python improve dataframe slicing: closes #1082

py-polars/polars/eager/frame.py

@@ -932,6 +932,10 @@ def __getitem__(self, item: Any) -> Any:
 
         # df[:]
         if isinstance(item, slice):
+            # special case df[::-1]
+            if item.start is None and item.stop is None and item.step == -1:
+                return self.select(pl.col("*").reverse())
+
             if getattr(item, "end", False):
                 raise ValueError("A slice with steps larger than 1 is not supported.")
             if item.start is None:
@@ -942,8 +946,16 @@ def __getitem__(self, item: Any) -> Any:
                 stop = self.height
             else:
                 stop = item.stop
+
             length = stop - start
-            return self.slice(start, length)
+            if item.step is None:
+                # df[start:stop]
+                return self.slice(start, length)
+            else:
+                # df[start:stop:step]
+                return self.select(
+                    pl.col("*").slice(start, length).take_every(item.step)
+                )
 
         # select multiple columns
         # df["foo", "bar"]

py-polars/tests/test_df.py

@@ -248,6 +248,13 @@ def test_selection():
     assert df[2, "c"] == "c"
     assert df[0, "a"] == 1
 
+    # more slicing
+    expect = pl.DataFrame({"a": [3, 2, 1], "b": [3.0, 2.0, 1.0], "c": ["c", "b", "a"]})
+    assert df[::-1].frame_equal(expect)
+
+    expect = pl.DataFrame({"a": [1, 3], "b": [1.0, 3.0], "c": ["a", "c"]})
+    assert df[::2].frame_equal(expect)
+
 
 def test_from_arrow():
     tbl = pa.table(