fix(python): Raise properly for slices not supported by LazyFrame (#…

…15331)

Co-authored-by: alexander-beedie <alexander.m.beedie@icloud.com>

py-polars/polars/slice.py

@@ -146,7 +146,8 @@ def apply(self, s: slice) -> LazyFrame:
         # [i:<=i]
         # [i:>=i:-k]
         if (step > 0 and (s.stop is not None and start >= s.stop)) or (
-            step < 0 and (s.stop is not None and s.stop >= s.start >= 0)
+            step < 0
+            and (s.start is not None and s.stop is not None and s.stop >= s.start >= 0)
         ):
             return self.obj.clear()
 
@@ -192,7 +193,7 @@ def apply(self, s: slice) -> LazyFrame:
         # ---------------------------------------
         # [-i:]    => tail(abs(i))
         # [-i::k]  => tail(abs(i)).gather_every(k)
-        elif start < 0 and s.stop is None:
+        elif start < 0 and s.stop is None and step > 0:
             obj = self.obj.tail(abs(start))
             return obj if (step == 1) else obj.gather_every(step)
 

py-polars/tests/parametric/test_lazyframe.py

@@ -1,7 +1,7 @@
 # ----------------------------------------------------
 # Validate LazyFrame behaviour with parametric tests
 # ----------------------------------------------------
-from hypothesis import given
+from hypothesis import example, given
 from hypothesis.strategies import integers
 
 import polars as pl
@@ -16,25 +16,35 @@
             column(
                 "start",
                 dtype=pl.Int8,
-                null_probability=0.15,
-                strategy=integers(min_value=-4, max_value=6),
+                null_probability=0.3,
+                strategy=integers(min_value=-3, max_value=4),
             ),
             column(
                 "stop",
                 dtype=pl.Int8,
-                null_probability=0.15,
-                strategy=integers(min_value=-2, max_value=8),
+                null_probability=0.3,
+                strategy=integers(min_value=-2, max_value=6),
             ),
             column(
                 "step",
                 dtype=pl.Int8,
-                null_probability=0.15,
+                null_probability=0.3,
                 strategy=integers(min_value=-3, max_value=3).filter(lambda x: x != 0),
             ),
             column("misc", dtype=pl.Int32),
         ],
     )
 )
+@example(
+    ldf=pl.LazyFrame(
+        {
+            "start": [-1, None, 1, None, 1, -1],
+            "stop": [None, 0, -1, -1, 2, 1],
+            "step": [-1, -1, 1, None, -1, 1],
+            "misc": [1, 2, 3, 4, 5, 6],
+        }
+    )
+)
 def test_lazyframe_slice(ldf: pl.LazyFrame) -> None:
     py_data = ldf.collect().rows()
 

py-polars/tests/unit/operations/test_slice.py

@@ -222,3 +222,21 @@ def test_slice_pushdown_literal_projection_14349() -> None:
     plan = q.explain()
     assert plan.index("WITH_COLUMNS") < plan.index("SLICE")
     assert q.collect().height == 0
+
+
+@pytest.mark.parametrize(
+    "input_slice",
+    [
+        (-1, None, -1),
+        (None, 0, -1),
+        (1, -1, 1),
+        (None, -1, None),
+        (1, 2, -1),
+        (-1, 1, 1),
+    ],
+)
+def test_slice_lazy_frame_raises_proper(input_slice: tuple[int | None]) -> None:
+    ldf = pl.LazyFrame({"a": [1, 2, 3]})
+    s = slice(*input_slice)
+    with pytest.raises(ValueError, match="not supported"):
+        ldf[s].collect()