Control flow and functions

What you will be able to do

• Write if/elif/else and for loops over lists and dicts
• Define functions with arguments and return values
• Keep business logic in small testable helpers

Why this matters for data work

Pipelines are loops over files or rows, with branches for bad data, wrapped in functions you can reuse and test. Giant scripts without def become unmaintainable on day three.

Concepts

if / elif / else — branch on conditions.

for — iterate a sequence: for row in rows:.

while — rarer in data scripts; prefer for when the collection is known.

def — name a block of code. return sends a value back. Prefer pure helpers (same inputs → same outputs) for transforms.

f-strings — f"count={n}" for readable messages.

Practice on your machine

cd ~/dcubes/python-lab
source .venv/bin/activate
cat > functions_lab.py << 'EOF'
def line_total(quantity: int, unit_price: float) -> float:
    return quantity * unit_price


def normalize_status(raw: str) -> str:
    value = raw.strip().lower()
    if value in {"done", "complete", "completed"}:
        return "completed"
    if value in {"cancel", "cancelled", "canceled"}:
        return "cancelled"
    return value


orders = [
    {"sku": "SKU-1", "quantity": 2, "unit_price": 10.0, "status": "Done"},
    {"sku": "SKU-2", "quantity": 1, "unit_price": 5.5, "status": "CANCEL"},
]

for order in orders:
    total = line_total(order["quantity"], order["unit_price"])
    status = normalize_status(order["status"])
    print(f"{order['sku']} total={total} status={status}")
EOF
python functions_lab.py

SKU-1 total=20.0 status=completed
SKU-2 total=5.5 status=cancelled

What you should see

• Two printed lines with computed totals and normalized statuses.
• Type hints on def lines are optional at runtime — they document intent (more in a later lesson).

Common mistakes

• Mutating global lists inside functions without clear ownership.
• Deep nesting instead of early continue / helper calls.
• No return — function falls off the end and returns None by accident.

Next

Errors and logging