Repository: narwhals-dev/narwhals (python)

Issue:
api: use `large_string` / `large_list` when converting Narwhals types -> PyArrow?
Currently we convert `nw.String` to pyarrow string. Polars would convert `pl.String` to pyarrow large_string. We should probably be doing the same?

Pull request description:
fix: don't downcast `large_string` to `string` unnecessarily in `concat_str` for PyArrow
closes #2097 

<!--
# Thanks for contributing a pull request! 
## Please make sure you see our contribution guidelines: https://github.com/narwhals-dev/narwhals/blob/main/CONTRIBUTING.md
-->

## What type of PR is this? (check all applicable)

- [ ] 💾 Refactor
- [ ] ✨ Feature
- [x] 🐛 Bug Fix
- [ ] 🔧 Optimization
- [ ] 📝 Documentation
- [ ] ✅ Test
- [ ] 🐳 Other

## Related issues

- Related issue #\<issue number\>
- Closes #\<issue number\>

## Checklist

- [ ] Code follows style guide (ruff)
- [ ] Tests added
- [ ] Documented the changes

## If you have comments or can explain your changes, please do so below


Patch:
diff --git a/narwhals/_arrow/group_by.py b/narwhals/_arrow/group_by.py
index aefade0d..25ec346d 100644
--- a/narwhals/_arrow/group_by.py
+++ b/narwhals/_arrow/group_by.py
@@ -5,11 +5,11 @@ import re
 from typing import TYPE_CHECKING
 from typing import Any
 from typing import Iterator
-from typing import cast
 
 import pyarrow as pa
 import pyarrow.compute as pc
 
+from narwhals._arrow.utils import cast_to_comparable_string_types
 from narwhals._arrow.utils import extract_py_scalar
 from narwhals._expression_parsing import evaluate_output_names_and_aliases
 from narwhals._expression_parsing import is_elementary_expression
@@ -146,15 +146,17 @@ class ArrowGroupBy:
 
         table = self._df._native_frame
         # NOTE: stubs fail in multiple places for `ChunkedArray`
-        it = cast(
-            "Iterator[pa.StringArray]",
-            (table[key].cast(pa.string()) for key in self._keys),
+        it, separator_scalar = cast_to_comparable_string_types(
+            *(table[key] for key in self._keys), separator=""
         )
         # NOTE: stubs indicate `separator` must also be a `ChunkedArray`
         # Reality: `str` is fine
         concat_str: Incomplete = pc.binary_join_element_wise
         key_values = concat_str(
-            *it, "", null_handling="replace", null_replacement=null_token
+            *it,
+            separator_scalar,
+            null_handling="replace",
+            null_replacement=null_token,
         )
         table = table.add_column(i=0, field_=col_token, column=key_values)
         for v in pc.unique(key_values):
diff --git a/narwhals/_arrow/namespace.py b/narwhals/_arrow/namespace.py
index 22e2e561..7097c377 100644
--- a/narwhals/_arrow/namespace.py
+++ b/narwhals/_arrow/namespace.py
@@ -19,6 +19,7 @@ from narwhals._arrow.expr import ArrowExpr
 from narwhals._arrow.selectors import ArrowSelectorNamespace
 from narwhals._arrow.series import ArrowSeries
 from narwhals._arrow.utils import align_series_full_broadcast
+from narwhals._arrow.utils import cast_to_comparable_string_types
 from narwhals._arrow.utils import diagonal_concat
 from narwhals._arrow.utils import extract_dataframe_comparand
 from narwhals._arrow.utils import horizontal_concat
@@ -288,27 +289,27 @@ class ArrowNamespace(EagerNamespace[ArrowDataFrame, ArrowSeries, ArrowExpr]):
         separator: str,
         ignore_nulls: bool,
     ) -> ArrowExpr:
-        dtypes = import_dtypes_module(self._version)
-
         def func(df: ArrowDataFrame) -> list[ArrowSeries]:
             compliant_series_list = align_series_full_broadcast(
-                *(chain.from_iterable(expr.cast(dtypes.String())(df) for expr in exprs))
+                *(chain.from_iterable(expr(df) for expr in exprs))
             )
+            name = compliant_series_list[0].name
             null_handling: Literal["skip", "emit_null"] = (
                 "skip" if ignore_nulls else "emit_null"
             )
-            it = (s._native_series for s in compliant_series_list)
+            it, separator_scalar = cast_to_comparable_string_types(
+                *(s.native for s in compliant_series_list), separator=separator
+            )
             # NOTE: stubs indicate `separator` must also be a `ChunkedArray`
             # Reality: `str` is fine
             concat_str: Incomplete = pc.binary_join_element_wise
-            return [
-                ArrowSeries(
-                    native_series=concat_str(*it, separator, null_handling=null_handling),
-                    name=compliant_series_list[0].name,
-                    backend_version=self._backend_version,
-                    version=self._version,
-                )
-            ]
+            compliant = self._series(
+                concat_str(*it, separator_scalar, null_handling=null_handling),
+                name=name,
+                backend_version=self._backend_version,
+                version=self._version,
+            )
+            return [compliant]
 
