Python Control Flow

Python's syntax for control flow is famously minimal — no curly braces, no semicolons. Indentation is the language.

if / elif / else

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

The condition doesn't need parentheses. Each branch is the indented block underneath. Conditions can be any expression — Python's truthiness rules decide.

for loops

for iterates over anything iterable — lists, dicts, strings, files, ranges, generators:

for x in [1, 2, 3]:        # list
    print(x)

for ch in "abc":           # string
    print(ch)

for k, v in user.items():  # dict (key + value via items())
    print(k, v)

for i in range(5):         # range — 0..4
    print(i)

for i, x in enumerate(xs): # index + value
    print(i, x)

Need to iterate two collections in parallel? zip:

for name, age in zip(names, ages):
    print(name, age)

while loops

For when you don't know the iteration count up front:

while not done:
    chunk = fetch()
    process(chunk)
    done = chunk.is_last

while True: plus break is a common pattern when you need a "loop until X happens" with exit condition mid-body.

break, continue, else (yes really)

for line in lines:
    if line.startswith("#"):
        continue            # skip
    if line == "END":
        break               # stop
    process(line)
else:
    print("loop completed without break")

Yes, for and while loops can have an else. It runs only when the loop finished without a break. Useful for search loops:

for x in candidates:
    if matches(x):
        result = x
        break
else:
    raise NotFound

match (3.10+)

Python's structural pattern matching:

match command.split():
    case ["quit"]:
        return None
    case ["go", direction]:
        return move(direction)
    case ["take", *items]:
        return take(items)
    case _:
        return error("unknown command")

Each case can match literal values, bind variables (direction), or destructure (*items, dict keys, class instances).

Comprehensions are conditions too

A list comprehension with if filters:

evens = [x for x in xs if x % 2 == 0]

Equivalent loop:

evens = []
for x in xs:
    if x % 2 == 0:
        evens.append(x)

The comprehension is denser, no faster — but it scans more like its English description: "the even x in xs".

Conditional expressions

Python has a ternary, written with English keywords:

status = "active" if user.logged_in else "anonymous"

Read it as: "value if condition else other-value". Convention: keep them short. If the branches need to be more than a few words, use a regular if statement.

Tip: don't compare booleans to True

# Don't:
if user.is_admin == True:
# Do:
if user.is_admin:

The first reads as "ask whether is_admin equals True"; the second reads as "ask whether is_admin". The latter matches the way you'd say it out loud.