docs: Python development docs (#35)

Author: kylebarron

python/DEVELOP.md

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+# Python Development Guide
+
+This guide covers Python-specific development for the `geodatafusion` Python package, which provides Python bindings for the Rust `geodatafusion` library using PyO3.
+
+This guide is **in addition** to the top-level [DEVELOP.md](../DEVELOP.md) that covers Rust development. You should read that first and all Rust-related instructions also apply here.
+
+## Overview
+
+The Python package is a separate workspace that wraps the Rust library using:
+
+- **PyO3** - Rust bindings for Python (automatically installed via Cargo)
+- **Maturin** - Build system for PyO3 packages (automatically installed in the dev environment by uv)
+- **uv** - Python package and virtual environment manager
+
+## Prerequisites
+
+**uv** is the recommended package manager ([install instructions](https://docs.astral.sh/uv/)).
+
+## Project Structure
+
+```
+python/
+├── Cargo.toml              # Rust package configuration
+├── pyproject.toml          # Python package configuration
+├── src/                    # Rust source (PyO3 bindings)
+│   ├── lib.rs             # Main module entry point
+│   ├── udf/               # UDF registration modules
+│   └── utils.rs           # Helper utilities
+├── python/                 # Pure Python source
+│   └── geodatafusion/
+│       └── __init__.py    # Python API
+├── tests/                  # Python tests
+│   └── udf/               # UDF tests
+└── examples/               # Example scripts
+```
+
+## Getting Started
+
+### Clone and Setup
+
+```bash
+# From the repository root
+cd python
+
+# Create virtual environment and install dependencies
+uv sync --no-install-package geodatafusion
+```
+
+The `--no-install-package geodatafusion` avoids building `geodatafusion` itself (in release mode) during setup. **Maturin** is automatically installed in the dev environment by uv.
+
+### Build the Package
+
+There are two ways to build the package:
+
+#### Development Build (Fast, Debug Mode)
+
+```bash
+# Build and install in development mode
+uv run --no-project maturin develop --uv
+```
+
+**Note**: Debug builds will show a performance warning at runtime. This is expected during development.
+
+#### Release Build (Optimized)
+
+```bash
+# Build optimized release version
+uv run --no-project maturin develop --uv --release
+
+# Or build wheel for distribution
+uv run --no-project maturin build --uv --release
+```
+
+## Development Workflow
+
+### Running Tests
+
+```bash
+# Run all tests
+uv run --no-project pytest
+
+# Run specific test file
+uv run --no-project pytest tests/test_register.py
+
+# Run with verbose output
+uv run --no-project pytest -v
+
+# Run with output capture disabled (see print statements)
+uv run --no-project pytest -s
+```
+
+## Adding a new UDF
+
+When a new UDF is added to the Rust library, you need to expose it in Python:
+
+### 0. Implement the UDF in Rust
+
+Follow the instructions in the top-level [DEVELOP.md](../DEVELOP.md) to implement the UDF in Rust first.
+
+### 1. Update Rust Bindings
+
+The UDF modules are in `src/udf/`. Each module corresponds to a category:
+
+- `src/udf/native/` - Native implementations
+- `src/udf/geo/` - Geo trait implementations
+- `src/udf/geohash/` - GeoHash functions
+
+Wrap the UDF, using one of our existing macros, if possible.
+
+- `impl_udf!`: for UDFs without any instantiation arguments
+- `impl_udf_coord_type_arg!`: for UDFs taking a `CoordType` argument upon instantiation
+
+### 2. Add to Python Module
+
+For example, in `src/udf/geo/mod.rs`, register the new function:
+
+```rs
+#[pymodule]
+pub(crate) fn geo(m: &Bound<PyModule>) -> PyResult<()> {
+    m.add_class::<NewUdf>()?;
+```
+
+### 3. Update Python API
+
+The Python API is in `python/geodatafusion/__init__.py`. Update `register_all()` or add specific registration functions if needed.
+
+This ensures that the UDF is easily injected onto a DataFusion `SessionContext`.
+
+### 4. Add Tests
+
+Create tests in `tests/udf/` following the existing structure:
+
+```python
+from datafusion import SessionContext
+from geodatafusion import register_all
+
+
+def test_my_new_function():
+    ctx = SessionContext()
+    register_all(ctx)
+
+    sql = "SELECT my_new_function(ST_GeomFromText('POINT(1 2)'));"
+    result = ctx.sql(sql)
+    assert result.to_arrow_table().columns[0][0].as_py() == expected_value
+```
+
+## Package Building
+
+Push a new git tag to GitHub, and the CI workflow will build and publish wheels automatically.