[Python] Fixes for datetime subtract (#3630)

* Fixes for datetime subtract

* Fix error in days to microseconds calculation

* Fix constant casing

* Update changelog

src/Fable.Cli/CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Fix #3598: Using obj () now generated an empty dict instead of None
 * Fix #3597: Do not translate .toString methods to str
 * Fix #3610: Cleanup Python regex handling
+* Fix #3628: System.DateTime.Substract not correctly transpiled
 
 ## 4.6.0 - 2023-11-27
 

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

@@ -5,21 +5,28 @@
 from re import Match
 from typing import Any
 
-from .time_span import TimeSpan
+from .time_span import TimeSpan, total_microseconds
 from .time_span import create as create_time_span
 from .types import FSharpRef
 from .util import DateKind
 
 
-formatRegExp = re.compile(r"(\w)\1*")
+FORMAT_REGEXP = re.compile(r"(\w)\1*")
 
 
-def op_subtraction(x: datetime, y: datetime) -> TimeSpan:
-    delta = x - y
-    # ts.microseconds only contains the microseconds provided to the constructor
-    # so we need to calculate the total microseconds ourselves
-    delta_microseconds = delta.days * (24 * 3600) + delta.seconds * 10**6 + delta.microseconds
-    return create_time_span(0, 0, 0, 0, 0, delta_microseconds)
+def subtract(x: datetime, y: datetime | TimeSpan) -> datetime | TimeSpan:
+    if isinstance(y, datetime):
+        delta = x - y
+        # ts.microseconds only contains the microseconds provided to the constructor
+        # so we need to calculate the total microseconds ourselves
+        delta_microseconds = delta.days * (24 * 3600 * 10**6) + delta.seconds * 10**6 + delta.microseconds
+        return create_time_span(0, 0, 0, 0, 0, delta_microseconds)
+
+    return x - timedelta(microseconds=total_microseconds(y))
+
+
+def op_subtraction(x: datetime, y: datetime | TimeSpan) -> datetime | TimeSpan:
+    return subtract(x, y)
 
 
 def create(
@@ -79,7 +86,7 @@ def match(match: Match[str]) -> str:
 
         return group
 
-    ret = formatRegExp.sub(match, format)
+    ret = FORMAT_REGEXP.sub(match, format)
     return ret
 
     # return format.replace(/(\w)\1*/g, (match) => {
@@ -228,3 +235,29 @@ def try_parse(string: str, style: int, unsigned: bool, bitsize: int, defValue: F
         return True
     except Exception:
         return False
+
+
+def add_milliseconds(d: datetime, v: int) -> datetime:
+    return d + timedelta(milliseconds=v)
+
+
+__all__ = [
+    "op_subtraction",
+    "subtract",
+    "create",
+    "year",
+    "date_to_string_with_custom_format",
+    "date_to_string_with_offset",
+    "date_to_string_with_kind",
+    "to_string",
+    "now",
+    "utc_now",
+    "to_local_time",
+    "compare",
+    "equals",
+    "max_value",
+    "min_value",
+    "op_addition",
+    "parse",
+    "try_parse",
+]

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

@@ -1,9 +1,9 @@
 from __future__ import annotations
 
+from math import ceil, floor, fmod
 from typing import Any
 
 from .util import pad_left_and_right_with_zeros, pad_with_zeros
-from math import fmod, ceil, floor
 
 
 # TimeSpan is represented as an int which is the Tick value

tests/Python/TestDateTime.fs

@@ -74,6 +74,33 @@ let ``test DateTime.MinValue works in pattern match`` () =
     | Some date when date <> DateTime.MinValue -> ()
     | _ -> failwith "expected pattern match above"
 
+[<Fact>]
+let ``test DateTime Subtraction with TimeSpan works`` () =
+    let test ms expected =
+        let dt = DateTime(2014,9,12,0,0,0,DateTimeKind.Utc)
+        let ts = TimeSpan.FromMilliseconds(ms)
+        let res1 = dt.Subtract(ts) |> thatYearSeconds
+        let res2 = (dt - ts) |> thatYearSeconds
+        equal true (res1 = res2)
+        equal expected res1
+    test 1000. 21945599.0
+    test -1000. 21945601.0
+    test 0. 21945600.0
+
+[<Fact>]
+let ``test DateTime Subtraction with DateTime works`` () =
+    let test ms expected =
+        let dt1 = DateTime(2014, 10, 9, 13, 23, 30, 234, DateTimeKind.Utc)
+        let dt2 = dt1.AddMilliseconds(ms)
+        let res1 = dt1.Subtract(dt2).TotalSeconds
+        let res2 = (dt1 - dt2).TotalSeconds
+        equal true (res1 = res2)
+        equal expected res1
+    test 1000. -1.0
+    test -1000. 1.0
+    test 0. 0.0
+
+
 // [<Fact>]
 // let ``test DateTime.ToLocalTime works`` () =
 //     let d = DateTime(2014, 10, 9, 13, 23, 30, DateTimeKind.Utc)