[Python] Type annotation fixes (#4332)

Author: dbrattli

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

@@ -12,13 +12,18 @@ open Fable.Transforms.Python.AST
 open Fable.Transforms.Python.Types
 open Fable.Transforms.Python.Util
 
-/// Check if type is an inref (in-reference) or Any type.
+/// Check if type is an inref wrapping a struct/value type.
 /// In F#, struct instance method's `this` parameter is represented as inref<StructType>,
 /// but in Python the struct is passed directly, not wrapped in FSharpRef.
-let isInRefOrAnyType (com: IPythonCompiler) =
+/// For regular inref<T> parameters (where T is a primitive or non-struct), we keep FSharpRef.
+let isStructInRefType (com: IPythonCompiler) =
     function
-    | Replacements.Util.IsInRefType com _ -> true
-    | Fable.Any -> true
+    | Replacements.Util.IsInRefType com innerType ->
+        match innerType with
+        | Fable.DeclaredType(entRef, _) ->
+            let ent = com.GetEntity(entRef)
+            ent.IsValueType
+        | _ -> false
     | _ -> false
 
 let tryPyConstructor (com: IPythonCompiler) ctx ent =
@@ -698,9 +703,10 @@ let makeBuiltinTypeAnnotation com ctx typ repeatedGenerics kind =
     match kind with
     | Replacements.Util.BclGuid -> stdlibModuleTypeHint com ctx "uuid" "UUID" [] repeatedGenerics
     | Replacements.Util.FSharpReference genArg ->
-        // In F#, struct instance method's `this` parameter is represented as inref<StructType>,
+        // For struct instance methods, `this` is represented as inref<StructType> in F#,
         // but in Python the struct is passed directly, not wrapped in FSharpRef.
-        if isInRefOrAnyType com typ then
+        // For regular byref/inref/outref parameters, we use FSharpRef.
+        if isStructInRefType com typ then
             typeAnnotation com ctx repeatedGenerics genArg
         else
             let resolved, stmts = resolveGenerics com ctx [ genArg ] repeatedGenerics

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

@@ -82,7 +82,6 @@ let getMemberArgsAndBody (com: IPythonCompiler) ctx kind hasSpread (args: Fable.
                 Set.difference (Annotation.getGenericTypeParams [ thisArg.Type ]) ctx.ScopedTypeParams
 
             let body =
-                // TODO: If ident is not captured maybe we can just replace it with "this"
                 if isIdentUsed thisArg.Name body then
                     let thisKeyword = Fable.IdentExpr { thisArg with Name = "self" }
 
@@ -365,14 +364,23 @@ let transformCast (com: IPythonCompiler) (ctx: Context) t e : Expression * State
 
         // Wrap ResizeArray (Python list) when cast to IEnumerable
         // Python lists don't implement IEnumerable_1, so they need wrapping
+        // Optimization: If ResizeArray was created via of_seq from IEnumerable, use the original arg
         | Types.ienumerableGeneric, _ when
             match e.Type with
             | Fable.Array(_, Fable.ArrayKind.ResizeArray) -> true
             | Fable.DeclaredType(entRef, _) when entRef.FullName = Types.resizeArray -> true
             | _ -> false
             ->
             let listExpr, stmts = com.TransformAsExpr(ctx, e)
-            libCall com ctx None "util" "to_enumerable" [ listExpr ], stmts
+            // Check if the expression is of_seq(arg) - if so, use arg directly (already IEnumerable)
+            match listExpr with
+            | Expression.Call {
+                                  Func = Expression.Name { Id = Identifier "of_seq" }
+                                  Args = [ innerArg ]
+                              } ->
+                // Skip both of_seq and to_enumerable - use original IEnumerable directly
+                innerArg, stmts
+            | _ -> libCall com ctx None "util" "to_enumerable" [ listExpr ], stmts
 
         | _ -> com.TransformAsExpr(ctx, e)
     | Fable.Number(Float32, _), _ ->
@@ -601,7 +609,59 @@ let transformObjectExpr
     // A generic class nested in another generic class cannot use same type variables. (PEP-484)
     let ctx = { ctx with TypeParamsScope = ctx.TypeParamsScope + 1 }
 
+    // Check if any member body uses ThisValue from an outer scope (e.g., inside a constructor).
+    // ThisValue specifically represents `self` in a constructor/method context.
+    // Note: IsThisArgument identifiers are captured via default arguments (x: Any=x),
+    // so we only need to handle explicit ThisValue here.
+    let usesOuterThis =
+        members
+        |> List.exists (fun memb ->
+            memb.Body
+            |> deepExists (
+                function
+                | Fable.Value(Fable.ThisValue _, _) -> true
+                | _ -> false
+            )
+        )
+
+    // Only generate capture statement if outer this is actually used.
+    // This allows inner class methods to reference the outer instance via "_this"
+    // while using standard "self" for the inner instance (satisfies Pylance).
+    let thisCaptureStmts =
+        if usesOuterThis then
+            let anyType = stdlibModuleAnnotation com ctx "typing" "Any" []
+
+            [
+                Statement.assign (Expression.name "_this", anyType, value = Expression.name "self")
+            ]
+        else
+            []
+
+    // Replace ThisValue in the body with an identifier reference to "_this"
+    // This ensures that outer self references correctly bind to the captured variable
+    let replaceThisValue (body: Fable.Expr) =
+        if usesOuterThis then
+            body
+            |> visitFromInsideOut (
+                function
+                | Fable.Value(Fable.ThisValue typ, r) ->
+                    Fable.IdentExpr
+                        {
+                            Name = "_this"
+                            Type = typ
+                            IsMutable = false
+                            IsThisArgument = false
+                            IsCompilerGenerated = true
+                            Range = r
+                        }
+                | e -> e
+            )
+        else
+            body
+
     let makeMethod prop hasSpread (fableArgs: Fable.Ident list) (fableBody: Fable.Expr) decorators =
+        let fableBody = replaceThisValue fableBody
+
         let args, body, returnType =
             getMemberArgsAndBody com ctx (Attached(isStatic = false)) hasSpread fableArgs fableBody
 
