141 lines
6.5 KiB
Markdown
141 lines
6.5 KiB
Markdown
# Development Guidelines
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## Branching Model
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<!-- TODO: drop this section once v2 ships and main becomes the stable line -->
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- `main` is currently the V2 rework. Breaking changes are expected here — when removing or
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replacing an API, delete it outright and document the change in
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`docs/migration.md`. Do not add `@deprecated` shims or backward-compat layers
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on `main`.
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- `v1.x` is the release branch for the current stable line. Backport PRs target
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this branch and use a `[v1.x]` title prefix.
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- `README.md` is frozen at v1 (a pre-commit hook rejects edits). Edit
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`README.v2.md` instead.
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## Package Management
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- ONLY use uv, NEVER pip
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- Installation: `uv add <package>`
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- Running tools: `uv run --frozen <tool>`. Always pass `--frozen` so uv doesn't
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rewrite `uv.lock` as a side effect.
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- Cross-version testing: `uv run --frozen --python 3.10 pytest ...` to run
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against a specific interpreter (CI covers 3.10–3.14).
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- Upgrading: `uv lock --upgrade-package <package>`
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- FORBIDDEN: `uv pip install`, `@latest` syntax
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- Don't raise dependency floors for CVEs alone. The `>=` constraint already
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lets users upgrade. Only raise a floor when the SDK needs functionality from
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the newer version, and don't add SDK code to work around a dependency's
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vulnerability. See Kludex/uvicorn#2643 and python-sdk #1552 for reasoning.
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## Code Quality
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- Type hints required for all code
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- Public APIs must have docstrings. When a public API raises exceptions a
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caller would reasonably catch, document them in a `Raises:` section. Don't
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list exceptions from argument validation or programmer error.
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- `src/mcp/__init__.py` defines the public API surface via `__all__`. Adding a
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symbol there is a deliberate API decision, not a convenience re-export.
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- IMPORTANT: All imports go at the top of the file — inline imports hide
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dependencies and obscure circular-import bugs. Only exception: when a
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top-level import genuinely can't work (lazy-loading optional deps, or
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tests that re-import a module).
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## Testing
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- Framework: `uv run --frozen pytest`
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- Async testing: use anyio, not asyncio
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- Do not use `Test` prefixed classes — write plain top-level `test_*` functions.
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Legacy files still contain `Test*` classes; do NOT follow that pattern for new
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tests even when adding to such a file.
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- IMPORTANT: Tests should be fast and deterministic. Prefer in-memory async execution;
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reach for threads only when necessary, and subprocesses only as a last resort.
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- For end-to-end behavior, an in-memory `Client(server)` is usually the
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cleanest approach (see `tests/client/test_client.py` for the canonical
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pattern). For narrower changes, testing the function directly is fine. Use
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judgment.
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- Test files mirror the source tree: `src/mcp/client/stdio.py` →
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`tests/client/test_stdio.py`. Add tests to the existing file for that module.
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- Avoid `anyio.sleep()` with a fixed duration to wait for async operations. Instead:
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- Use `anyio.Event` — set it in the callback/handler, `await event.wait()` in the test
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- For stream messages, use `await stream.receive()` instead of `sleep()` + `receive_nowait()`
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- Exception: `sleep()` is appropriate when testing time-based features (e.g., timeouts)
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- Wrap indefinite waits (`event.wait()`, `stream.receive()`) in `anyio.fail_after(5)` to prevent hangs
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- Pytest is configured with `filterwarnings = ["error"]`, so warnings fail
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tests. Don't silence warnings from your own code; fix the underlying cause.
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Scoped `ignore::` entries for upstream libraries are acceptable in
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`pyproject.toml` with a comment explaining why.
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### Coverage
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CI requires 100% (`fail_under = 100`, `branch = true`).
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- Full check: `./scripts/test` (~23s). Runs coverage + `strict-no-cover` on the
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default Python. Not identical to CI: CI runs 3.10–3.14 × {ubuntu, windows}
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× {locked, lowest-direct}, and some branch-coverage quirks only surface on
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specific matrix entries.
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- Targeted check while iterating (~4s, deterministic):
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```bash
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uv run --frozen coverage erase
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uv run --frozen coverage run -m pytest tests/path/test_foo.py
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uv run --frozen coverage combine
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uv run --frozen coverage report --include='src/mcp/path/foo.py' --fail-under=0
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# UV_FROZEN=1 propagates --frozen to the uv subprocess strict-no-cover spawns
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UV_FROZEN=1 uv run --frozen strict-no-cover
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```
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Partial runs can't hit 100% (coverage tracks `tests/` too), so `--fail-under=0`
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and `--include` scope the report. `strict-no-cover` has no false positives on
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partial runs — if your new test executes a line marked `# pragma: no cover`,
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even a single-file run catches it.
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Avoid adding new `# pragma: no cover`, `# type: ignore`, or `# noqa` comments.
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In tests, use `assert isinstance(x, T)` to narrow types instead of
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`# type: ignore`. In library code (`src/`), a `# pragma: no cover` needs very
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good reasoning — it usually means a test is missing. Audit before pushing:
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```bash
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git diff origin/main... | grep -E '^\+.*(pragma|type: ignore|noqa)'
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```
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What the existing pragmas mean:
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- `# pragma: no cover` — line is never executed. CI's `strict-no-cover` (skipped
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on Windows runners) fails if it IS executed. When your test starts covering
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such a line, remove the pragma.
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- `# pragma: lax no cover` — excluded from coverage but not checked by
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`strict-no-cover`. Use for lines covered on some platforms/versions but not
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others.
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- `# pragma: no branch` — excludes branch arcs only. coverage.py misreports the
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`->exit` arc for nested `async with` on Python 3.11+ (worse on 3.14/Windows).
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## Breaking Changes
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When making breaking changes, document them in `docs/migration.md`. Include:
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- What changed
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- Why it changed
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- How to migrate existing code
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Search for related sections in the migration guide and group related changes together
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rather than adding new standalone sections.
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## Formatting & Type Checking
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- Format: `uv run --frozen ruff format .`
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- Lint: `uv run --frozen ruff check . --fix`
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- Type check: `uv run --frozen pyright`
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- Pre-commit runs all of the above plus markdownlint, a `uv.lock` consistency
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check, and README checks — see `.pre-commit-config.yaml`
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## Exception Handling
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- **Always use `logger.exception()` instead of `logger.error()` when catching exceptions**
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- Don't include the exception in the message: `logger.exception("Failed")` not `logger.exception(f"Failed: {e}")`
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- **Catch specific exceptions** where possible:
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- File ops: `except (OSError, PermissionError):`
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- JSON: `except json.JSONDecodeError:`
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- Network: `except (ConnectionError, TimeoutError):`
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- **FORBIDDEN** `except Exception:` - unless in top-level handlers
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