Python If-Else, Loops and Comprehensions

Part 5 of the Python for Everyone track. Last updated: September 2026.

Control flow in Python reads almost like English — but the details (the for-else clause, comprehensions, enumerate/zip) are what separate beginners from fluent writers.

If / elif / else

score = 85
if score >= 90:
    grade = "A"
elif score >= 75:
    grade = "B"
else:
    grade = "C"

# One-line ternary
status = "pass" if score >= 40 else "fail"

Indentation is the block structure — 4 spaces per level, no braces, no exceptions.

For loops

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

# Need the index too? Use enumerate — never range(len(...))
for i, fruit in enumerate(["apple", "banana"]):
    print(i, fruit)   # 0 apple / 1 banana

# Loop two lists together
names = ["Ada", "Grace"]
ages = [36, 85]
for name, age in zip(names, ages):
    print(f"{name} is {age}")

# range() for counted loops
for i in range(3):       # 0, 1, 2
    print(i)
for i in range(2, 10, 3):  # start, stop, step → 2, 5, 8
    print(i)

While loops

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

break, continue, and the for-else surprise

for n in range(10):
    if n == 3:
        continue   # skip this iteration
    if n == 7:
        break      # exit the loop entirely
    print(n)

# else runs only if the loop finished WITHOUT break
for n in [2, 4, 6]:
    if n % 2 == 1:
        print("found odd")
        break
else:
    print("all even")   # ← this prints

The for-else pattern is the idiomatic "search with not-found handling" — common in interviews.

Comprehensions: build collections in one line

squares = [x**2 for x in range(6)]            # [0, 1, 4, 9, 16, 25]
evens = [x for x in range(10) if x % 2 == 0]  # [0, 2, 4, 6, 8]

# Dict and set comprehensions exist too
square_map = {x: x**2 for x in range(4)}      # {0: 0, 1: 1, 2: 4, 3: 9}
unique_lens = {len(w) for w in ["hi", "hello", "hey"]}  # {2, 5, 3}

Comprehensions are faster than equivalent append loops and are expected style in Python code reviews.

Key takeaways

• Indentation defines blocks — 4 spaces, consistently.
• enumerate() for index+value; zip() for parallel loops; never range(len(x)).
• for-else: else runs when no break occurred.
• Comprehensions replace most simple build-a-list loops.

Next in this series: Python Functions: Arguments, *args, **kwargs and Lambdas.