# AGENTS.md - AI Agent Development Guide
This document helps AI agents understand the pytest-language-server codebase structure, architecture, and development practices.
## Project Overview
**pytest-language-server** is a Language Server Protocol (LSP) implementation for pytest fixtures, written in Rust. It provides IDE features like go-to-definition, find-references, and hover documentation for pytest fixtures.
- **Language**: Rust (Edition 2021, MSRV 1.83)
- **Lines of Code**: ~4,000 lines (2,501 in fixtures.rs, 1,574 in main.rs)
- **Architecture**: Async LSP server using tower-lsp
- **Key Features**: Fixture go-to-definition, find-references, hover docs, fixture overriding, undeclared fixture diagnostics
## Core Architecture
### Module Structure
```
src/
├── lib.rs # Library exports (3 lines)
├── main.rs # LSP server implementation (~1,574 lines)
└── fixtures.rs # Fixture analysis engine (~2,501 lines)
```
### Key Components
1. **FixtureDatabase** (`src/fixtures.rs`)
- Central data structure for storing fixture definitions and usages
- Uses `DashMap` for lock-free concurrent access
- Handles workspace scanning, file analysis, and fixture resolution
- Implements pytest's fixture priority/shadowing rules
2. **Backend** (`src/main.rs`)
- LSP server implementation using `tower-lsp`
- Handles LSP protocol requests (initialize, goto_definition, references, hover)
- Coordinates with FixtureDatabase for fixture information
- Manages text document lifecycle (did_open, did_change)
### Data Structures
```rust
// Core types from src/fixtures.rs:
pub struct FixtureDefinition {
pub name: String,
pub file_path: PathBuf,
pub line: usize,
pub docstring: Option<String>,
}
pub struct FixtureUsage {
pub name: String,
pub file_path: PathBuf,
pub line: usize,
pub start_char: usize, // Character position on line
pub end_char: usize, // Character position on line
}
pub struct UndeclaredFixture {
pub name: String,
pub file_path: PathBuf,
pub line: usize,
pub start_char: usize,
pub end_char: usize,
pub function_name: String, // Name of test/fixture where used
pub function_line: usize, // Line where function is defined
}
pub struct FixtureDatabase {
// Map: fixture name -> all definitions (multiple conftest.py files)
definitions: Arc<DashMap<String, Vec<FixtureDefinition>>>,
// Map: file path -> usages in that file
usages: Arc<DashMap<PathBuf, Vec<FixtureUsage>>>,
// Cache of analyzed file contents
file_cache: Arc<DashMap<PathBuf, String>>,
// Map: file path -> undeclared fixtures in function bodies
undeclared_fixtures: Arc<DashMap<PathBuf, Vec<UndeclaredFixture>>>,
}
```
## Pytest Fixture Resolution Rules
The LSP correctly implements pytest's fixture priority/shadowing rules:
1. **Same file**: Fixtures defined in the same file have highest priority
2. **Closest conftest.py**: Walk up directory tree looking for conftest.py
3. **Virtual environment**: Third-party plugin fixtures (pytest-mock, pytest-asyncio, etc.)
### Character-Position Awareness
A critical feature added in v0.4.0: when a fixture overrides another fixture with the same name, the LSP distinguishes between the function name and parameter:
```python
@pytest.fixture
def cli_runner(cli_runner): # Self-referencing fixture
return cli_runner
```
- Cursor at position 4 (function name) → refers to child fixture
- Cursor at position 16+ (parameter) → refers to parent fixture
This is handled by `start_char` and `end_char` in `FixtureUsage`.
