refi-calculator 0.2.0

Refi Calculator Package.

Refi-Calculator

Refi-Calculator helps borrowers analyze whether refinancing makes sense by comparing current and proposed loan terms through breakeven, NPV, amortization, and holding-period views inside a Tkinter GUI.

Features

• Side-by-side analysis of current vs. proposed mortgage payments (nominal and after-tax)
• Breakeven and holding-period tables with clear recommendations
• Accelerated payoff planning for maintaining current payment levels
• Visual amortization comparisons plus cumulative savings (nominal/NPV) chart
• Exportable CSV data for every tab
• Live market tab (when FRED_API_KEY is configured) surfaces historical 30-year fixed rates and pre-fills the refinance rate so you start with a realistic benchmark.
• Interactive history chart in the Market tab plus the value table below it with 30Y/15Y toggles for quick comparisons.

Getting Started

Requirements

• Python 3.13+
• Poetry for dependency management (the virtual environment lives in .venv/)

Installation

poetry install

This command creates the virtual environment, installs dependencies, and prepares the hooks defined in .pre-commit-config.yaml.

Running the GUI

poetry run python bin/refi-calculator.py

The script boots the Tkinter application (src/refi_calculator/ui/app.py) that ties together all the analysis helpers.

Testing & Quality

• poetry run pre-commit run --all-files (runs formatting, linting, safety hooks, etc.)
• poetry run pytest (not yet populated, but ready for future tests)

Market Data Tab

• Set the FRED_API_KEY environment variable to pull historical 30-year fixed-rate data when you launch the app. The Market tab will populate a refreshable table and update the default refinance rate using the latest observation. If the key is missing or the FRED request fails, the tab keeps working but reports the reason for the skip.
• Use the selectors inside the Market section to start with one year of history or extend the view to two years, five years, or all available data. The chart overlays both tenors with a legend, and the table below shows one column per tenor for the selected time window.

Add new tests under tests/ following the test_*.py pattern whenever you enhance functionality.

Code Structure

• src/refi_calculator/models.py: Dataclasses for LoanParams and RefinanceAnalysis.
• src/refi_calculator/calculations.py: All refinance math helpers (NPV, amortization, sensitivity, etc.).
• src/refi_calculator/ui/: Tkinter GUI composed of:
  • app.py: The main application wiring.
  • chart.py: Custom savings chart canvas.
  • builders/: Tab-specific builders and helpers for clean separation of UI concerns.
• bin/refi-calculator.py: Entry point that runs the GUI.

Contributing

• Keep imports grouped by functionality (allow isort to do the heavy lifting through pre-commit).
• Update or add tests in tests/ before opening a pull request.
• Run poetry run pre-commit run --all-files to catch formatting/lint issues early.

License

See LICENSE.txt for licensing details.