O(n²) string concatenation
A CSV renderer that builds the output with `+=`. Looks fine; runs slow on a 100k-row job. What does a senior flag FIRST?
The snippet
# Render a list of rows as CSV. Slow at 100k rows.
def render_csv(rows):
out = ""
for row in rows:
out += ",".join(str(c) for c in row) + "\n"
return out