[Python] More typing fixes (#4334)

Author: dbrattli

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

@@ -306,11 +306,16 @@ let stdlibModuleAnnotation (com: IPythonCompiler) ctx moduleName memberName args
             match args with
             | Expression.Name { Id = Identifier Ellipsis } :: _xs -> Expression.ellipsis
             | _ ->
-                args
-                |> List.removeAt (args.Length - 1)
+                let argsWithoutReturn = args |> List.removeAt (args.Length - 1)
+
+                argsWithoutReturn
                 |> List.choose (
                     function
-                    | Expression.Name { Id = Identifier "None" } when args.Length = 2 -> None
+                    // Filter out None (unit) only when it's the sole argument.
+                    // F# `unit -> T` means "takes no args" in Python: Callable[[], T]
+                    // But `unit -> 'a -> T` uncurried to `(unit, 'a) -> T` must keep
+                    // None to match the actual function signature with unit parameter.
+                    | Expression.Name { Id = Identifier "None" } when argsWithoutReturn.Length = 1 -> None
                     | x -> Some x
                 )
                 |> Expression.list
@@ -441,7 +446,7 @@ let rec typeAnnotation
         fableModuleAnnotation com ctx "option" "Option" [ Expression.none ], []
     | Fable.Option(genArg, _) ->
         // Must match mustWrapOption logic in Transforms.Util.fs
-        // Wrap when: Any, Unit, GenericParam, or nested Option
+        // Wrap when: Any, Unit, GenericParam, nested Option, or callable with generic params
         match genArg with
         | Fable.Option _
         | Fable.Any
@@ -450,8 +455,15 @@ let rec typeAnnotation
             // Use full Option type annotation (code will use SomeWrapper)
             let resolved, stmts = resolveGenerics com ctx [ genArg ] repeatedGenerics
             fableModuleAnnotation com ctx "option" "Option" resolved, stmts
+        | Fable.LambdaType _
+        | Fable.DelegateType _ when containsGenericParams genArg ->
+            // Callable types with generic parameters (e.g., Callable[[_A], _B])
+            // Must use Option[T] form because runtime wraps with SomeWrapper
+            let resolved, stmts = resolveGenerics com ctx [ genArg ] repeatedGenerics
+            fableModuleAnnotation com ctx "option" "Option" resolved, stmts
         | _ ->
-            // For concrete types, erase to T | None (simpler, no wrapper needed)
+            // For concrete types (including DeclaredTypes with generics like FSharpList[T]),
+            // erase to T | None (simpler, no wrapper needed)
             let resolved, stmts = typeAnnotation com ctx repeatedGenerics genArg
             Expression.binOp (resolved, BitOr, Expression.none), stmts
     | Fable.Tuple(genArgs, _) -> makeGenericTypeAnnotation com ctx "tuple" genArgs repeatedGenerics, []

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

@@ -1109,19 +1109,36 @@ let transformCall (com: IPythonCompiler) ctx range callee (callInfo: Fable.CallI
 let transformCurriedApply com ctx range (TransformExpr com ctx (applied, stmts)) args =
     ((applied, stmts), args)
     ||> List.fold (fun (applied, stmts) arg ->
-        let args, stmts' =
+        let args, argStmts =
             match arg with
             // TODO: If arg type is unit but it's an expression with potential
             // side-effects, we need to extract it and execute it before the call
 
             // TODO: discardUnitArg may still be needed in some cases
             | Fable.Value(Fable.UnitConstant, _) -> [], []
             | Fable.IdentExpr ident when ident.Type = Fable.Unit -> [], []
-            | TransformExpr com ctx (arg, stmts') -> [ arg ], stmts'
+            | arg ->
+                let argExpr, transformStmts = com.TransformAsExpr(ctx, arg)
+                // When a Call or CurriedApply result is passed as an argument,
+                // check if we need to erase Option[T] to T | None
+                let argExpr =
+                    if needsOptionEraseForBinding arg arg.Type then
+                        wrapInOptionErase com ctx argExpr
+                    else
+                        argExpr
+
+                [ argExpr ], transformStmts
 
-        callFunction range applied args [], stmts @ stmts'
+        callFunction range applied args [], stmts @ argStmts
     )
 
+/// Extract the expected return type from a return strategy, unwrapping ResourceManager if needed
+let rec private getExpectedReturnType (strategy: ReturnStrategy option) =
+    match strategy with
+    | Some(Return(Some expectedType)) -> Some expectedType
+    | Some(ResourceManager inner) -> getExpectedReturnType inner
+    | _ -> None
+
 let transformCallAsStatements com ctx range t returnStrategy callee callInfo =
     let argsLen (i: Fable.CallInfo) =
         List.length i.Args
@@ -1141,17 +1158,18 @@ let transformCallAsStatements com ctx range t returnStrategy callee callInfo =
     | _ ->
         let expr, stmts = transformCall com ctx range callee callInfo
         // Check if we need to cast Option[T] to T | None for return statements
+        // Also handles ResourceManager-wrapped return strategies (e.g., inside `with` blocks)
         let expr =
-            match returnStrategy with
-            | Some(Return(Some expectedType)) ->
+            match getExpectedReturnType returnStrategy with
+            | Some expectedType ->
                 // Create a temporary Fable.Call to check if erase is needed
                 let callExpr = Fable.Call(callee, callInfo, t, range)
 
                 if needsOptionEraseForReturn callExpr expectedType then
                     wrapInOptionErase com ctx expr
                 else
                     expr
-            | _ -> expr
+            | None -> expr
 
         stmts @ (expr |> resolveExpr ctx t returnStrategy)
 
