[Repo Assist] fix(python): DateTime.TryParse preserves Unspecified kind for naive strings; use fromisoformat in DateTimeOffset.parse (#4491)

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Dag Brattli <dag@brattli.net>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 4 people

pyproject.toml

@@ -9,9 +9,7 @@ authors = [
 requires-python = ">= 3.12, < 4.0"
 readme = "README.md"
 license = "MIT"
-dependencies = [
-    "python-dateutil>=2.9.0,<3",
-]
+dependencies = []
 
 [project.urls]
 Homepage = "https://fable.io"

src/Fable.Cli/CHANGELOG.md

@@ -20,6 +20,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [Python] Fix `DateTime.TryParse` incorrectly assigning `DateTimeKind.Local` to naive datetime strings (should be `DateTimeKind.Unspecified`) (fixes #3654)
+* [Python] Remove `python-dateutil` dependency from fable-library; use stdlib `datetime.fromisoformat` with `strptime` fallback
 * [All] Fix unnecessary object allocations during AST traversal when visiting `Import` expressions (by Repo Assist)
 * [Beam] Fix `System.Random.Next(0)` implementation (by @ncave)
 * [Python] Fix `System.Random` seeded implementation  (by @ncave)

src/Fable.Compiler/CHANGELOG.md

@@ -20,6 +20,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [Python] Fix `DateTime.TryParse` incorrectly assigning `DateTimeKind.Local` to naive datetime strings (should be `DateTimeKind.Unspecified`) (fixes #3654)
+* [Python] Remove `python-dateutil` dependency from fable-library; use stdlib `datetime.fromisoformat` with `strptime` fallback
 * [All] Fix unnecessary object allocations during AST traversal when visiting `Import` expressions (by Repo Assist)
 * [Beam] Fix `System.Random.Next(0)` implementation (by @ncave)
 * [Python] Fix `System.Random` seeded implementation  (by @ncave)

src/fable-library-py/fable_library/date.py

@@ -510,7 +510,12 @@ def add(x: datetime, y: TimeSpan) -> datetime:
 
 def parse(string: str) -> datetime:
     try:
-        return datetime.fromisoformat(string).astimezone()
+        parsed = datetime.fromisoformat(string)
+        # Only convert to local time when the string had an explicit timezone.
+        # Naive strings represent DateTimeKind.Unspecified and must stay naive.
+        if parsed.tzinfo is not None:
+            return parsed.astimezone()
+        return parsed
     except ValueError:
         pass
 

src/fable-library-py/fable_library/date_offset.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import re
 from datetime import UTC, datetime, timedelta, timezone
 from math import fmod
 from typing import Any, SupportsFloat, SupportsIndex, SupportsInt, overload
@@ -189,10 +190,26 @@ def timedelta_total_microseconds(td: timedelta) -> int:
     return td.days * (24 * 3600) + td.seconds * 10**6 + td.microseconds
 
 
-def parse(string: str, detectUTC: bool = False) -> DateTimeOffset:
-    from dateutil import parser  # noqa: PLC0415 - lazy import to avoid top-level dependency
+_NON_ISO_FORMATS: dict[str, str] = {
+    # 9/10/2014 1:50:34 PM
+    r"^(0?[1-9]|1[0-2])\/(0?[1-9]|1[0-2])\/\d{4} ([0-9]|(0|1)[0-9]|2[0-4]):([0-5][0-9]|0?[0-9]):([0-5][0-9]|0?[0-9]) [AP]M$": "%m/%d/%Y %I:%M:%S %p",
+    # 9/10/2014 1:50:34
+    r"^(0?[1-9]|1[0-2])\/(0?[1-9]|1[0-2])\/\d{4} ([0-9]|(0|1)[0-9]|2[0-4]):([0-5][0-9]|0?[0-9]):([0-5][0-9]|0?[0-9])$": "%m/%d/%Y %H:%M:%S",
+}
+
+
+def _parse_non_iso(string: str) -> datetime:
+    for pattern, fmt in _NON_ISO_FORMATS.items():
+        if re.fullmatch(pattern, string):
+            return datetime.strptime(string, fmt)
+    raise ValueError(f"Unsupported format by Fable: {string}")
 
-    parsed_dt = parser.parse(string)
+
+def parse(string: str, detectUTC: bool = False) -> DateTimeOffset:
+    try:
+        parsed_dt = datetime.fromisoformat(string)
+    except ValueError:
+        parsed_dt = _parse_non_iso(string)
 
     # Calculate offset in milliseconds
     if parsed_dt.tzinfo is not None:

src/fable-library-py/pyproject.toml

@@ -8,9 +8,7 @@ authors = [
 requires-python = ">= 3.10, < 4.0"
 readme = "README.md"
 license = "MIT License"
-dependencies = [
-    "python-dateutil>=2.9.0,<3"
-]
+dependencies = []
 
 [project.urls]
 Homepage = "https://fable.io"
@@ -23,7 +21,6 @@ dev = [
     "pytest-benchmark>=5.1.0,<6",
     "pyright>=1.1.401,<2",
     "pydantic>=2.12.5",
-    "python-dateutil>=2.9.0,<3",
 ]
 
 [tool.hatch.build.targets.sdist]

src/fable-library-py/uv.lock

@@ -787,6 +787,18 @@ let ``test DateTime.TryParse works`` () =
     dateTime.Year + dateTime.Month + dateTime.Day + dateTime.Hour + dateTime.Minute + dateTime.Second |> equal 2130
 
 
+[<Fact>]
+let ``test DateTime.TryParse preserves Unspecified kind for naive strings`` () =
+    let (isSuccess, dt) = DateTime.TryParse("2018-10-01T11:12:55")
+    isSuccess |> equal true
+    dt.Kind |> equal DateTimeKind.Unspecified
+    dt.Year |> equal 2018
+    dt.Month |> equal 10
+    dt.Day |> equal 1
+    dt.Hour |> equal 11
+    dt.Minute |> equal 12
+    dt.Second |> equal 55
+
 [<Fact>]
 let ``test "Parsing doesn't succeed for invalid dates`` () =
     let invalidAmericanDate = "13/1/2020"

tests/Python/TestDateTime.fs

@@ -471,6 +471,13 @@ let ``test DateTimeOffset.TryParse works`` () =
     f "9/10/2014 1:50:34" |> equal true
     f "2014-09-10T13:50:34" |> equal true
 
+[<Fact>]
+let ``test DateTimeOffset.TryParse with timezone offset returns correct value`` () =
+    let expected = DateTimeOffset(2018, 7, 2, 12, 23, 45, 0, TimeSpan.FromHours(2.))
+    let (isSuccess, actual) = DateTimeOffset.TryParse("2018-07-02T12:23:45+02:00")
+    isSuccess |> equal true
+    actual |> equal expected
+
 [<Fact>]
 let ``test DateTimeOffset.ToString() default works`` () =
     let d = DateTimeOffset(2014, 10, 9, 13, 23, 30, TimeSpan.Zero)