8 Control flow

Use if for the first condition, any number of elif branches for alternatives, and an optional else for the “none of the above” case. Only the first true branch runs; the rest are skipped.

if x < 0:
    print("negative")
elif x == 0:
    print("zero")
else:
    print("positive")

Both elif and else are optional. If a block is required but you have nothing to do yet, use the no-op placeholder pass.

if condition:
    pass  # TODO: implement later

match/case (Python 3.10+) selects a branch by matching the structure or type of a value, not just equality. You can match literals, types, multiple options with |, and use _ as a catch-all.

match x:
    case "A":
        print("letter A")
    case str():
        print("some other string")
    case 1 | 2 | 3:
        print("small int")
    case int():
        print("some other int")
    case _:
        print("no match")

Be careful: patterns are powerful but can be subtle; test edge cases thoroughly.

A while loop runs its body while the condition is True. The optional else block runs only if the loop ends normally (i.e., not via break).

while condition:
    ...  # body
else:
    ...  # runs if no break occurred

break exits the loop immediately (skipping the else), and continue skips to the next iteration.

while True:
    s = input("> ")
    if s == "B":
        break      # else won't run
    if not s:
        continue   # skip rest of body
# else would go here if present

Python’s for iterates over any iterable (lists, strings, range objects, generators, etc.), not over an index counter by default.

for ch in "spam":
    print(ch)

An optional else runs only if the loop wasn’t terminated by break.

range(stop) yields 0..stop-1. range(start, stop) yields start..stop-1. range(start, stop, step) increments by step (use a negative step to count down). The stop value is excluded.

list(range(5))        # [0, 1, 2, 3, 4]
list(range(3, 7))     # [3, 4, 5, 6]
list(range(0, 10, 2)) # [0, 2, 4, 6, 8]
list(range(5, 0, -1)) # [5, 4, 3, 2, 1]

range returns a lightweight range object that generates numbers on demand (it doesn’t build a full list).

Use enumerate() to get pairs of (index, item) and unpack them. Use zip() to pair items from multiple iterables (stops at the shortest).

for i, n in enumerate([10, 20, -5]):
    if n < 0:
        print("negative at", i)

for a, b in zip([1, 2, 3], ["a", "b"]):
    print(a, b)  # (1, "a"), (2, "b")

Tuple unpacking also works directly in for loops when iterating sequences of tuples.

Comprehensions are concise “one-line loops” that build new collections, optionally filtering items.

# list comprehension
squares = [x*x for x in nums if x > 0]

# set comprehension
unique_pos = {x for x in nums if x > 0}

# dict comprehension
square_map = {x: x*x for x in nums}

They’re ideal when transforming/filtering data into a new collection in a clear, compact way.

Generator expressions look like list comprehensions but use parentheses. They produce values on demand, saving memory for large data.

squares = (x*x for x in nums)  # generator
for v in squares:
    print(v)

Use them when you only need to iterate once and don’t need the whole list in memory.

Indentation delimits blocks; use consistent indentation within a block (PEP 8 recommends 4 spaces). Avoid mixing tabs and spaces (can cause IndentationError). You can place multiple simple statements on one line with semicolons, but it’s discouraged for readability.

Break long statements with a backslash or, preferably, inside (), [], or {} which allow implicit line continuation.

total = (a + b + c
         + d + e)

When splitting long strings with \, indentation becomes part of the string; instead, concatenate adjacent string literals.

In Boolean contexts, 0, 0.0, 0+0j, empty collections/strings, and None are False; most other values are True. Compare with <, <=, >, >=, ==, !=; use in/not in for membership and is/is not for identity.

if 0 < x < 10:  # chained comparison
    ...

and/or short-circuit and return one of their operands (not just True/False): and returns the first falsy value or the last value; or returns the first truthy value or the last value.

[] and 5    # []
[2] or [3] # [2]

# identity vs equality
a = [0]; b = [0]
a == b     # True (same contents)
a is b     # False (different objects)