fix(python): avoid union case field name collision with Union.name (#4647)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dbrattli

src/Fable.Transforms/Python/Fable2Python.Reflection.fs

@@ -40,7 +40,7 @@ let private transformRecordReflectionInfo com ctx r (ent: Fable.Entity) generics
 
                 let name =
                     if Util.shouldUseRecordFieldNaming ent then
-                        fi.Name |> Naming.toRecordFieldSnakeCase |> Helpers.clean
+                        fi.Name |> Naming.toFieldSnakeCase |> Helpers.clean
                     else
                         fi.Name |> Naming.toSnakeCase |> Helpers.clean
 

src/Fable.Transforms/Python/Fable2Python.Transforms.fs

@@ -3223,7 +3223,7 @@ let getUnionFieldsAsIdents (_com: IPythonCompiler) _ctx (_ent: Fable.Entity) =
 let getEntityFieldsAsIdents (com: IPythonCompiler) (ent: Fable.Entity) =
     let entityNamingConvention =
         if shouldUseRecordFieldNaming ent then
-            Naming.toRecordFieldSnakeCase
+            Naming.toFieldSnakeCase
         else
             Naming.toPythonNaming
 
@@ -3248,7 +3248,7 @@ let getEntityFieldsAsProps (com: IPythonCompiler) ctx (ent: Fable.Entity) =
     else
         let namingConvention =
             if shouldUseRecordFieldNaming ent then
-                Naming.toRecordFieldSnakeCase
+                Naming.toFieldSnakeCase
             else
                 Naming.toPythonNaming
 
@@ -3290,7 +3290,7 @@ let declareDataClassType
                     consArgs.Args.[i].Arg
                 else
                     // Fallback to field name if consArgs doesn't have enough args
-                    com.GetIdentifier(ctx, field.Name |> Naming.toRecordFieldSnakeCase |> Helpers.clean)
+                    com.GetIdentifier(ctx, field.Name |> Naming.toFieldSnakeCase |> Helpers.clean)
 
             // Uncurry lambda types for field annotations since fields store uncurried functions
             let fieldType =
@@ -3943,7 +3943,7 @@ let transformAttachedProperty
             // Apply the same naming convention as record fields for record types
             let propertyName =
                 //if shouldUseRecordFieldNaming ent then
-                //    memb.Name |> Naming.toRecordFieldSnakeCase |> Helpers.clean
+                //    memb.Name |> Naming.toFieldSnakeCase |> Helpers.clean
                 //else
                 memb.Name |> Naming.toPropertyNaming
 
@@ -4097,9 +4097,10 @@ let transformUnion (com: IPythonCompiler) ctx (ent: Fable.Entity) (entName: stri
             let fieldAnnotations =
                 uci.UnionCaseFields
                 |> List.map (fun field ->
-                    // Convert to snake_case and clean to remove invalid characters like apostrophes
-                    // Handles: "Item" -> "item", "Item1" -> "item1", "MyField" -> "my_field"
-                    let fieldName = field.Name |> Naming.toSnakeCase |> Helpers.clean
+                    // toFieldSnakeCase appends '_' to camelCase names ("name" -> "name_") so a
+                    // field can't collide with the inherited `Union.name` property, which @dataclass
+                    // would treat as a default value ("non-default argument follows default"). See #4645.
+                    let fieldName = field.Name |> Naming.toFieldSnakeCase |> Helpers.clean
                     // Uncurry lambda types for field annotations since union case fields
                     // store uncurried functions at runtime (same as record fields)
                     let fieldType =
@@ -4238,7 +4239,7 @@ let transformClassWithCompilerGeneratedConstructor
                     |> List.collecti (fun i field ->
                         let fieldName =
                             if shouldUseRecordFieldNaming ent then
-                                field.Name |> Naming.toRecordFieldSnakeCase |> Helpers.clean
+                                field.Name |> Naming.toFieldSnakeCase |> Helpers.clean
                             else
                                 match Util.getFieldNamingKind com field.FieldType field.Name with
                                 | InstancePropertyBacking -> field.Name |> Naming.toPropertyBackingFieldNaming
@@ -4680,7 +4681,7 @@ let transformStaticProperty
     // printfn "transformStaticProperty: %A" propName
     let propertyName =
         if shouldUseRecordFieldNaming ent then
-            propName |> Naming.toRecordFieldSnakeCase |> Helpers.clean
+            propName |> Naming.toFieldSnakeCase |> Helpers.clean
         else
             propName |> Naming.toPropertyNaming
 

