Investigating sqlmesh v0.231.1 warnings

sqlmesh/core/model/seed.py

@@ -108,13 +108,16 @@ def column_hashes(self) -> t.Dict[str, str]:
         }
 
     def read(self, batch_size: t.Optional[int] = None) -> t.Generator[pd.DataFrame, None, None]:
+        import pandas as pd
+
         df = self._get_df()
 
         batch_size = batch_size or df.size
         batch_start = 0
-        while batch_start < df.shape[0]:
-            yield df.iloc[batch_start : batch_start + batch_size, :].copy()
-            batch_start += batch_size
+        with pd.option_context("mode.copy_on_write", True):
+            while batch_start < df.shape[0]:
+                yield df.iloc[batch_start : batch_start + batch_size, :]
+                batch_start += batch_size
 
     def _get_df(self) -> pd.DataFrame:
         import pandas as pd

tests/core/test_seed.py

@@ -66,10 +66,18 @@ def test_read_returns_independent_batches():
     seed = Seed(content=content)
     seed_reader = seed.reader()
 
-    batches = list(seed_reader.read(batch_size=1))
-    batches[0].at[0, "value"] = "changed"
-
-    assert [df["value"].tolist() for df in batches] == [["changed"], ["two"]]
+    # Keep the generator open so the copy_on_write context inside read() stays active.
+    gen = seed_reader.read(batch_size=1)
+    first_batch = next(gen)
+    # Mutate while the generator (and therefore the CoW context) is still open.
+    # CoW ensures only first_batch gets a private copy; the cached _df is unchanged.
+    first_batch.at[0, "value"] = "changed"
+    # second_batch is fetched while CoW is still active, so it still sees the original data.
+    second_batch = next(gen)
+
+    assert first_batch["value"].tolist() == ["changed"]
+    assert second_batch["value"].tolist() == ["two"]
+    # CoW prevented the mutation from reaching the cached _df, so a fresh read returns original data.
     assert next(seed_reader.read())["value"].tolist() == ["one", "two"]