RFC, feat: infer datetime format for pyarrow backend

narwhals/_arrow/series.py

@@ -12,6 +12,7 @@
 from narwhals._arrow.utils import floordiv_compat
 from narwhals._arrow.utils import narwhals_to_native_dtype
 from narwhals._arrow.utils import native_to_narwhals_dtype
+from narwhals._arrow.utils import parse_datetime_format
 from narwhals._arrow.utils import validate_column_comparand
 from narwhals.utils import Implementation
 from narwhals.utils import generate_temporary_column_name
@@ -1115,8 +1116,7 @@ def to_datetime(self: Self, format: str | None) -> ArrowSeries:  # noqa: A002
         import pyarrow.compute as pc  # ignore-banned-import()
 
         if format is None:
-            msg = "`format` is required for pyarrow backend."
-            raise ValueError(msg)
+            format = parse_datetime_format(self._arrow_series._native_series)
 
         return self._arrow_series._from_native_series(
             pc.strptime(self._arrow_series._native_series, format=format, unit="us")

narwhals/_arrow/utils.py

@@ -335,3 +335,88 @@ def convert_str_slice_to_int_slice(
     stop = columns.index(str_slice.stop) + 1 if str_slice.stop is not None else None
     step = str_slice.step
     return (start, stop, step)
+
+
+# Regex for date, time, separator and timezone components
+DATE_RE = r"(?P<date>\d{1,4}[-/.]\d{1,2}[-/.]\d{1,4})"
+SEP_RE = r"(?P<sep>\s|T)"
+TIME_RE = r"(?P<time>\d{2}:\d{2}:\d{2})"  # \s*(?P<period>[AP]M)?)?
+TZ_RE = r"(?P<tz>Z|[+-]\d{2}:?\d{2})"  # Matches 'Z', '+02:00', '+0200', '+02', etc.
+FULL_RE = rf"{DATE_RE}{SEP_RE}?{TIME_RE}?{TZ_RE}?$"
+
+# Separate regexes for different date formats
+YMD_RE = r"^(?P<year>(?:[12][0-9])?[0-9]{2})(?P<sep1>[-/.])(?P<month>0[1-9]|1[0-2])(?P<sep2>[-/.])(?P<day>0[1-9]|[12][0-9]|3[01])$"
+DMY_RE = r"^(?P<day>0[1-9]|[12][0-9]|3[01])(?P<sep1>[-/.])(?P<month>0[1-9]|1[0-2])(?P<sep2>[-/.])(?P<year>(?:[12][0-9])?[0-9]{2})$"
+MDY_RE = r"^(?P<month>0[1-9]|1[0-2])(?P<sep1>[-/.])(?P<day>0[1-9]|[12][0-9]|3[01])(?P<sep2>[-/.])(?P<year>(?:[12][0-9])?[0-9]{2})$"
+
+DATE_FORMATS = (
+    (YMD_RE, "%Y-%m-%d"),
+    (DMY_RE, "%d-%m-%Y"),
+    (MDY_RE, "%m-%d-%Y"),
+)
+
+
+def parse_datetime_format(arr: pa.StringArray) -> str:
+    """Try to infer datetime format from StringArray."""
+    import pyarrow as pa  # ignore-banned-import
+    import pyarrow.compute as pc  # ignore-banned-import
+
+    matches = pa.concat_arrays(  # converts from ChunkedArray to StructArray
+        pc.extract_regex(pc.drop_null(arr).slice(0, 10), pattern=FULL_RE).chunks
+    )
+
+    if not pc.all(matches.is_valid()).as_py():
+        msg = (
+            "Unable to infer datetime format, provided format is not supported. "
+            "Please report a bug to https://github.com/narwhals-dev/narwhals/issues"
+        )
+        raise NotImplementedError(msg)
+
+    dates = matches.field("date")
+    separators = matches.field("sep")
+    times = matches.field("time")
+    tz = matches.field("tz")
+
+    # separators and time zones must be unique
+    if pc.count(pc.unique(separators)).as_py() > 1:
+        msg = "Found multiple separator values while inferring datetime format."
+        raise ValueError(msg)
+
+    if pc.count(pc.unique(tz)).as_py() > 1:
+        msg = "Found multiple timezone values while inferring datetime format."
+        raise ValueError(msg)
+
+    date_value = _parse_date_format(dates)
+    time_value = _parse_time_format(times)
+
+    sep_value = separators[0].as_py()
+    tz_value = "%z" if tz[0].as_py() else ""
+
+    return f"{date_value}{sep_value}{time_value}{tz_value}"
+
+
+def _parse_date_format(arr: pa.Array) -> str:
+    import pyarrow.compute as pc  # ignore-banned-import
+
+    for date_rgx, date_fmt in DATE_FORMATS:
+        matches = pc.extract_regex(arr, pattern=date_rgx)
+        if (
+            pc.all(matches.is_valid()).as_py()
+            and pc.count(pc.unique(sep1 := matches.field("sep1"))).as_py() == 1
+            and pc.count(pc.unique(sep2 := matches.field("sep2"))).as_py() == 1
+            and (date_sep_value := sep1[0].as_py()) == sep2[0].as_py()
+        ):
+            return date_fmt.replace("-", date_sep_value)
+
+    msg = (
+        "Unable to infer datetime format. "
+        "Please report a bug to https://github.com/narwhals-dev/narwhals/issues"
+    )
+    raise ValueError(msg)
+
+
+def _parse_time_format(arr: pa.Array) -> str:
+    import pyarrow.compute as pc  # ignore-banned-import
+
+    matches = pc.extract_regex(arr, pattern=TIME_RE)
+    return "%H:%M:%S" if pc.all(matches.is_valid()).as_py() else ""

