[Python] Fix modulo with negative numbers for bigint (#4463)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dbrattli

src/Fable.Cli/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [Python] Fix modulo with negative numbers using Python floored semantics instead of .NET truncated semantics for bigint (fixes #4462) (by @dbrattli)
 * [Beam] Fix `System.String.Concat` with 4+ arguments not being supported (by @dbrattli)
 * [TS/Python] Fix invalid `this` argument type in structs (#4453) (by @ncave)
 * [JS/TS] Fix `N` format specifier (`ToString("N0")`, `String.Format("{0:N0}", ...)`) producing a trailing dot when precision is 0 (fix #2582) (by @MangelMaxime)

src/Fable.Compiler/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [Python] Fix modulo with negative numbers using Python floored semantics instead of .NET truncated semantics for bigint (fixes #4462) (by @dbrattli)
 * [Beam] Fix `System.String.Concat` with 4+ arguments not being supported (by @dbrattli)
 * [TS/Python] Fix invalid `this` argument type in structs (#4453) (by @ncave)
 * [JS/TS] Fix `N` format specifier (`ToString("N0")`, `String.Format("{0:N0}", ...)`) producing a trailing dot when precision is 0 (fix #2582) (by @MangelMaxime)

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

@@ -81,7 +81,12 @@ def op_division(a: int, b: int) -> int:
 
 
 def op_modulus(a: int, b: int) -> int:
-    return a % b
+    # .NET uses truncated remainder, Python uses floored remainder.
+    # They differ when the dividend and divisor have different signs.
+    r = a % b
+    if r != 0 and ((a < 0) != (b < 0)):
+        r -= b
+    return r
 
 
 def op_right_shift(a: int, num_bits: int) -> int:

tests/Python/TestArithmetic.fs

@@ -60,6 +60,12 @@ let ``test Integer division doesn't produce floats`` () =
 let ``test Infix modulo can be generated`` () =
     4 % 3 |> equal 1
 
+[<Fact>]
+let ``test Infix modulo with negative numbers`` () =
+    -5 % 3 |> equal -2
+    5 % -3 |> equal 2
+    -5 % -3 |> equal -2
+
 // [<Fact>]
 // let ``test Math.DivRem works with bytes`` () =
 //     Math.DivRem(5y, 2y) |> equal struct (2y, 1y)
@@ -384,6 +390,12 @@ let ``test Int64 Integer division doesn't produce floats`` () =
 let ``test Int64 Infix modulo can be generated`` () =
     4L % 3L |> equal 1L
 
+[<Fact>]
+let ``test Int64 Infix modulo with negative numbers`` () =
+    -5L % 3L |> equal -2L
+    5L % -3L |> equal 2L
+    -5L % -3L |> equal -2L
+
 [<Fact>]
 let ``test Int64 Evaluation order is preserved by generated code`` () =
     (4L - 2L) * 2L + 1L |> equal 5L
@@ -446,6 +458,12 @@ let ``test BigInt Integer division doesn't produce floats`` () =
 let ``test BigInt Infix modulo can be generated`` () =
     4I % 3I |> equal 1I
 
+[<Fact>]
+let ``test BigInt Infix modulo with negative numbers`` () =
+    -5I % 3I |> equal -2I
+    5I % -3I |> equal 2I
+    -5I % -3I |> equal -2I
+
 // [<Fact>]
 // let ``test BigInt.DivRem works`` () = // See #1744
 //     let quotient,remainder = bigint.DivRem(5I,2I)