elif, and, or

More than two branches

Real rules rarely come in pairs. Letter grades have five bands, shipping costs have weight tiers, and tax has brackets. Programs need a clean way to pick one outcome out of several, and nesting separate if/else statements inside each other gets unreadable fast, so Python provides a flat chain instead.

elif, short for "else if", chains extra conditions. Python checks them top to bottom and runs only the first one that is True:

score = 72
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
else:
    print("F")

With score = 72: is 72 ≥ 90? No. Is 72 ≥ 80? No. Is 72 ≥ 70? Yes, so print C and skip the rest.

Order matters. Put the strictest condition first, or an earlier looser one will steal the match. Reversing this chain to check >= 70 first would print C for every passing score, including a 95.

The chain picking one branch

score is 72, so the third condition is the first one that holds.

score = 72
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
else:
    print("F")

Output

C

Exactly one branch runs, the first whose condition is True. Walking other values through the same chain: 91 prints A, 80 prints B on the second check, and 12 falls through every condition to the else and prints F.

The 80 case is the one to watch, because >= 80 accepts it while >= 90 rejects it. Every band boundary in a chain like this belongs to the branch above it, which is exactly the kind of detail a grading system gets complaints about.

Combining conditions

Three words let you build compound conditions from the comparisons in lesson 4-1:

• and is True only when both sides are True
• or is True when at least one side is True
• not flips True to False and back

age = 25
has_ticket = True
if age >= 18 and has_ticket:
    print("Welcome in!")
else:
    print("Sorry, no entry.")

Note that has_ticket holds a boolean directly, so it can be used as a condition all by itself. There is no need for has_ticket == True, because the variable already is True or False, and writing the comparison just adds a step that answers a question you already have the answer to.

That is also a hint about naming. Boolean variables read best when their names sound like yes-or-no questions, which is why has_ticket and is_weekend are the conventional shapes.

Evaluating an or

With is_weekend = False and is_holiday = True, the expression (is_weekend or is_holiday) evaluates to True.

or needs only one side to be True, and is_holiday supplies it. The False on the left does not matter.

and is the strict one that requires both sides, and swapping the operator here would give False.

A reliable way to keep them straight is to read them as everyday English promises. "Free entry on a weekend or a holiday" lets you in on either, and "bring your ID and your ticket" requires both.

A three-band temperature chain

temperature is 5, which fails both conditions and lands in the else.

temperature = 5
if temperature >= 30:
    print("hot")
elif temperature >= 15:
    print("warm")
else:
    print("cold")

Output

cold

Reading the pieces

• Three branches: the if, one elif, and a final else with no condition at all. The else is the catch-all, so the chain always prints exactly one thing.
• The >= 30 check has to come first. If >= 15 came first, a temperature of 35 would satisfy it and wrongly print warm, because the chain stops at the first match.
• With temperature = 5 both conditions are False, so the else branch prints cold. That branch is what stops the program from silently printing nothing for cold days.