fix: check current session's pending-write queue when recalling snaps…

…hots (e.g. diffing)

src/syrupy/assertion.py

@@ -377,11 +377,7 @@ def _recall_data(
     ) -> Tuple[Optional["SerializableData"], bool]:
         try:
             return (
-                self.extension.read_snapshot(
-                    test_location=self.test_location,
-                    index=index,
-                    session_id=str(id(self.session)),
-                ),
+                self.session.recall_snapshot(self.extension, self.test_location, index),
                 False,
             )
         except SnapshotDoesNotExist:

src/syrupy/session.py

@@ -67,17 +67,25 @@ class SnapshotSession:
         List[Tuple["SerializedData", "PyTestLocation", "SnapshotIndex"]],
     ] = field(default_factory=dict)
 
-    def queue_snapshot_write(
+    def _snapshot_write_queue_key(
         self,
         extension: "AbstractSyrupyExtension",
         test_location: "PyTestLocation",
-        data: "SerializedData",
         index: "SnapshotIndex",
-    ) -> None:
+    ) -> Tuple[Type["AbstractSyrupyExtension"], str]:
         snapshot_location = extension.get_location(
             test_location=test_location, index=index
         )
-        key = (extension.__class__, snapshot_location)
+        return (extension.__class__, snapshot_location)
+
+    def queue_snapshot_write(
+        self,
+        extension: "AbstractSyrupyExtension",
+        test_location: "PyTestLocation",
+        data: "SerializedData",
+        index: "SnapshotIndex",
+    ) -> None:
+        key = self._snapshot_write_queue_key(extension, test_location, index)
         queue = self._queued_snapshot_writes.get(key, [])
         queue.append((data, test_location, index))
         self._queued_snapshot_writes[key] = queue
@@ -93,6 +101,27 @@ def flush_snapshot_write_queue(self) -> None:
                 )
         self._queued_snapshot_writes = {}
 
+    def recall_snapshot(
+        self,
+        extension: "AbstractSyrupyExtension",
+        test_location: "PyTestLocation",
+        index: "SnapshotIndex",
+    ) -> Optional["SerializedData"]:
+        """Find the current value of the snapshot, for this session, either a pending write or the actual snapshot."""
+
+        key = self._snapshot_write_queue_key(extension, test_location, index)
+        queue = self._queued_snapshot_writes.get(key)
+        if queue:
+            # find the last (i.e. most recent) write to this index in the queue:
+            for queue_data, queue_test_location, queue_index in reversed(queue):
+                if queue_index == index and queue_test_location == test_location:
+                    return queue_data
+
+        # no queue, or no matching write, so just read the snapshot directly:
+        return extension.read_snapshot(
+            test_location=test_location, index=index, session_id=str(id(self))
+        )
+
     @property
     def update_snapshots(self) -> bool:
         return bool(self.pytest_session.config.option.update_snapshots)

tests/integration/test_snapshot_diff.py

@@ -0,0 +1,56 @@
+import pytest
+
+_TEST = """
+def test_foo(snapshot):
+    assert {**base} == snapshot(name="a")
+    assert {**base, **extra} == snapshot(name="b", diff="a")
+"""
+
+
+def _make_file(testdir, base, extra):
+    testdir.makepyfile(
+        test_file="\n\n".join([f"base = {base!r}", f"extra = {extra!r}", _TEST])
+    )
+
+
+def _run_test(testdir, base, extra, expected_update_lines):
+    _make_file(testdir, base=base, extra=extra)
+
+    # Run with --snapshot-update, to generate/update snapshots:
+    result = testdir.runpytest(
+        "-v",
+        "--snapshot-update",
+    )
+    result.stdout.re_match_lines((expected_update_lines,))
+    assert result.ret == 0
+
+    # Run without --snapshot-update, to validate the snapshots are actually up-to-date
+    result = testdir.runpytest("-v")
+    result.stdout.re_match_lines((r"2 snapshots passed\.",))
+    assert result.ret == 0
+
+
+def test_diff_lifecycle(testdir) -> pytest.Testdir:
+    # first: create both snapshots completely from scratch
+    _run_test(
+        testdir,
+        base={"A": 1},
+        extra={"X": 10},
+        expected_update_lines=r"2 snapshots generated\.",
+    )
+
+    # second: edit the base data, to change the data for both snapshots (only changes the serialized output for the base snapshot `a`).
+    _run_test(
+        testdir,
+        base={"A": 1, "B": 2},
+        extra={"X": 10},
+        expected_update_lines=r"1 snapshot passed. 1 snapshot updated\.",
+    )
+
+    # third: edit just the extra data (only changes the serialized output for the diff snapshot `b`)
+    _run_test(
+        testdir,
+        base={"A": 1, "B": 2},
+        extra={"X": 10, "Y": 20},
+        expected_update_lines=r"1 snapshot passed. 1 snapshot updated\.",
+    )