Batches never split a postcode 🟥

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/modelling/batching.py

@@ -0,0 +1,19 @@
+"""Pack a Modelling Run's resolved properties into SQS-message-sized batches.
+
+Batches stay postcode-grouped — properties sharing a postcode ride the same
+message so the workers' prediction cohort cache keeps paying (ADR-0055), the
+same packing the trigger script has been running.
+"""
+
+from backend.app.modelling.property_filters import FilteredProperty
+
+BATCH_SIZE = 50
+
+
+def pack_postcode_batches(
+    properties: list[FilteredProperty], batch_size: int = BATCH_SIZE
+) -> list[list[FilteredProperty]]:
+    """Pack *properties* into batches of ~*batch_size*, never splitting a
+    postcode across batches. A single postcode larger than *batch_size*
+    becomes its own oversized batch."""
+    raise NotImplementedError

tests/backend/app/modelling/test_batching.py

@@ -0,0 +1,30 @@
+from backend.app.modelling.batching import pack_postcode_batches
+from backend.app.modelling.property_filters import FilteredProperty
+
+
+def _properties(postcode: str, count: int, start_id: int) -> list[FilteredProperty]:
+    return [
+        FilteredProperty(property_id=start_id + i, postcode=postcode)
+        for i in range(count)
+    ]
+
+
+def test_postcodes_are_never_split_across_batches() -> None:
+    # arrange — 30 + 30 + 10 with a cap of 50: the second postcode won't fit
+    # alongside the first, the third rides with the second
+    properties = (
+        _properties("B93 8SU", 30, start_id=0)
+        + _properties("M20 4TF", 30, start_id=100)
+        + _properties("SW1A 1AA", 10, start_id=200)
+    )
+
+    # act
+    batches = pack_postcode_batches(properties, batch_size=50)
+
+    # assert
+    assert [len(b) for b in batches] == [30, 40]
+    for batch in batches:
+        for postcode in {p.postcode for p in batch}:
+            in_batch = [p for p in batch if p.postcode == postcode]
+            everywhere = [p for p in properties if p.postcode == postcode]
+            assert len(in_batch) == len(everywhere)  # whole postcode, one batch