Python If, Elif, and Else

A program becomes useful when it can react to different values. Conditionals run a block only when a test is true. Python uses indentation to show which lines belong to each branch. You will also replace deep nesting with guard clauses and test boundary values.

Comparisons produce booleans

Use == for equality, != for difference, and <, <=, >, >= for ordering. A comparison produces True or False. Do not confuse = (assignment) with == (comparison).

score = 85
print(score >= 80)
print(score == 100)
print(score != 0)

Comparison results

Choose one branch

if runs when its condition is true. elif is checked only when the previous conditions were false. else catches everything that remains. Order matters: put the narrowest or highest threshold first.

score = 85
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
else:
    print("C")

A grade ladder

Combine conditions

and is true only when both sides are true. or is true when at least one side is true. not flips a boolean. Parentheses make complex conditions easier to read.

age = 20
member = True
print(age >= 18 and member)
print(age < 18 or member)
print(not member)

Boolean logic

Guard clauses keep nesting shallow

When a function has several preconditions, check them first and return early. This is a guard clause. The main path stays flat, and each rejection is close to the condition that caused it.

def shipping(total, member=False):
    if total <= 0:
        return "invalid total"
    if total >= 50 or member:
        return "free shipping"
    return "shipping: 5.00"

for total in [0, 42, 50]:
    print(total, shipping(total))
print(50, shipping(50, member=True))

Early returns instead of nested branches

Boundaries and explicit truth tests

Boundaries deserve explicit tests. if total treats 0 as false, which may hide an invalid order. Prefer a comparison such as total > 0 when zero means missing data. Keep mutually exclusive cases in one if/elif/else chain.

for total in [0, 49.99, 50]:
    if total <= 0:
        print(total, "invalid")
    elif total < 50:
        print(total, "paid shipping")
    else:
        print(total, "free shipping")

Test the values around a threshold

Use guard clauses

Deep nesting makes the main path hard to see. A guard clause handles invalid input first and returns immediately, so the rest of the function stays flat. Use guards for preconditions, not for hiding complex branches. The reader should be able to scan from the top and see one normal path.

def shipping(weight):
    if weight <= 0:
        return "invalid"
    if weight < 1:
        return "small"
    return "standard"

for w in (0, 0.5, 3):
    print(w, shipping(w))

Invalid cases leave early

Try it yourself

Change the values, predict the output, then run the code. The Python runtime is downloaded the first time you press Run, so the first run can take a few seconds.