Operators and Conditionals

Operators build expressions. Conditionals use the resulting values to choose which statements run.

Arithmetic operators

print(7 + 3)   # 10
print(7 - 3)   # 4
print(7 * 3)   # 21
print(7 / 3)   # 2.333... true division always returns float
print(7 // 3)  # 2 floor division
print(7 % 3)   # 1 remainder
print(7 ** 3)  # 343 exponentiation

// rounds down, not toward zero:

print(-7 // 3)  # -3

Use divmod when both quotient and remainder matter:

hours, minutes = divmod(135, 60)
print(hours, minutes)  # 2 15

Parentheses make precedence explicit:

average = (8 + 9 + 10) / 3

Comparison and logical operators

Comparisons produce booleans:

age = 20
print(age >= 18)  # True
print(age != 21)  # True
print(18 <= age < 65)  # True: chained comparison

Combine conditions with and, or, and not:

has_ticket = True
is_banned = False
may_enter = age >= 18 and has_ticket and not is_banned

Precedence is not, then and, then or. Use parentheses when a reader could hesitate.

Membership

in and not in test membership:

role = "editor"
allowed_roles = {"editor", "admin"}

if role in allowed_roles:
    print("Access granted")

Membership in a dictionary checks keys, not values.

Truthiness

Conditions need not be literal booleans. These values are falsy:

• False and None
• numeric zero, such as 0 and 0.0
• empty containers and text, such as "", [], {}, and set()

Most other values are truthy:

name = ""

if name:
    print(f"Hello, {name}")
else:
    print("A name is required")

Use explicit comparisons when zero and absence have different meanings:

discount = 0

if discount is None:
    print("Discount not calculated")
else:
    print(f"Discount: {discount}%")

if, elif, and else

Only the first matching branch runs:

score = 84

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "Needs improvement"

print(grade)  # B

Order matters. Put narrower or higher-priority conditions before broader ones.

Nested conditions are sometimes necessary, but combined conditions or early returns inside functions often read more clearly.

Conditional expressions

Use a conditional expression for one simple value choice:

status = "adult" if age >= 18 else "minor"

Do not nest conditional expressions. A regular if statement is clearer when there are several conditions or actions.

Short-circuit evaluation

and stops at the first falsy operand; or stops at the first truthy operand. This can safely guard later work:

items = ["pen", "book"]

if items and items[0] == "pen":
    print("The first item is a pen")

The expression avoids indexing an empty list.

Logical operators return an operand, not necessarily True or False:

display_name = "" or "Anonymous"
print(display_name)  # Anonymous

Use this defaulting pattern only when every falsy value should trigger the default.

Assignment operators

count = 5
count += 2
count *= 3
print(count)  # 21

For mutable objects, augmented assignment may mutate in place. For immutable values, it binds a new object.

Common mistakes

• Writing = instead of == in a comparison causes SyntaxError.
• Using & or | for boolean logic; those are bitwise operators.
• Putting a broad condition before a narrower elif, making the latter unreachable.
• Writing if x == "a" or "b":; "b" is always truthy.
• Treating every falsy value as missing when 0 or an empty collection is meaningful.
• Comparing floats for exact equality after arithmetic.

Correct a multi-value comparison with membership:

if answer in {"yes", "y"}:
    print("Confirmed")

Decision guidance

• Use chained comparisons for ranges: 0 <= percentage <= 100.
• Use a set for membership among several fixed choices.
• Use if/elif for mutually exclusive choices and separate if statements for independent checks.
• Use a conditional expression only when it improves a short assignment.
• Prefer explicit conditions when truthiness would hide an important distinction.

Practice

1. Classify a temperature as freezing, cool, warm, or hot with ordered branches.
2. Determine whether a year is a leap year: divisible by 4, except centuries unless divisible by 400.
3. Convert total seconds to minutes and remaining seconds with divmod.
4. Fix if command == "start" or "run": and explain the bug.