Start with an input contract
Define accepted file types, required columns, date formats, units and what counts as a valid row. Without an input contract, every export becomes a new hidden assumption.
Fail visibly, not silently
A workflow that quietly replaces bad values with zero may finish successfully and still produce a dangerous report. Validation should stop critical failures and isolate non-critical exceptions for review.
Make the output answer a business question
A cleaned workbook is not automatically a useful report. The output should make the recurring decision easier: monthly change, category performance, missing records or another defined operational question.
Test the ugly inputs
The normal sample proves very little. Tests should include missing columns, empty files, invalid dates, duplicates, negative quantities and output-path failures.
A small, reliable reporting workflow is more valuable than a large script whose numbers cannot be explained.