score = 85

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

print(f"Grade: {grade}")   # Grade: B

• Conditions are evaluated top-to-bottom.
• Only the first matching branch runs.
• else is optional; catches everything remaining.
• Any expression that is truthy/falsy works as a condition.

Nested (avoid when possible)

if x > 0:
    if x < 10:
        print("single digit")

Better: logical operators

if 0 < x < 10:       # chained comparison
    print("single digit")

if x > 0 and x < 10: # equivalent
    print("single digit")

Common patterns

# Guard clause — exit early
def process(data):
    if not data:
        return []
    # ... rest of function

# Membership test
if color in {"red", "green", "blue"}:
    print("primary")

# Iterate over a sequence
for fruit in ["apple", "banana", "cherry"]:
    print(fruit)

# Iterate with index
for i, fruit in enumerate(["apple", "banana"]):
    print(i, fruit)

# Range
for n in range(1, 6):
    print(n)   # 1 2 3 4 5

# Iterate over dict
scores = {"alice": 92, "bob": 85}
for name, score in scores.items():
    print(f"{name}: {score}")

# List comprehension (Pythonic)
squares = [x**2 for x in range(10)]
evens   = [x for x in range(20) if x % 2 == 0]

# while loop
n = 1
while n < 100:
    n *= 2
print(n)   # 128

# break — exit the loop early
for i in range(10):
    if i == 5:
        break
    print(i)   # 0 1 2 3 4

# continue — skip to next iteration
for i in range(10):
    if i % 2 == 0:
        continue
    print(i)   # 1 3 5 7 9

Accumulator pattern

# Sum (accumulate a number)
total = 0
for score in scores:
    total += score

# Collect (accumulate a list)
passing = []
for score in scores:
    if score >= 60:
        passing.append(score)

# Count (accumulate a tally)
count = 0
for score in scores:
    if score >= 90:
        count += 1