test: Add various tests (#20768)

Author: lukemanley

py-polars/tests/unit/dataframe/test_serde.py

@@ -202,3 +202,10 @@ def test_df_serialize_invalid_type() -> None:
         ComputeError, match="serializing data of type Object is not supported"
     ):
         df.serialize()
+
+
+def test_df_serde_list_of_null_17230() -> None:
+    df = pl.Series([[]], dtype=pl.List(pl.Null)).to_frame()
+    ser = df.serialize(format="json")
+    result = pl.DataFrame.deserialize(io.StringIO(ser), format="json")
+    assert_frame_equal(result, df)

py-polars/tests/unit/datatypes/test_enum.py

@@ -180,6 +180,31 @@ def test_nested_enum_concat() -> None:
     assert_series_equal(s1.extend(s2), expected)
 
 
+def test_nested_enum_agg_sort_18026() -> None:
+    df = (
+        pl.DataFrame({"a": [1, 1, 2, 2], "b": ["Y", "Z", "Z", "Y"]})
+        .cast({"b": pl.Enum(["Z", "Y"])})
+        .with_columns(pl.struct("b", "a").alias("c"))
+    )
+    result = df.group_by("a").agg("c").sort("a")
+    expected = pl.DataFrame(
+        {
+            "a": [1, 2],
+            "c": [
+                [{"b": "Y", "a": 1}, {"b": "Z", "a": 1}],
+                [{"b": "Z", "a": 2}, {"b": "Y", "a": 2}],
+            ],
+        },
+        schema={
+            "a": pl.Int64,
+            "c": pl.List(
+                pl.Struct([pl.Field("b", pl.Enum(["Z", "Y"])), pl.Field("a", pl.Int64)])
+            ),
+        },
+    )
+    assert_frame_equal(result, expected)
+
+
 def test_casting_to_an_enum_from_utf() -> None:
     dtype = pl.Enum(["a", "b", "c"])
     s = pl.Series([None, "a", "b", "c"])

py-polars/tests/unit/io/test_csv.py

@@ -2501,3 +2501,10 @@ def test_trailing_separator_8240() -> None:
 
     result = pl.scan_csv(io.StringIO(csv), separator="|", has_header=False).collect()
     assert_frame_equal(result, expected)
+
+
+def test_header_only_column_selection_17173() -> None:
+    csv = "A,B"
+    result = pl.read_csv(io.StringIO(csv), columns=["B"])
+    expected = pl.Series("B", [], pl.String()).to_frame()
+    assert_frame_equal(result, expected)

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

@@ -61,3 +61,10 @@ def test_fill_null_lit_() -> None:
         df.fill_null(pl.lit(0)).select(pl.all().null_count()).transpose().sum().item()
         == 0
     )
+
+
+def test_fill_null_decimal_with_int_14331() -> None:
+    s = pl.Series("a", ["1.1", None], dtype=pl.Decimal(precision=None, scale=5))
+    result = s.fill_null(0)
+    expected = pl.Series("a", ["1.1", "0.0"], dtype=pl.Decimal(precision=None, scale=5))
+    assert_series_equal(result, expected)

py-polars/tests/unit/series/test_item.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import datetime
+
 import pytest
 
 import polars as pl
@@ -36,3 +38,9 @@ def test_series_item_with_index(index: int, expected: int, s: pl.Series) -> None
 def test_df_item_out_of_bounds(index: int, s: pl.Series) -> None:
     with pytest.raises(IndexError, match="out of bounds"):
         s.item(index)
+
+
+def test_series_item_out_of_range_date() -> None:
+    s = pl.Series([datetime.date(9999, 12, 31)]).dt.offset_by("1d")
+    with pytest.raises(ValueError, match="out of range"):
+        s.item()