Control flow: decisions and loops

So far our programs run from top to bottom, once. Real programs need to choose — only charge VAT for some customers, only alert when disk usage is high — and to repeat — process every row of a file, retry a failed request. Python’s control-flow statements handle both, with very little syntax.

Decisions: if, elif and else

usage = 91  # percent

if usage >= 90:
    print("Critical: free up disk space now")
elif usage >= 75:
    print("Warning: disk filling up")
else:
    print("Disk usage normal")

Python checks each condition in order and runs the first block whose condition is true, then skips the rest. The indented lines are the block. Python uses indentation — conventionally four spaces — instead of braces, so consistent indentation is not style, it is syntax.

Drag the disk usage and watch which conditions Python actually checks. Then swap the order of the tests to see a bug that hides until the right input arrives:

Conditions are built from comparisons (==, !=, <, <=, >, >=, in) and combined with and, or and not:

if role == "admin" or (role == "editor" and not account_locked):
    allow_publish()

Truthiness

Any object can be used as a condition. Python treats these as false: False, None, 0, 0.0, empty strings, and empty collections — [], {}, set(), (). Almost everything else is true. That allows concise checks:

if not errors:
    print("All checks passed")

Test your intuition. For each value, guess whether if value: runs its block:

Loops: for and while

A for loop runs its block once for each item of an iterable — a list, a string, a file, a range of numbers:

for server in ["web-1", "web-2", "db-1"]:
    print("Checking", server)

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

for index, name in enumerate(["asha", "ravi"], start=1):
    print(index, name)       # 1 asha, 2 ravi

A while loop repeats as long as a condition stays true. Use it when you don’t know in advance how many iterations you need:

attempts = 0
while not connected() and attempts < 5:
    attempts += 1
    wait_and_retry()

Always make sure something inside a while loop eventually makes the condition false — otherwise the loop never ends.

Trace a loop, step by step

The code below counts how many scores pass a threshold. Step through it and watch the current item, the condition and the variables change. This is exactly how an experienced developer “runs” code in their head.

Four patterns that cover most loops

Counting

Start a counter at 0 and add 1 when a condition holds — as in the trace above.

Accumulating

Start a total (0, an empty list, an empty string) and add each item’s contribution.

Filtering

Build a new list containing only the items that pass a test.

Searching

Stop as soon as you find what you need, with break — no point checking the rest.

# Searching with break
for order in orders:
    if order["id"] == target_id:
        print("Found:", order)
        break
else:
    print("No such order")   # runs only if the loop did not break

The else on a loop is unusual but handy: it runs only when the loop finished without break, which makes “found / not found” logic tidy. continue, by contrast, skips the rest of the current iteration and moves to the next item.

Loops and conditions let you write longer programs — which soon become hard to read. The next lecture introduces functions: the main tool for organizing code into named, reusable pieces.