         return self._expr._from_callable(
             func=func,
diff --git a/narwhals/_arrow/utils.py b/narwhals/_arrow/utils.py
index a1fd03f4..2d4f8544 100644
--- a/narwhals/_arrow/utils.py
+++ b/narwhals/_arrow/utils.py
@@ -5,6 +5,7 @@ from itertools import chain
 from typing import TYPE_CHECKING
 from typing import Any
 from typing import Iterable
+from typing import Iterator
 from typing import Sequence
 from typing import cast
 from typing import overload
@@ -543,6 +544,19 @@ def pad_series(
     return series._from_native_series(concat), offset_left + offset_right
 
 
+def cast_to_comparable_string_types(
+    *chunked_arrays: ArrowChunkedArray,
+    separator: str,
+) -> tuple[Iterator[ArrowChunkedArray], pa.Scalar[Any]]:
+    # Ensure `chunked_arrays` are either all `string` or all `large_string`.
+    dtype = (
+        pa.string()  # (PyArrow default)
+        if not any(pa.types.is_large_string(ca.type) for ca in chunked_arrays)
+        else pa.large_string()
+    )
+    return (ca.cast(dtype) for ca in chunked_arrays), lit(separator, dtype)
+
+
 class ArrowSeriesNamespace(_SeriesNamespace["ArrowSeries", "ArrowChunkedArray"]):
     def __init__(self: Self, series: ArrowSeries, /) -> None:
         self._compliant_series = series
diff --git a/narwhals/functions.py b/narwhals/functions.py
index db29692b..f2512dfe 100644
--- a/narwhals/functions.py
+++ b/narwhals/functions.py
@@ -309,7 +309,8 @@ def from_dict(
 
     Arguments:
         data: Dictionary to create DataFrame from.
-        schema: The DataFrame schema as Schema or dict of {name: type}.
+        schema: The DataFrame schema as Schema or dict of {name: type}. If not
+            specified, the schema will be inferred by the native library.
         backend: specifies which eager backend instantiate to. Only
             necessary if inputs are not Narwhals Series.
 
@@ -1593,7 +1594,7 @@ def lit(value: Any, dtype: DType | type[DType] | None = None) -> Expr:
     Arguments:
         value: The value to use as literal.
         dtype: The data type of the literal value. If not provided, the data type will
-            be inferred.
+            be inferred by the native library.
 
     Returns:
         A new expression.
diff --git a/narwhals/stable/v1/__init__.py b/narwhals/stable/v1/__init__.py
index da25bf60..e8e1af32 100644
--- a/narwhals/stable/v1/__init__.py
+++ b/narwhals/stable/v1/__init__.py
@@ -1869,7 +1869,7 @@ def lit(value: Any, dtype: DType | type[DType] | None = None) -> Expr:
     Arguments:
         value: The value to use as literal.
         dtype: The data type of the literal value. If not provided, the data type will
-            be inferred.
+            be inferred by the native library.
 
