True)     # True
print(True and False)    # False
print(False and False)   # False

print(True or True)      # True
print(True or False)     # True
print(False or False)    # False

print(not True)          # False
print(not False)         # True

In words:

and — strict parent: *both* must be True

or — lenient parent: *at least one* True is enough

not — the rebel: flips whatever it sees

The real usage: combining comparisons

code

age = 20
has_ticket = True

# Both conditions must hold
can_enter = age >= 18 and has_ticket
print(can_enter)          # True

# Any one is enough
is_weekend = True
is_holiday = False
can_relax = is_weekend or is_holiday
print(can_relax)          # True

# Flip a flag
is_raining = False
need_umbrella = not is_raining
print(need_umbrella)      # True... wait, that's wrong on purpose!

That last one is a bug I planted — if it's NOT raining, do you need an umbrella? Read not is_raining out loud: "not raining" → True → but you need an umbrella when it IS raining. The correct line: need_umbrella = is_raining. Read every `not` out loud. It catches these instantly.

Real conditions from real apps

code

# Login check
username_ok = username == "admin"
password_ok = password == "secret123"
if username_ok and password_ok:
    print("Welcome!")

# Range check (the readable way)
if 0 < marks <= 100:
    print("Valid marks")

# Discount eligibility
if is_student or is_senior:
    price = price * 0.8

# Guard clause
if not is_logged_in:
    print("Please log in first")

That last pattern — if not something: exit early — is called a guard clause, and professional code uses it everywhere.

Truthiness — the sneaky superpower

Python can treat ANY value as True or False:

code

# These are all "falsy" (act like False):
print(bool(0))           # False
print(bool(""))          # False — empty string
print(bool([]))          # False — empty list
print(bool(None))        # False

# These are all "truthy" (act like True):
print(bool(42))          # True
print(bool("hi"))        # True — non-empty string
print(bool([-1]))        # True — non-empty list

Which enables beautifully short code:

code

name = input("Name: ")

if name:                    # "if name is non-empty"
    print(f"Hello, {name}")
else:
    print("You typed nothing!")

if name: reads as "if there's a name" — Pythonic and clean.

Short-circuit evaluation

Python stops evaluating as soon as the answer is known:

code

print(False and crash_the_program)   # False — crash_the_program never runs!
print(True or crash_the_program)     # True  — same, skipped

and short-circuits on the first False; or short-circuits on the first True. Practical use — safe checks:

code

# If user is None, the second condition is never evaluated → no crash
if user is not None and user.age >= 18:
    print("Adult user")

Precedence: not > and > or

code

print(True or False and False)    # True — "and" binds tighter: True or (False and False)
print((True or False) and False)  # False — parentheses change it

Same advice as arithmetic: when mixing, use parentheses to say what you mean.

Common Errors & Fixes

`and` where you meant `or` — read the condition aloud; "and" means BOTH.

`SyntaxError` on `if x > 5 and < 10:` — each side needs the variable: if x > 5 and x < 10: (or chain: 5 < x < 10).

Double negatives confusing you — if not is_invalid: — consider renaming the flag is_valid and simplifying.

✅ Checkpoint

True and not False? *(True — not False is True, then True and True)*

age < 18 or has_id — True if the user is 16 with no ID? *(False — both sides False)*

What's bool("")? *(False — empty string is falsy)*

Why does if user is not None and user.age >= 18 not crash when user is None? *(Short-circuit: the second check never runs)*

Next: assignment operators — the shorthand that makes updates one character shorter.