15.1 Loops and Comprehensions

Before you start

Work through the earlier fundamentals chapters. Use a Python 3 interpreter and run each example locally.

Core concepts

A for loop visits an iterable’s items. range(n) yields integers from zero up to, but excluding, n. Use enumerate() when you need both an index and a value. A while loop repeats while a condition holds; make sure it can terminate.

break stops the loop and continue skips to the next iteration. A comprehension creates a new collection; prefer a normal loop when the body needs multiple steps or error handling.

Worked example

servers = ["web-01", "db-01", "web-02"]
web_servers = [name for name in servers if name.startswith("web-")]
for number, name in enumerate(web_servers, start=1):
    print(number, name)

Mini lab

From ports [22, 80, 443, 8080], collect only ports greater than 100. Expected: [443, 8080]. Repeat with an empty list.

ports = [22, 80, 443, 8080]
selected = [port for port in ports if port > 100]
assert selected == [443, 8080]
assert [port for port in [] if port > 100] == []
print(selected)

Knowledge check

Does range(3) include the number 3?

Common mistake

Avoid removing items from a list while iterating over it. Build a filtered collection instead.

Completion checklist

• Run the example and explain its output.
• Complete the mini lab without copying the solution.
• Test an edge case and explain how the code handles it.
• Answer the knowledge check in your own words.

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