Control flow

Python expresses the three universal shapes of logic — sequence, selection, and iteration — with notably clean syntax. The rule that surprises newcomers: indentation is significant. The block belonging to a statement is defined by how far it's indented, not by braces or keywords.

Selection: if / elif / else

temperature = 22

if temperature > 30:
    print("hot")
elif temperature > 15:
    print("pleasant")
else:
    print("cold")

The colon opens a block; the indented lines below form its body. elif ("else if") lets you chain conditions; only the first matching branch runs. Mis-indent and you'll get an IndentationError, or — worse — code that runs but belongs to the wrong block.

Iteration: for

A for loop walks through the items of any collection:

for name in ["Ada", "Alan", "Grace"]:
    print(f"Hello, {name}")

To repeat a fixed number of times, loop over range:

for i in range(3):   # 0, 1, 2
    print(i)

Note range(3) yields 0, 1, 2 — it starts at 0 and stops before 3. That half-open convention is everywhere in Python; it's worth internalising early.

Iteration: while

A while loop repeats as long as its condition stays true, so something inside must eventually make it false:

count = 3
while count > 0:
    print(count)
    count = count - 1   # the line that guarantees termination

Changing the flow: break and continue

Two tools adjust loops mid-stream:

• break exits the loop entirely.
• continue skips to the next iteration.

for n in range(10):
    if n == 5:
        break        # stop at 5
    if n % 2 == 0:
        continue     # skip even numbers
    print(n)         # prints 1, 3

Where to go next

You can now make decisions and repeat work. Combined with variables and types, that's enough to express real logic — and the foundation for the intermediate track's comprehensions.