## Key Methods & Logic
### src/fixtures.rs
**Core Methods:**
- `scan_workspace(&self, root_path: &Path)` - Walks directory tree, finds test files
- `analyze_file(&self, file_path: PathBuf, content: &str)` - Parses Python AST, extracts fixtures
- `find_fixture_definition(&self, file_path: &Path, fixture_name: &str, line: usize, char: usize)` - Resolves fixture based on priority rules
- `find_fixture_at_position(&self, file_path: &Path, line: usize, char: usize)` - Finds fixture name at cursor
- `find_all_references(&self, fixture_name: &str, def_file: &Path)` - Finds all usages of a fixture
- `get_char_position_from_offset(&self, file_path: &Path, line: usize, char_offset: usize)` - Converts byte offset to character position
- `get_undeclared_fixtures(&self, file_path: &Path)` - Gets all undeclared fixture usages in a file
- `scan_function_body_for_undeclared_fixtures()` - Detects fixtures used in function bodies without parameter declaration
**AST Parsing:**
- Uses `rustpython-parser` to parse Python files
- Looks for `@pytest.fixture` decorators
- Handles assignment-style fixtures (pytest-mock pattern: `mocker = pytest.fixture()(_mocker)`)
- Extracts function signatures, docstrings, and parameter dependencies
- Walks function body AST to find Name expressions that reference available fixtures
**Undeclared Fixture Detection:**
- Scans test and fixture function bodies for name references
- Checks if each name is an available fixture (respects hierarchy)
- Excludes declared parameters and built-in names (self, request)
- Tracks line/character position for diagnostics
- Only reports fixtures that are actually available in the current scope
### src/main.rs
**LSP Handlers:**
- `initialize()` - Scans workspace on startup
- `goto_definition()` - Calls `find_fixture_at_position()` then `find_fixture_definition()`
- `references()` - Finds all references, ensures current position is included (LSP spec compliance)
- `hover()` - Shows fixture signature and docstring in Markdown format
- `did_open()`, `did_change()` - Re-analyzes files when opened/modified, publishes diagnostics
- `code_action()` - Provides quick fixes to add missing fixture parameters
- `publish_diagnostics_for_file()` - Publishes warnings for undeclared fixtures
## Testing
### Test Structure
```
tests/
├── test_project/ # Fixture test files for integration tests
│ ├── conftest.py
│ ├── test_example.py
│ ├── test_parent_usage.py
│ └── subdir/
│ ├── conftest.py
│ ├── test_hierarchy.py
│ └── test_override.py
└── test_parser_api.rs # Integration tests
```
### Running Tests
```bash
cargo test # Run all tests (40 tests)
cargo test --lib # Run library tests (fixtures.rs: 28 tests)
cargo test --bin # Run binary tests (main.rs: 12 tests)
RUST_LOG=debug cargo test # Run with debug logging
```
### Test Coverage
- **40 total tests passing**
- 28 tests in `src/fixtures.rs`
- 12 tests in `src/main.rs`
Key test areas:
- Fixture definition extraction from various patterns
- Fixture usage detection in test functions and other fixtures
- Fixture priority/shadowing rules
- Character-position awareness for self-referencing fixtures
- LSP spec compliance (references always include current position)
- Multiline function signatures
- Third-party fixture detection
- Undeclared fixture detection (5 new tests)
- Hierarchy-aware undeclared fixture reporting
## Development Workflow
### Build & Run
```bash
# Development build
cargo build
# Release build (optimized)
cargo build --release
# Run with logging
RUST_LOG=debug cargo run
# Format code
cargo fmt
# Lint
cargo clippy
# Security audit
cargo audit
```
### Version Bumping
Use the provided script to bump versions across all files:
```bash
./bump-version.sh 0.5.0 # Updates Cargo.toml, pyproject.toml, zed-extension/
```
This updates:
- `Cargo.toml`
- `pyproject.toml`
- `zed-extension/Cargo.toml`
- `zed-extension/extension.toml`
### Pre-commit Hooks
The project uses pre-commit hooks defined in `.pre-commit-config.yaml`:
- `cargo fmt --check` - Format checking
- `cargo clippy` - Linting
- `cargo audit` - Security vulnerability scanning
Install with: `pre-commit install`
## Common Development Tasks
### Adding a New LSP Feature
1. Add the capability in `main.rs` `initialize()` method's `ServerCapabilities`
2. Implement the handler method (async trait impl)
3. Add necessary methods to `FixtureDatabase` in `fixtures.rs`
4. Write integration tests in `main.rs` (see existing tests for patterns)
5. Update README.md with feature documentation
### Modifying Fixture Resolution Logic
1. Edit `src/fixtures.rs` methods:
- `find_fixture_definition()` for go-to-definition
- `find_all_references()` for find-references
- `analyze_file()` if changing what fixtures are detected
2. Add test cases to `src/fixtures.rs` tests
3. Run `cargo test` to ensure all 35 tests pass
4. Consider edge cases: self-referencing fixtures, multiline signatures, conftest.py hierarchy
