String Builder Usage (list + join, avoid quadratic +=)

Building strings efficiently in Python means collecting parts in a list and ''.join once. Repeated s += chunk in a tight loop copies the whole string each time—quadratic cost.

Why this shows up in the real world

HTTP response serializers concatenate many small fragments. CSV writers buffer rows as lists of fields before joining.

Core idea (explained for students)

Pattern: parts: list[str] = []; ... parts.append(x); return ''.join(parts). For very large data, write to io.StringIO.

Try this in Python

def build_csv_row(fields: list[str]) -> str:
    return ",".join(fields)


print(build_csv_row(["a", "b", "c"]))

Common mistakes

• join on a generator that performs heavy side effects—materialize intentionally.
• Forgetting sep.join when you need commas between items.

Key takeaways

• Default mental model: list buffer + join for unknown iteration counts.
• ''.join(map(str, nums)) avoids per-element str concatenation loops.