Raise better error for repartition on divisions with unknown divisions (#980)

Author: phofl

dask_expr/_repartition.py

@@ -116,6 +116,10 @@ def _lower(self):
         elif self.new_divisions:
             if tuple(self.new_divisions) == self.frame.divisions:
                 return self.frame
+            elif self.frame.divisions[0] is None:
+                raise ValueError(
+                    "Cannot repartition on divisions with unknown divisions"
+                )
             return RepartitionDivisions(self.frame, self.new_divisions, self.force)
         elif self.partition_size is not None:
             return RepartitionSize(self.frame, partition_size=self.partition_size)

dask_expr/tests/test_repartition.py

@@ -131,3 +131,12 @@ def test_repartition_filter_pushdown():
     expected = df[["x", "y"]]
     expected = expected[expected.x > 5.0].repartition(npartitions=5)
     assert result.simplify()._name == expected.simplify()._name
+
+
+def test_repartition_unknown_divisions():
+    pdf = pd.DataFrame({"x": [1, 2, 3, 4, 5, 6, 7, 8] * 10, "y": 1, "z": 2})
+    df = from_pandas(pdf, npartitions=5).clear_divisions()
+    with pytest.raises(
+        ValueError, match="Cannot repartition on divisions with unknown divisions"
+    ):
+        df.repartition(divisions=(0, 100)).compute()