tests/expr_and_series/str/to_datetime_test.py

@@ -1,14 +1,19 @@
 from __future__ import annotations
 
+from datetime import datetime
 from typing import TYPE_CHECKING
 
+import pyarrow as pa
 import pytest
 
 import narwhals.stable.v1 as nw
+from narwhals._arrow.utils import parse_datetime_format
+from tests.utils import assert_equal_data
 
 if TYPE_CHECKING:
     from tests.utils import Constructor
     from tests.utils import ConstructorEager
+
 data = {"a": ["2020-01-01T12:34:56"]}
 
 
@@ -42,12 +47,7 @@ def test_to_datetime_series(constructor_eager: ConstructorEager) -> None:
     assert str(result) == expected
 
 
-def test_to_datetime_infer_fmt(
-    request: pytest.FixtureRequest, constructor: Constructor
-) -> None:
-    if "pyarrow_table" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
-
+def test_to_datetime_infer_fmt(constructor: Constructor) -> None:
     if "cudf" in str(constructor):  # pragma: no cover
         expected = "2020-01-01T12:34:56.000000000"
     else:
@@ -63,12 +63,7 @@ def test_to_datetime_infer_fmt(
     assert str(result) == expected
 
 
-def test_to_datetime_series_infer_fmt(
-    request: pytest.FixtureRequest, constructor_eager: ConstructorEager
-) -> None:
-    if "pyarrow_table" in str(constructor_eager):
-        request.applymarker(pytest.mark.xfail)
-
+def test_to_datetime_series_infer_fmt(constructor_eager: ConstructorEager) -> None:
     if "cudf" in str(constructor_eager):  # pragma: no cover
         expected = "2020-01-01T12:34:56.000000000"
     else:
@@ -78,3 +73,48 @@ def test_to_datetime_series_infer_fmt(
         nw.from_native(constructor_eager(data), eager_only=True)["a"].str.to_datetime()
     ).item(0)
     assert str(result) == expected
+
+
+def test_to_datetime_infer_fmt_from_date(constructor: Constructor) -> None:
+    data = {"z": ["2020-01-01", "2020-01-02", None]}
+    expected = [datetime(2020, 1, 1), datetime(2020, 1, 2), None]
+    result = (
+        nw.from_native(constructor(data))
+        .lazy()
+        .select(nw.col("z").str.to_datetime())
+        .collect()
+    )
+    assert_equal_data(result, {"z": expected})
+
+
+def test_pyarrow_infer_datetime_raise_invalid() -> None:
+    with pytest.raises(
+        NotImplementedError,
+        match="Unable to infer datetime format, provided format is not supported.",
+    ):
+        parse_datetime_format(pa.chunked_array([["2024-01-01", "abc"]]))
+
+
+@pytest.mark.parametrize(
+    ("data", "duplicate"),
+    [
+        (["2024-01-01T00:00:00", "2024-01-01 01:00:00"], "separator"),
+        (["2024-01-01 00:00:00+01:00", "2024-01-01 01:00:00+02:00"], "timezone"),
+    ],
+)
+def test_pyarrow_infer_datetime_raise_not_unique(
+    data: list[str | None], duplicate: str
+) -> None:
+    with pytest.raises(
+        ValueError,
+        match=f"Found multiple {duplicate} values while inferring datetime format.",
+    ):
+        parse_datetime_format(pa.chunked_array([data]))
+
+
+@pytest.mark.parametrize("data", [["2024-01-01", "2024-12-01", "02-02-2024"]])
+def test_pyarrow_infer_datetime_raise_inconsistent_date_fmt(
+    data: list[str | None],
+) -> None:
+    with pytest.raises(ValueError, match="Unable to infer datetime format. "):
+        parse_datetime_format(pa.chunked_array([data]))