Earlier quoted context omitted.
Here the tricky part is to make tests that it work correctly. I did this upgrade a few times, and works for simple stuff like charm (e.g. removing requirements.txt and adding proper pyproject.toml). Even in Claude Code, it takes some prompting and CLAUDE.md so that it consistently runs uv, rather sometimes `uv run python`, other times `python3 -m` being surprised that some dependency is not available.
I am doing this now. What are your instructions in CLAUDE.md? thx
Usually are long and project-dependent, so I won't share here.
But just a copy-paste of a piece of a project of mine
## Core Principles (IMPORTANT)
- **NO FALLBACKS EVER** - Fail fast, fail hard. If something is missing, crash immediately
- **NO SILENT DEFAULTS** - Never use default values when files/data is missing
- **NO TRY/EXCEPT WRAPPING** - Let exceptions propagate for easier debugging
- **PROPER TYPES ONLY** - Use Literal types for enums, Pydantic models for structures. No tuples for structured data
- **NO JAVA-STYLE ABSTRACTIONS** - Don't create pointless constants like STATUS_OK = "OK". Just use literals or proper types
- **PROPER PARSING** - No regex for structured formats, use real parsers
- **CRASH EARLY** - Things should crash as soon and as hard as possible for easy debugging
- **NO DEFENSIVE PROGRAMMING** - Don't worry about "backwards compatibility" during refactoring - just redesign things to be better.
## Development
**IMPORTANT: Always use `uv run` for all Python commands. Never use plain `python` or `python3`.**
```bash
# Format code
uv run ruff format .
# Check linting
uv run ruff check .
# Type checking
uv run ty check
# Run tests
uv run pytest tests/
```