### Debugging LSP Issues
1. Set `RUST_LOG=debug` or `RUST_LOG=trace` environment variable
2. Check logs in stderr (LSP uses stdout for protocol communication)
3. Key log points:
- "goto_definition request" - shows incoming requests
- "Looking for fixture definition" - shows resolution logic
- "Found fixture at position" - shows what fixture was detected
- "Resolved fixture definition" - shows final resolution
4. Use editor's LSP client logs to see request/response JSON
### Testing in an Editor
1. Build release binary: `cargo build --release`
2. Binary location: `target/release/pytest-language-server`
3. Configure editor to use this binary
4. Test on `tests/test_project/` for quick iteration
## Dependencies
Core dependencies (from `Cargo.toml`):
- **tower-lsp** (0.20.0) - LSP framework
- **tokio** (1.48) - Async runtime
- **rustpython-parser** (0.4.0) - Python AST parsing
- **dashmap** (6.1) - Concurrent hash map
- **walkdir** (2.5) - Directory traversal
- **tracing** (0.1) - Logging framework
## File Naming Conventions
Python test discovery patterns:
- `conftest.py` - Fixture configuration files
- `test_*.py` - Test files (prefix pattern)
- `*_test.py` - Test files (suffix pattern)
## Known Edge Cases
1. **Self-referencing fixtures**: Fixtures that override a parent fixture with the same name
- Handled via character-position awareness (`start_char`, `end_char`)
2. **Multiline function signatures**: Function definitions spanning multiple lines
- Handled in `analyze_file()` by checking line bounds during AST traversal
3. **Assignment-style fixtures**: `mocker = pytest.fixture()(_mocker)` pattern
- Detected in `analyze_file()` via AST pattern matching
4. **Async fixtures**: `async def` fixtures
- Treated the same as regular fixtures
5. **Third-party fixtures**: pytest-mock, pytest-asyncio, pytest-django
- Scanned from virtual environment site-packages
## LSP Spec Compliance
Critical LSP specification requirements:
1. **References must include current position** (added in v0.4.0)
- When user invokes find-references, the cursor position MUST be in results
- Handled in `main.rs` references handler
2. **Character ranges must be accurate** (added in v0.4.0)
- Use actual fixture name character positions, not line start (0)
- Stored in `FixtureUsage.start_char` and `end_char`
3. **Hover uses Markdown format**
- Documentation formatted with proper code blocks
- Docstrings dedented and cleaned
## Performance Considerations
- **Concurrent workspace scanning**: Uses `DashMap` for lock-free parallel file processing
- **Incremental updates**: Re-analyzes only changed files on `did_change`
- **Efficient lookup**: HashMap-based fixture lookup by name
- **AST parsing**: Cached in `file_cache` for re-analysis
## Release Process
1. Make changes and commit
2. Run `./bump-version.sh X.Y.Z` to update version
3. Update CHANGELOG or release notes
4. Create GitHub release with `gh release create vX.Y.Z`
5. CI automatically:
- Builds binaries for all platforms
- Publishes to PyPI
- Publishes to crates.io
- Updates Homebrew formula
## Troubleshooting
### Tests failing after fixture logic changes
- Check that all 40 tests pass: `cargo test`
- Focus on failing tests in `fixtures.rs` (fixture resolution) or `main.rs` (LSP handlers)
- Common issue: fixture priority rules not respecting conftest.py hierarchy
### LSP not finding fixtures
- Check `RUST_LOG=debug` logs for "Scanning workspace" messages
- Verify workspace root is correct
- Check if files match test patterns (`test_*.py`, `conftest.py`)
- Verify Python AST parsing isn't failing (look for parse errors in logs)
### References not including current position
- Added in v0.4.0 to fix LSP spec violation
- Check that `start_char` and `end_char` are correctly set in `FixtureUsage`
- Verify `find_fixture_at_position()` is checking character bounds
## Additional Resources
- **README.md** - User-facing documentation and setup instructions
- **SECURITY.md** - Security policy and vulnerability reporting
- **RELEASE.md** - Release process documentation
- **tests/test_project/** - Example pytest project for testing
- **zed-extension/** - Zed editor extension source
## Contributing Guidelines
1. Run tests: `cargo test`
2. Run lints: `cargo clippy`
3. Format code: `cargo fmt`
4. Run security audit: `cargo audit`
5. Install pre-commit hooks: `pre-commit install`
6. Write tests for new features
7. Update AGENTS.md if adding significant architectural changes
## Version History
- **v0.5.0** (In Development) - Undeclared fixture diagnostics and code actions
- **v0.4.0** - Character-position aware references, LSP spec compliance
- **v0.3.1** - Previous stable release
- See GitHub releases for full changelog
---
**Last Updated**: v0.4.0 (November 2025)
This document should be updated when making significant architectural changes or adding new features.