024. Logical Operators

RAM Manager — Kill Decision Logic

Aryan’s RAM manager has a should_kill(process) function that decides whether to terminate a process. The rules are:

Auto-kill: RSS memory > 500 MB AND the process is marked as killable
Safe to skip: The process name is "system" OR it has pid == 1

He writes both functions, but swaps and for or in the auto-kill check. Now a process with 600 MB RSS is killed even if it is not marked killable (because or only needs one condition to be true). Critical system processes get killed.

The lesson: and requires both conditions; or requires at least one. In security and resource management, using or instead of and almost always means you are too permissive — and the consequences can be severe.

💡 Fun fact: Python uses and, or, not as keywords rather than C’s &&, ||, !. Guido van Rossum chose English words to make code read like prose. Additionally, Python’s logical operators use short-circuit evaluation — False and expensive_call() never evaluates expensive_call(), which can be used deliberately to avoid unnecessary computation.

⚠️ Watch out: Replacing and with or in a multi-condition check is the classic security mistake — it makes your gate too permissive. A password validator that uses or instead of and will accept any string that has at least one good property, not all of them.

🤔 Think about it: `not` has higher precedence than `and`, which has higher precedence than `or`. So `not a or b and c` parses as `(not a) or (b and c)`. How would you rewrite this expression with explicit parentheses to make the intended meaning unambiguous?

Learning objectives

Use and for “all conditions must be true”
Use or for “at least one condition must be true”
Use not to invert a boolean

Key concepts

logical operators
and
or
not

Try it

Concept detail

Logical operators: and (all True), or (at least one True), not (invert). Short-circuit evaluation: ‘False and anything’ → False (skips right side). ‘True or anything’ → True (skips right side). Truth table: and needs both True; or needs at least one True. not True == False, not False == True, not 0 == True, not “” == True. All non-zero, non-empty objects are truthy in Python.

Solution

def is_valid_password(password):
    has_length = len(password) >= 8
    has_digit = any(c.isdigit() for c in password)
    has_upper = any(c.isupper() for c in password)
    return has_length and has_digit and has_upper

def is_locked_out(attempts, last_attempt_mins_ago):
    return attempts >= 3 and last_attempt_mins_ago <= 30

Tests

def test_valid_password():
    assert is_valid_password("Secure123") == True

def test_too_short():
    assert is_valid_password("Ab1") == False

def test_no_digit():
    assert is_valid_password("SecurePass") == False

def test_no_upper():
    assert is_valid_password("secure123") == False

def test_locked_out():
    assert is_locked_out(3, 10) == True

def test_not_locked_few_attempts():
    assert is_locked_out(2, 5) == False

def test_not_locked_old_attempt():
    assert is_locked_out(5, 60) == False