Follow-up to #8540, which turned on `PLC0415` (import-outside-top-level)
for checkpoint-postgres and checkpoint-sqlite. This does the remaining
six packages: checkpoint, checkpoint-conformance, langgraph, prebuilt,
cli, sdk-py.
Scoped to tests, per @sydney-runkle's call on #8540: library code is
exempted with `per-file-ignores`, since it still has deferred imports
nobody has reviewed and mixing that in would make this hard to read.
## What changed
Function-level imports across 56 test files moved to module level. Nine
could not move and carry an explicit `# noqa: PLC0415` with a reason:
| File | Why it stays local |
|---|---|
| `libs/langgraph/tests/test_deprecation.py` (4) | the import has to run
inside `pytest.warns` for the warning to be observed |
| `libs/langgraph/tests/test_serde_allowlist.py` | try/except guard,
skips when langchain_core is absent |
| `libs/langgraph/tests/test_delta_channel_benchmark.py` | optional
psycopg probe |
| `libs/checkpoint/tests/test_conformance_delta.py` (3) | protected by a
module-level `pytest.importorskip`; hoisting past the guard turns a skip
into a collection error |
That last one is the trap: an import moved above `pytest.importorskip`
silently defeats the guard. I hit it locally and it turned the skip into
a `ModuleNotFoundError` at collection. Every file with an `importorskip`
or `except ImportError` was checked by hand for this.
## Verification
`make lint` and `make test` in each of the six:
| Package | Tests |
|---|---|
| checkpoint | 156 passed, 17 skipped |
| checkpoint-conformance | 1 passed |
| langgraph | 1968 passed, 4 skipped |
| prebuilt | 284 passed |
| cli | 336 passed |
| sdk-py | 493 passed |
Also confirmed the rule actually fires: a throwaway test file with a
function-level import is flagged in all six packages, and the source
exemption holds.
## Summary
Refactors the four (now five) sdk-py streaming projections into
reusable, transport-agnostic `Decoder` classes and adds a new
`interleave_projections(channels)` method to `AsyncThreadStream` and
`SyncThreadStream` that drives multiple decoders from one shared
subscription, yielding `(channel_name, item)` tuples in arrival order
(the SDK analog of local `GraphRunStream.interleave`).
- **New `langgraph_sdk/stream/decoders.py`**: pure `feed(event) ->
Iterable[item]` state machines — `ValuesDecoder`, `MessagesDecoder`,
`ToolCallsDecoder`, `SubgraphsDecoder`, `ExtensionsDecoder` — behind a
`Decoder` Protocol. No subscription/queue/thread access.
- **Projection migration (async + sync)**: the five existing projections
now delegate their per-event logic to the decoders. Behavior-preserving
— the existing test suite is the regression net. Thread-coupled side
effects (active-stream registration, terminal-error-on-close, root-inbox
forwarding) stay in the projection wrappers; sync messages/tool_calls
keep their pre-dispatch contract (handle/stream resolved on yield) via a
FIFO-head buffer.
- **`interleave_projections`**: flat-namespace channel list (built-ins +
extension names), `tool_calls`↔`tools` wire mapping, subgraphs fed every
event, extensions keyed by bare name.
### Notable
- Fixes a latent sync bug surfaced by the refactor: two tool calls /
messages whose events interleave previously dropped the second; both now
surface. Locked in with a regression test.
- `Decoder.feed` takes `Mapping[str, Any]` (read-only), so the Protocol
is load-bearing in both stream files.
### Deferred (not in this PR)
- Wiring `RemoteGraph._RemoteGraphRunStream.interleave` to
`interleave_projections` (gated on #7927).
- Migrating the handle-scoped projections (`_Handle*Projection`) to the
decoders — hence the small, verified-identical helper duplication
between `decoders.py` and `_async/stream.py`.
- `interleave_projections` handles aren't registered for thread-close
cleanup (additive follow-up).
## Test Plan
- [x] `make test` in `libs/sdk-py` — 464 passed, 0 failures
- [x] `make format` / `make lint` (ruff + ty) clean
- [x] Per-decoder unit tests in `tests/streaming/test_decoders.py`
- [x] `interleave_projections` tests (single-channel, multi-channel
arrival order, builtin+extension mix, tool_calls public-name, subgraphs
discovery) async + sync
- [x] Existing projection suites pass unchanged (regression net for the
migration)
