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description: "Control Flow — Write a program that prints FizzBuzz for numbers 1–30: multiples of 3 print 'Fizz', multiples of 5 print 'Buzz', multiples of both print…"---

03 · Control Flow

if / elif / else

score = 82

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

print(grade)  # B

while loops

count = 0
while count < 5:
    print(count)
    count += 1

for loops

Python's for iterates over sequences, not index counters:

for fruit in ["apple", "banana", "cherry"]:
    print(fruit)

for i in range(5):        # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 10, 2): # 2, 4, 6, 8
    print(i)

break, continue, else

for n in range(2, 20):
    if n % 7 == 0:
        print(f"first multiple of 7: {n}")
        break
else:
    # runs only if the loop finished WITHOUT break
    print("no multiple of 7 found")

for n in range(10):
    if n % 2 != 0:
        continue  # skip odd numbers
    print(n)

Match statement (Python 3.10+)

Structural pattern matching for cleaner multi-branch logic:

def describe(value):
    match value:
        case 0:
            return "zero"
        case int() if value < 0:
            return "negative int"
        case [x, y]:
            return f"pair: {x}, {y}"
        case {"name": name}:
            return f"has a name: {name}"
        case _:
            return "something else"

print(describe(0))            # zero
print(describe([1, 2]))       # pair: 1, 2
print(describe({"name": "Ada"}))  # has a name: Ada

How It Actually Works

Control flow is a compile-time transformation into jumps in bytecode. Disassemble an if/else with dis.dis and you'll see it:

• if score >= 90: compiles to COMPARE_OP >= followed by POP_JUMP_IF_FALSE — evaluate the condition, and if the result is falsy, jump straight to the address where the next elif/else branch begins. There is no "if statement" at runtime, only conditional jumps between bytecode offsets.
• "Falsy" is decided by calling the object's __bool__ (or __len__ if __bool__ is absent). 0, 0.0, "", [], {}, None all return False; almost everything else returns True.
• A while loop is the same conditional jump plus an unconditional JUMP_BACKWARD to re-test the condition.
• A for loop is different: GET_ITER calls iter() on the sequence to get an iterator object, then FOR_ITER repeatedly calls __next__ on it, binding each result to the loop variable. When __next__ raises StopIteration internally, FOR_ITER catches it and jumps past the loop body. This is why for works identically over lists, files, generators, and any custom iterable.
• The loop else clause compiles as code placed after the normal loop exit but before the target of any break jump — so break skips it and natural completion falls into it.
• match (3.10+) compiles to a decision tree of type checks, length checks, attribute/key extraction, and equality tests using dedicated opcodes like MATCH_CLASS, MATCH_MAPPING, and MATCH_SEQUENCE — not a hash-based jump table like C's switch.

🔀 See this in another language

• Java — Control Flow
• Go — Control Flow
• Scala — Control Flow

Exercise

Write a program that prints FizzBuzz for numbers 1–30: multiples of 3 print "Fizz", multiples of 5 print "Buzz", multiples of both print "FizzBuzz", otherwise print the number.