@@ -3996,7 +4014,14 @@ let transformUnion (com: IPythonCompiler) ctx (ent: Fable.Entity) (entName: stri
                     // 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
-                    let ta, _ = Annotation.typeAnnotation com caseCtx None field.FieldType
+                    // Uncurry lambda types for field annotations since union case fields
+                    // store uncurried functions at runtime (same as record fields)
+                    let fieldType =
+                        match field.FieldType with
+                        | Fable.LambdaType _ -> FableTransforms.uncurryType field.FieldType
+                        | _ -> field.FieldType
+
+                    let ta, _ = Annotation.typeAnnotation com caseCtx None fieldType
                     let target = Expression.name (fieldName, Store)
                     // Use annAssign to generate: field: type (not field = type)
                     Statement.annAssign (target, annotation = ta, simple = true)
@@ -4763,6 +4788,12 @@ let rec transformDeclaration (com: IPythonCompiler) ctx (decl: Fable.Declaration
                     let value, stmts = transformAsExpr com ctx decl.Body
                     let name = com.GetIdentifier(ctx, Naming.toPythonNaming decl.Name)
                     let ta, _ = Annotation.typeAnnotation com ctx None decl.Body.Type
+                    // Erase Option wrapper if needed (Call/CurriedApply returning Option[T] to T | None)
+                    let value =
+                        if needsOptionEraseForBinding decl.Body decl.Body.Type then
+                            wrapInOptionErase com ctx value
+                        else
+                            value
 
                     stmts @ declareModuleMember com ctx info.IsPublic name (Some ta) value
                 else

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

@@ -100,34 +100,56 @@ module Util =
         || hasAttribute Atts.emitIndexer atts
         || hasAttribute Atts.emitProperty atts
 
+    /// Check if a type contains any generic parameters (recursively).
+    /// Used to determine if Option<T> should use Option[T] annotation vs T | None.
+    /// Note: This is duplicated from Annotation.fs due to compilation order (Util.fs compiles first).
+    let private containsGenericParams (t: Fable.Type) =
+        FSharp2Fable.Util.getGenParamNames [ t ] |> List.isEmpty |> not
+
     /// Check if an Option type has a concrete inner type (erases to T | None).
     /// Returns true when inner type is concrete, meaning Option[T] -> T | None.
     let hasConcreteOptionInner (typ: Fable.Type) =
         match typ with
         | Fable.Option(innerType, _) -> not (mustWrapOption innerType)
         | _ -> false
 
+    /// Check if inner type of Option is a callable with generic params (needs wrapping)
+    let private isCallableWithGenerics (t: Fable.Type) =
+        match t with
+        | Fable.LambdaType _
+        | Fable.DelegateType _ -> containsGenericParams t
+        | _ -> false
+
     /// Check if a call expression needs Option erase from Option[T] to T | None.
     /// When target type is Option<ConcreteType> (erased to T | None), we need to erase
     /// because library functions use Option[T] (wrapped form) in their signatures,
     /// but actual runtime values are not wrapped for concrete types.
-    let private needsOptionEraseForCall (targetType: Fable.Type) =
+    /// Also handles function types (LambdaType) where the return type contains a concrete Option,
+    /// using the erase() overload for Callable[..., Option[T]] -> Callable[..., T | None].
+    let rec private needsOptionEraseForCall (targetType: Fable.Type) =
         match targetType with
-        | Fable.Option(tgtInner, _) when not (mustWrapOption tgtInner) -> true
+        // Don't erase if inner type is callable with generic params - both annotation and runtime use wrapped form
+        | Fable.Option(tgtInner, _) when not (mustWrapOption tgtInner) && not (isCallableWithGenerics tgtInner) -> true
+        // Handle function types where return type is a concrete Option
+        // erase() has an overload: Callable[..., Option[T]] -> Callable[..., T | None]
+        | Fable.LambdaType(_, returnType) -> needsOptionEraseForCall returnType
+        | Fable.DelegateType(_, returnType) -> needsOptionEraseForCall returnType
         | _ -> false
 
     /// Check if binding needs Option erase from Option[T] to T | None.
     let needsOptionEraseForBinding (value: Fable.Expr) (targetType: Fable.Type) =
         match value with
-        | Fable.Call _ -> needsOptionEraseForCall targetType
+        | Fable.Call _
+        | Fable.CurriedApply _ -> needsOptionEraseForCall targetType
         | _ -> false
 
     /// Check if return expression needs Option erase from Option[T] to T | None.
     /// Used when returning from a function where the expected return type is T | None
     /// but the actual expression returns Option[T] (wrapped form).
     let needsOptionEraseForReturn (value: Fable.Expr) (expectedReturnType: Fable.Type) =
         match value with
-        | Fable.Call _ -> needsOptionEraseForCall expectedReturnType
+        | Fable.Call _
+        | Fable.CurriedApply _ -> needsOptionEraseForCall expectedReturnType
         | _ -> false
 
     /// Recursively check if a type contains Option (wrapped or erased).