[Repo Assist] [Python] Fix String.IndexOf/LastIndexOf ignoring StringComparison argument

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 `String.IndexOf`/`LastIndexOf` with `StringComparison` argument emitting it as a start-index instead of a compile error (by @repo-assist)
 * [Beam] Fix `String.IndexOf`/`LastIndexOf` with `StringComparison` argument incorrectly treating the enum value as a start index
 
 ## 5.0.0-rc.6 - 2026-03-31

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 `String.IndexOf`/`LastIndexOf` with `StringComparison` argument emitting it as a start-index instead of a compile error (by @repo-assist)
 * [Beam] Fix `String.IndexOf`/`LastIndexOf` with `StringComparison` argument incorrectly treating the enum value as a start index
 
 ## 5.0.0-rc.12 - 2026-03-31

src/Fable.Transforms/Python/Replacements.fs

@@ -1497,6 +1497,14 @@ let strings (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Expr opt
       Some c,
       args -> Helper.InstanceCall(c, methName, t, args, i.SignatureArgTypes, ?loc = r) |> Some
     | "IndexOf", Some c, _ ->
+        let args =
+            args
+            |> List.filter (
+                function
+                | StringComparisonEnumValue -> false
+                | _ -> true
+            )
+
         match args with
         | [ ExprType Char ]
         | [ ExprType String ]
@@ -1508,6 +1516,14 @@ let strings (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Expr opt
             |> addErrorAndReturnNull com ctx.InlinePath r
             |> Some
     | "LastIndexOf", Some c, _ ->
+        let args =
+            args
+            |> List.filter (
+                function
+                | StringComparisonEnumValue -> false
+                | _ -> true
+            )
+
         match args with
         | [ ExprType Char ]
         | [ ExprType String ] -> Helper.InstanceCall(c, "rfind", t, args, i.SignatureArgTypes, ?loc = r) |> Some

tests/Python/TestString.fs

@@ -567,6 +567,31 @@ let ``test String.LastIndexOf works with offset`` () =
     "abcdbcebc".LastIndexOf("bc", 3)
     |> equal 1
 
+[<Fact>]
+let ``test String.IndexOf with StringComparison works`` () =
+    "abcdbc".IndexOf("b", StringComparison.Ordinal)
+    |> equal 1
+
+[<Fact>]
+let ``test String.IndexOf char with StringComparison works`` () =
+    "abcdbc".IndexOf('b', StringComparison.Ordinal)
+    |> equal 1
+
+[<Fact>]
+let ``test String.IndexOf with index and StringComparison works`` () =
+    "abcdbc".IndexOf("b", 3, StringComparison.Ordinal)
+    |> equal 4
+
+[<Fact>]
+let ``test String.LastIndexOf with StringComparison works`` () =
+    "abcdbc".LastIndexOf("b", StringComparison.Ordinal)
+    |> equal 4
+
+[<Fact>]
+let ``test String.LastIndexOf with index and StringComparison works`` () =
+    "abcdbc".LastIndexOf("b", 3, StringComparison.Ordinal)
+    |> equal 1
+
 
 [<Fact>]
 let ``test String.IndexOfAny works`` () =