feat: add deprecation warning for Expr in date functions

- Added a deprecation warning helper in `functions.py` to warn users when an `Expr` is passed to:
- `date_part`
- `datepart`
- `extract`
- `date_trunc`
- `datetrunc`
- Kept plain string paths unchanged.

test: enhance tests for deprecation warnings

- Added tests in `test_functions.py` to check for deprecation warnings.
- Implemented no-warning checks for native strings.
- Updated an existing temporal test to use strings to reduce warning noise.

Author: kosiew

python/datafusion/functions.py

@@ -39,6 +39,7 @@
 from __future__ import annotations
 
 import inspect
+import warnings
 from typing import TYPE_CHECKING, Any
 
 import pyarrow as pa
@@ -60,6 +61,16 @@
     sort_or_default,
 )
 
+
+def _warn_expr_for_literal_arg(function_name: str, arg_name: str) -> None:
+    warnings.warn(
+        f"Passing Expr for {function_name}() argument {arg_name!r} is deprecated; "
+        "pass a Python literal instead.",
+        DeprecationWarning,
+        stacklevel=3,
+    )
+
+
 __all__ = [
     "abs",
     "acos",
@@ -2575,7 +2586,7 @@ def datepart(part: Expr | str, date: Expr) -> Expr:
     See Also:
         This is an alias for :py:func:`date_part`.
     """
-    return date_part(part, date)
+    return _date_part(part, date, "datepart")
 
 
 def date_part(part: Expr | str, date: Expr) -> Expr:
@@ -2595,6 +2606,12 @@ def date_part(part: Expr | str, date: Expr) -> Expr:
         >>> result.collect_column("y")[0].as_py()
         2021
     """
+    return _date_part(part, date, "date_part")
+
+
+def _date_part(part: Expr | str, date: Expr, function_name: str) -> Expr:
+    if isinstance(part, Expr):
+        _warn_expr_for_literal_arg(function_name, "part")
     part = coerce_to_expr(part)
     return Expr(f.date_part(part.expr, date.expr))
 
@@ -2605,7 +2622,7 @@ def extract(part: Expr | str, date: Expr) -> Expr:
     See Also:
         This is an alias for :py:func:`date_part`.
     """
-    return date_part(part, date)
+    return _date_part(part, date, "extract")
 
 
 def date_trunc(part: Expr | str, date: Expr) -> Expr:
@@ -2626,6 +2643,12 @@ def date_trunc(part: Expr | str, date: Expr) -> Expr:
         >>> str(result.collect_column("t")[0].as_py())
         '2021-07-01 00:00:00'
     """
+    return _date_trunc(part, date, "date_trunc")
+
+
+def _date_trunc(part: Expr | str, date: Expr, function_name: str) -> Expr:
+    if isinstance(part, Expr):
+        _warn_expr_for_literal_arg(function_name, "part")
     part = coerce_to_expr(part)
     return Expr(f.date_trunc(part.expr, date.expr))
 
@@ -2636,7 +2659,7 @@ def datetrunc(part: Expr | str, date: Expr) -> Expr:
     See Also:
         This is an alias for :py:func:`date_trunc`.
     """
-    return date_trunc(part, date)
+    return _date_trunc(part, date, "datetrunc")
 
 
 def date_bin(stride: Expr, source: Expr, origin: Expr) -> Expr:

python/tests/test_functions.py

@@ -15,6 +15,7 @@
 # specific language governing permissions and limitations
 # under the License.
 import math
+import warnings
 from datetime import date, datetime, time, timezone
 
 import numpy as np
@@ -1086,10 +1087,10 @@ def test_hash_functions(df):
 
 def test_temporal_functions(df):
     df = df.select(
-        f.date_part(literal("month"), column("d")),
-        f.datepart(literal("year"), column("d")),
-        f.date_trunc(literal("month"), column("d")),
-        f.datetrunc(literal("day"), column("d")),
+        f.date_part("month", column("d")),
+        f.datepart("year", column("d")),
+        f.date_trunc("month", column("d")),
+        f.datetrunc("day", column("d")),
         f.date_bin(
             literal("15 minutes").cast(pa.string()),
             column("d"),
@@ -1100,7 +1101,7 @@ def test_temporal_functions(df):
         f.to_timestamp_seconds(literal("2023-09-07 05:06:14.523952")),
         f.to_timestamp_millis(literal("2023-09-07 05:06:14.523952")),
         f.to_timestamp_micros(literal("2023-09-07 05:06:14.523952")),
-        f.extract(literal("day"), column("d")),
+        f.extract("day", column("d")),
         f.to_timestamp(
             literal("2023-09-07 05:06:14.523952000"), literal("%Y-%m-%d %H:%M:%S.%f")
         ),
@@ -2160,16 +2161,51 @@ def test_date_part_native_str(self):
         ctx = SessionContext()
         df = ctx.from_pydict({"a": ["2021-07-15T00:00:00"]})
         df = df.select(f.to_timestamp(column("a")).alias("a"))
-        result = df.select(f.date_part("year", column("a")).alias("y")).collect()
+        with warnings.catch_warnings():
+            warnings.simplefilter("error", DeprecationWarning)
+            result = df.select(f.date_part("year", column("a")).alias("y")).collect()
         assert result[0].column(0)[0].as_py() == 2021
 
+    @pytest.mark.parametrize(
+        ("func", "name"),
+        [
+            pytest.param(f.date_part, "date_part", id="date_part"),
+            pytest.param(f.datepart, "datepart", id="datepart"),
+            pytest.param(f.extract, "extract", id="extract"),
+        ],
+    )
+    def test_date_part_expr_part_warns_deprecated(self, func, name):
+        with pytest.warns(
+            DeprecationWarning,
+            match=rf"Passing Expr for {name}\(\) argument 'part' is deprecated",
+        ):
+            expr = func(literal("year"), column("a"))
+        assert expr is not None
+
     def test_date_trunc_native_str(self):
         ctx = SessionContext()
         df = ctx.from_pydict({"a": ["2021-07-15T12:34:56"]})
         df = df.select(f.to_timestamp(column("a")).alias("a"))
-        result = df.select(f.date_trunc("month", column("a")).alias("t")).collect()
+        with warnings.catch_warnings():
+            warnings.simplefilter("error", DeprecationWarning)
+            result = df.select(f.date_trunc("month", column("a")).alias("t")).collect()
         assert str(result[0].column(0)[0].as_py()) == "2021-07-01 00:00:00"
 
+    @pytest.mark.parametrize(
+        ("func", "name"),
+        [
+            pytest.param(f.date_trunc, "date_trunc", id="date_trunc"),
+            pytest.param(f.datetrunc, "datetrunc", id="datetrunc"),
+        ],
+    )
+    def test_date_trunc_expr_part_warns_deprecated(self, func, name):
+        with pytest.warns(
+            DeprecationWarning,
+            match=rf"Passing Expr for {name}\(\) argument 'part' is deprecated",
+        ):
+            expr = func(literal("month"), column("a"))
+        assert expr is not None
+
     def test_left_native_int(self):
         ctx = SessionContext()
         df = ctx.from_pydict({"a": ["the cat"]})