16.1 Functions

Before you start

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

Core concepts

Define a function with def, pass values through parameters, and use return to give a result to the caller. A function without an explicit return value returns None. Local names belong to that call unless you deliberately use a broader scope.

Keep filtering and validation separate from file or network operations so they can be tested easily. Type hints describe intended types but do not enforce them at runtime. Default arguments are evaluated once when the function is defined; avoid a mutable default such as items=[].

Worked example

def unhealthy(records, status="critical"):
    return [row for row in records if row.get("status") == status]

rows = [{"name": "web-01", "status": "critical"}]
print(unhealthy(rows))

Mini lab

Write is_healthy(status) returning a boolean. Check healthy, critical, and an empty string. The function must return a value rather than print it.

def is_healthy(status):
    return status == "healthy"

assert is_healthy("healthy") is True
assert is_healthy("critical") is False
assert is_healthy("") is False

Knowledge check

Why use None instead of [] as the default for an optional list?

Common mistake

Do not mix printing and returning accidentally. Test the returned value directly.

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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