src/Fable.Transforms/Python/Fable2Python.Util.fs

@@ -463,13 +463,13 @@ module Util =
 
         name
 
-    /// Determines if we should use the special record field naming convention (toRecordFieldSnakeCase)
+    /// Determines if we should use the special field naming convention (toFieldSnakeCase)
     /// for the given entity. Returns true for user-defined F# records, false for built-in types.
     let shouldUseRecordFieldNaming (ent: Fable.Entity) =
         ent.IsFSharpRecord
         && not (ent.FullName.StartsWith("Microsoft.FSharp.Core", StringComparison.Ordinal))
 
-    /// Determines if we should use the special record field naming convention (toRecordFieldSnakeCase)
+    /// Determines if we should use the special field naming convention (toFieldSnakeCase)
     /// for the given entity reference. Returns true for user-defined F# records, false for built-in types.
     let shouldUseRecordFieldNamingForRef (entityRef: Fable.EntityRef) (ent: Fable.Entity) =
         ent.IsFSharpRecord
@@ -510,7 +510,7 @@ module Util =
             | Fable.DeclaredType(entityRef, _) ->
                 match com.TryGetEntity entityRef with
                 | Some ent when shouldUseRecordFieldNamingForRef entityRef ent && isRecordField ent fieldName ->
-                    fieldName |> Naming.toRecordFieldSnakeCase |> Helpers.clean
+                    fieldName |> Naming.toFieldSnakeCase |> Helpers.clean
                 | _ -> fieldName |> Naming.toPythonNaming // Fallback to Python naming for other types
             | _ -> fieldName |> Naming.toPropertyNaming
 

src/Fable.Transforms/Python/Prelude.fs

@@ -33,10 +33,11 @@ module Naming =
     let toSnakeCase (name: string) =
         Naming.applyCaseRule CaseRules.SnakeCase name
 
-    /// Convert F# record field name to snake_case with special handling for camelCase/PascalCase conflicts.
+    /// Convert an F# field name (record or union case field) to snake_case with special handling
+    /// for camelCase/PascalCase conflicts.
     /// - If the name is PascalCase, convert to snake_case without suffix.
     /// - If the name is camelCase, convert to snake_case and add '_' suffix to avoid conflict with PascalCase.
-    let toRecordFieldSnakeCase (name: string) =
+    let toFieldSnakeCase (name: string) =
         let snakeCase = Naming.applyCaseRule CaseRules.SnakeCase name
 
         if name.Length > 0 && Char.IsLower(name.[0]) then

tests/Python/TestUnionType.fs

@@ -211,3 +211,17 @@ type S = S of string
 let ``test sprintf formats strings cases correctly`` () =
     let s = sprintf "%A" (S "1")
     s |> equal "S \"1\""
+
+// See https://github.com/fable-compiler/Fable/issues/4645
+// A named union case field called `name` collided with the inherited `Union.name`
+// property, which Python's @dataclass treated as a default value.
+type NamedFieldUnion =
+    | NamedCase of name: string * optional: string
+
+[<Fact>]
+let ``test Named union case fields work when a field is called name`` () =
+    let value = NamedCase("v1", "v2")
+    match value with
+    | NamedCase(name, optional) ->
+        name |> equal "v1"
+        optional |> equal "v2"