if, elif, else

A program that always does the same thing is not much use. Branching lets it react to its data. The fundamentals lesson on making decisions covers the logic: chains of tests, boolean conditions and guard clauses. This lesson shows the Python spelling.

The shape of an if statement

An if statement has a condition, a colon, and an indented block that runs only when the condition is truthy. Add elif for further conditions and else for whatever remains.

temperature = 28

if temperature > 30:
    print("Hot")
elif temperature > 20:
    print("Warm")
else:
    print("Cool")

Warm

Three Python details to notice:

1. The colon and the indent. Each condition ends with :, and the block under it is indented. Four spaces is the standard. There are no braces, so the indentation is the structure. Mixing tabs and spaces in one block is an error.
2. No parentheses needed. Write if temperature > 30:, not if (temperature > 30):.
3. Any object can be the condition. Truthiness from the previous lesson applies.

name = ""

if name:
    print("Hello,", name)
else:
    print("No name given")

No name given

An empty string is falsy, so the else block runs.

How an elif chain runs

Python tests the conditions from top to bottom and runs the first block whose condition is truthy. Then it skips the rest of the chain. else has no condition and catches whatever remains. Both elif and else are optional, and you can have as many elif branches as you like.

Because only the first match runs, the order matters. With temperature = 28, the test > 20 is true, but the stricter > 30 is not, so “Warm” prints. Put the broader test first and the stricter one can never be reached:

temperature = 35

if temperature > 20:
    print("Warm")
elif temperature > 30:
    print("Hot")

Warm

At 35 degrees the program says “Warm”, because the first test already matched. Put the strictest test first.

Separate if statements are different. Each one is tested independently, so several can run:

number = 12

if number % 2 == 0:
    print("even")
if number % 3 == 0:
    print("multiple of three")

even
multiple of three

The conditional expression

When you only need to choose between two values, a one-line form is neater than four lines of if:

age = 20
label = "adult" if age >= 18 else "minor"
print(label)

count = 1
print(count, "item" if count == 1 else "items")

adult
1 item

It reads as “the value is adult if the condition holds, otherwise minor”. It is an expression, so it can sit inside an argument to print. Keep it for simple choices. Once you need a third case, switch back to if/elif.

Handle the unusual cases first

Nested if statements push the code further right with every level:

age = 15

if age >= 0:
    if age < 13:
        print("child")
    else:
        if age < 20:
            print("teenager")
        else:
            print("adult")
else:
    print("invalid")

teenager

A flat chain reads straight down the page. Check the invalid case first, and each later test only needs one bound:

age = 15

if age < 0:
    print("invalid")
elif age < 13:
    print("child")
elif age < 20:
    print("teenager")
else:
    print("adult")

teenager

Both programs behave the same. This “deal with the bad cases first” shape is called a guard clause in the fundamentals track. Inside a function you can finish a guard with an early return, which the functions module covers.

An empty block: pass

A block cannot be empty. When you need a placeholder for code you have not written yet, use pass, which does nothing.

score = 75

if score > 90:
    pass
else:
    print("Keep going")

Keep going

The pass marks the spot where a “well done” branch will go later. It keeps the program valid until you fill it in.

Common pitfalls

• Using = instead of ==. if x = 5: is a syntax error. Assignment uses =; comparison uses ==.
• Forgetting the colon. The error message points at the end of the line.
• Chains that cannot reach later branches. If you test score >= 60 before score >= 90, an A is reported as a D. Go from strictest to loosest.
• Comparing against several values the long way. if x == 1 or x == 2 or x == 3 works, but later lessons show x in (1, 2, 3), which is clearer.
• A comment where a statement is needed. A block with only a comment is still empty and is a syntax error. Use pass.

year = 2000

if year % 400 == 0:
    print(year, "is a leap year")
elif year % 100 == 0:
    print(year, "is not a leap year")
elif year % 4 == 0:
    print(year, "is a leap year")
else:
    print(year, "is not a leap year")

2000 is a leap year

score = 89.5

if score < 0 or score > 100:
    print("invalid")
elif score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")
else:
    print("F")

B

score = 95

if score >= 60:
    print("D")
elif score >= 70:
    print("C")
elif score >= 80:
    print("B")
elif score >= 90:
    print("A")

D

score = 95

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")

A

Summary

• if, elif and else end with a colon and use an indented block. The first truthy branch wins.
• a if condition else b chooses between two values in one expression.
• Check the unusual cases first to keep the main path flat, and use pass for a block you have not written yet.

Next: while loops.