FastAPI Tutorial: Building Date-Range Reports and CSV Export with Streaming

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A developer building a personal expense tracker added date-range reporting and CSV export features to their FastAPI application in the project's sixth phase. The update introduced a new endpoint accepting start and end date query parameters, with FastAPI automatically validating date formats and returning a 422 error for malformed input, while business logic handles a 400 error when the start date falls after the end date. A second endpoint streams the filtered expenses as a downloadable CSV file using Python's StringIO and FastAPI's StreamingResponse, avoiding the need to build CSV strings manually. Shared filtering logic was refactored into a helper function so both endpoints stay consistent, and a React frontend button was wired up to trigger authenticated downloads. The author documented a bug where the CSV output contained only one row, caused by an indentation error, as a reference for future development.

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