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Python Test Infrastructure

docs/development/python-tests.md

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Python Test Infrastructure

There are two separate test systems for Python:

1. Python-native tests (pytest)

Located in api/python/slint/tests/. These test the slint Python API directly.

sh
cd api/python/slint && uv run pytest -s -v

This automatically builds the slint-python shared library via maturin if needed.

2. Rust test driver (test-driver-python)

The Rust test driver (tests/driver/python/python.rs) processes .slint test cases from tests/cases/:

  1. Compiles the .slint file using OutputFormat::Python (via compile_syntax_node + generator::generate). This goes through LLR lowering and generates a .py file.

  2. Runs the generated .py file as a subprocess using uv run. The subprocess loads the slint Python module, which re-compiles the .slint source using slint-interpreter (with full inlining enabled).

Run with:

sh
cargo test -p test-driver-python
# or with a filter:
cargo test -p test-driver-python -- test_name

Rebuilding slint-python

The slint-python shared library used by the Python subprocess is built by uv sync in api/python/slint/, not by cargo build. It's installed into a Python venv managed by uv.

To force a rebuild after code changes:

sh
cd api/python/slint && uv sync --reinstall-package slint && cd -

cargo build -p slint-python builds a separate artifact that is NOT used by the Python tests. The test driver's LazyLock<PYTHON_PATH> calls uv sync once per test run, but this may not detect source changes.

Debugging

Issues can occur in two places:

  • Test driver process (compilation): the test driver compiles the .slint source with OutputFormat::Python and generates the .py file. To debug, modify compiler code and rebuild with cargo build -p test-driver-python.

  • Python subprocess (runtime): the slint Python module re-compiles the .slint source via slint-interpreter and executes it. To debug, modify compiler/runtime code AND rebuild slint-python (cd api/python/slint && uv sync --reinstall-package slint).

The subprocess's STDERR/STDOUT is captured and printed in the test output under STDERR: / STDOUT: headers.