Control Flow

Conditionals: if / elif / else

Python conditionals use indentation instead of braces. Any expression that evaluates to a truthy or falsy value can be used as the condition.

score = 85

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

print(grade)   # B

There is no switch statement in Python — use if/elif chains or, since Python 3.10, structural pattern matching (match/case).

for loops over iterables

Python’s for loop iterates over any iterable — a list, string, range, file, dictionary, or custom object that implements __iter__.

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

# Iterate over characters in a string
for ch in "hello":
    print(ch)

Use enumerate() when you need both the index and the value:

for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")

range()

range(stop), range(start, stop), and range(start, stop, step) generate integer sequences without building a list in memory.

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)

while loops

while repeats as long as its condition is truthy.

count = 0
while count < 3:
    print(count)
    count += 1

break, continue, and loop else

• break exits the innermost loop immediately.
• continue skips the rest of the current iteration and moves to the next.
• The else clause on a loop runs only if the loop completed without hitting a break.

for n in [1, 3, 5, 7]:
    if n % 2 == 0:
        print("found even")
        break
else:
    print("no even number found")   # this runs

The walrus operator :=

Introduced in Python 3.8, := assigns a value and evaluates to that value in the same expression. It is most useful in while and if conditions to avoid repeating an expression.

data = [4, 7, 2, 9, 1]
if (largest := max(data)) > 5:
    print(f"Largest value {largest} exceeds 5")

Full runnable demo

Run ▸ Open in an online REPL

score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
print(f"Score {score} -> Grade {grade}")

total = 0
for i in range(1, 6):
    total += i
print(f"Sum 1..5 = {total}")

numbers = [1, 3, 5, 7]
for n in numbers:
    if n % 2 == 0:
        print(f"Found even: {n}")
        break
else:
    print("No even numbers found")

count = 0
while True:
    count += 1
    if count % 2 == 0:
        continue
    if count > 7:
        break
    print(f"odd: {count}")

data = [4, 7, 2, 9, 1]
if (largest := max(data)) > 5:
    print(f"Largest value {largest} exceeds 5")

🐍 for...else runs when there is no break

The else clause on a for or while loop is one of Python’s most surprising features. It does not mean “if the loop ran zero times.” It means “the loop finished normally — no break was executed.” Use it to replace a flag variable: instead of found = False + checking it after the loop, write the success branch inside the loop and the failure branch in else.

When does the `else` clause of a `for` loop execute?

When the iterable is empty When the loop completes without hitting a break Always, after the loop body When the loop body raises an exception

What does `range(2, 10, 3)` produce?

[2, 5, 8] [2, 4, 6, 8, 10] [2, 3, 4, 5, 6, 7, 8, 9] [3, 6, 9]

What does the walrus operator `:=` do?

Compares two values for equality Assigns a value and evaluates to that value in one expression Creates a new variable in a different scope Converts a value to a boolean

Which statement immediately ends the current loop iteration and moves to the next one?

break return continue pass

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