@@ -614,16 +674,7 @@ let transformObjectExpr
             com.GetIdentifier(ctx, Naming.toPythonNaming name)
 
         let self = Arg.arg "self"
-
-        let args =
-            match decorators with
-            // Remove extra parameters from getters, i.e __unit=None
-            | [ Expression.Name { Id = Identifier "property" } ] ->
-                { args with
-                    Args = [ self ]
-                    Defaults = []
-                }
-            | _ -> { args with Args = self :: args.Args }
+        let args = { args with Args = self :: args.Args }
 
         // Calculate type parameters for generic object expression methods
         let argTypes = fableArgs |> List.map _.Type
@@ -666,12 +717,16 @@ let transformObjectExpr
             | Fable.Lambda(arg, body, _) -> [ arg ], body
             | _ -> memb.Args, memb.Body
 
+        // Replace ThisValue with this_ identifier for outer self references
+        let body = replaceThisValue body
+
         let args', body', returnType =
             getMemberArgsAndBody com ctx (NonAttached memb.Name) false args body
 
         let name = com.GetIdentifier(ctx, Naming.toPythonNaming memb.Name)
         let self = Arg.arg "self"
         let args' = { args' with Args = self :: args'.Args }
+
         let argTypes = args |> List.map _.Type
         let typeParams = Annotation.calculateMethodTypeParams com ctx argTypes body.Type
 
@@ -716,7 +771,7 @@ let transformObjectExpr
 
     let stmt = Statement.classDef (name, body = classBody, bases = interfaces)
 
-    Expression.call (Expression.name name), [ stmt ] @ stmts
+    Expression.call (Expression.name name), thisCaptureStmts @ [ stmt ] @ stmts
 
 
 let transformCallArgs

src/Fable.Transforms/Python/Replacements.fs

@@ -1728,11 +1728,18 @@ let resizeArrays (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (this
     | ".ctor", _, [] -> makeResizeArray (getElementType t) [] |> Some
     | ".ctor", _, [ ExprType(Number _) ] -> makeResizeArray (getElementType t) [] |> Some
     | ".ctor", _, [ ArrayOrListLiteral(vals, _) ] -> makeResizeArray (getElementType t) vals |> Some
-    // Use Array constructor which accepts both Iterable and IEnumerable_1
-    | ".ctor", _, args ->
-        Helper.LibCall(com, "array", "Array", t, args, ?loc = r)
-        |> withTag "array"
-        |> Some
+    // When a ResizeArray is cast to IEnumerable and passed to ResizeArray constructor,
+    // unwrap the cast since list() can handle lists directly (avoids to_enumerable wrapper)
+    | ".ctor", _, [ TypeCast(innerExpr, DeclaredType(ent, _)) ] when
+        ent.FullName = Types.ienumerableGeneric
+        && match innerExpr.Type with
+           | Array(_, ResizeArray) -> true
+           | DeclaredType(entRef, _) when entRef.FullName = Types.resizeArray -> true
+           | _ -> false
+        ->
+        Helper.GlobalCall("list", t, [ innerExpr ], ?loc = r) |> Some
+    // Use resize_array.of_seq to create a list from IEnumerable_1 or any iterable
+    | ".ctor", _, args -> Helper.LibCall(com, "resize_array", "of_seq", t, args, ?loc = r) |> Some
     | "get_Item", Some ar, [ idx ] -> getExpr r t ar idx |> Some
     | "set_Item", Some ar, [ idx; value ] -> setExpr r ar idx value |> Some
     | "Add", Some ar, [ arg ] ->

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

@@ -9,6 +9,11 @@
 from .util import to_iterable
 
 
+def of_seq[T](items: Iterable[T] | IEnumerable_1[T]) -> list[T]:
+    """Create a ResizeArray (Python list) from an iterable or IEnumerable_1."""
+    return list(to_iterable(items))
+
+
 def exists[T](predicate: Callable[[T], bool], xs: list[T]) -> bool:
     """Test if a predicate is true for at least one element in a list."""
     return any(predicate(x) for x in xs)
@@ -146,6 +151,7 @@ def contains[T](value: T, xs: list[T], cons: Any | None = None) -> bool:
     "index_of",
     "insert_range_in_place",
     "iterate",
+    "of_seq",
     "remove",
     "remove_all_in_place",
     "remove_range",