Control Flow

Conditional Statements

if / elif / else

score = 85

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

print(f"Grade: {grade}")

Python uses indentation (4 spaces by convention) to define blocks. There are no curly braces or end keywords.

Truthiness

Values that evaluate to False in a boolean context:

bool(0)       # False
bool("")      # False
bool([])      # False — empty list
bool(None)    # False

Everything else is True.

Ternary Expression

status = "adult" if age >= 18 else "minor"

Loops

for Loop

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

# Iterating over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# With index
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

range()

range(stop)          # 0 to stop-1
range(start, stop)   # start to stop-1
range(start, stop, step)  # with custom step

while Loop

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

Loop Control

# break — exits the loop immediately
for n in range(10):
    if n == 5:
        break
    print(n)       # 0, 1, 2, 3, 4

# continue — skips to next iteration
for n in range(5):
    if n == 2:
        continue
    print(n)       # 0, 1, 3, 4

# else on loops — runs only if no break occurred
for n in range(3):
    print(n)
else:
    print("Loop completed normally")

The match Statement (Python 3.10+)

def describe(value):
    match value:
        case 0:
            return "Zero"
        case 1 | 2:
            return "One or Two"
        case int(n) if n > 10:
            return "Large integer"
        case str(s):
            return f"String: {s}"
        case _:
            return "Something else"

---

Next: Chapter 4 — Functions