     Returns:
         A new expression.
@@ -2228,7 +2228,8 @@ def from_dict(
 
     Arguments:
         data: Dictionary to create DataFrame from.
-        schema: The DataFrame schema as Schema or dict of {name: type}.
+        schema: The DataFrame schema as Schema or dict of {name: type}. If not
+            specified, the schema will be inferred by the native library.
         backend: specifies which eager backend instantiate to. Only
             necessary if inputs are not Narwhals Series.
 


Test patch:
diff --git a/tests/expr_and_series/concat_str_test.py b/tests/expr_and_series/concat_str_test.py
index 6b6e8907..ebcfb92e 100644
--- a/tests/expr_and_series/concat_str_test.py
+++ b/tests/expr_and_series/concat_str_test.py
@@ -1,5 +1,8 @@
 from __future__ import annotations
 
+from typing import Callable
+
+import pyarrow as pa
 import pytest
 
 import narwhals.stable.v1 as nw
@@ -71,3 +74,37 @@ def test_concat_str_with_lit(constructor: Constructor) -> None:
     result = df.with_columns(b=nw.concat_str("a", nw.lit("ab")))
     expected = {"a": ["cat", "dog", "pig"], "b": ["catab", "dogab", "pigab"]}
     assert_equal_data(result, expected)
+
+
+@pytest.mark.parametrize(
+    ("input_schema", "input_values", "expected_function"),
+    [
+        (
+            [("store", pa.large_string()), ("item", pa.large_string())],
+            ["a", "b"],
+            pa.types.is_large_string,
+        ),
+        (
+            [("store", pa.large_string()), ("item", pa.int32())],
+            [0, 1],
+            pa.types.is_large_string,
+        ),
+        ([("store", pa.string()), ("item", pa.int32())], [0, 1], pa.types.is_string),
+        ([("store", pa.string()), ("item", pa.string())], ["a", "b"], pa.types.is_string),
+    ],
+)
+def test_pyarrow_string_type(
+    input_schema: list[tuple[str, pa.DataType]],
+    input_values: list[object],
+    expected_function: Callable[[pa.DataType], bool],
+) -> None:
+    df = pa.table(
+        {"store": ["foo", "bar"], "item": input_values}, schema=pa.schema(input_schema)
+    )
+    result = (
+        nw.from_native(df)
+        .with_columns(store_item=nw.concat_str("store", "item", separator="-"))
+        .to_native()
+        .schema
+    )
+    assert expected_function(result.field("store_item").type)
diff --git a/tests/expr_and_series/lit_test.py b/tests/expr_and_series/lit_test.py
index 53fddcd8..cd09b390 100644
--- a/tests/expr_and_series/lit_test.py
+++ b/tests/expr_and_series/lit_test.py
@@ -5,6 +5,7 @@ from typing import TYPE_CHECKING
 from typing import Any
 
 import numpy as np
+import pyarrow as pa
 import pytest
 
 import narwhals.stable.v1 as nw
@@ -129,3 +130,11 @@ def test_date_lit(constructor: Constructor, request: pytest.FixtureRequest) -> N
         assert result == {"a": nw.Int64, "literal": nw.Datetime}
     else:
         assert result == {"a": nw.Int64, "literal": nw.Date}
+
+
+def test_pyarrow_lit_string() -> None:
+    df = nw.from_native(pa.table({"a": [1, 2, 3]}))
+    result = df.select(nw.lit("foo")).to_native().schema.field("literal")
+    assert pa.types.is_string(result.type)
+    result = df.select(nw.lit("foo", dtype=nw.String)).to_native().schema.field("literal")
+    assert pa.types.is_string(result.type)