Exercise 02 — Find the Bugs

This module (codebase.py) is a data processing pipeline that reads sales records from a CSV, filters them by date, summarizes revenue per category, and writes results to a new file.

It looks reasonable. It has docstrings. It runs without crashing on simple inputs.

It has 4 bugs. Find and fix all of them.

Rules

1. Read the code carefully before running anything.
2. For each bug, write down:
3. Which function contains the bug
4. What the bug is
5. What the correct behavior should be
6. Your fix
7. After fixing all 4, run the tests in tests/test_codebase.py to verify.

Hints (read one at a time, only if stuck)

Hint 1: Think about resources

One function opens something but never closes it. What happens if this function is called many times?

Hint 2: Think about indexing

The paginate function claims to be 1-indexed. Is it really?

Hint 3: Think about whitespace

CSV data from the real world is messy. What if there are trailing spaces in a column value?

Hint 4: Think about boundaries

The docstring for filter_by_date says the range is [start, end). Read the code. Does the implementation match that contract?

What you are practicing

• Reading code critically (not trusting that it works just because it looks clean)
• Identifying common bug patterns: resource leaks, off-by-one errors, missing input sanitization, contract violations
• Writing fixes and verifying them with tests