[Python] Fix exception variable captured in deferred closures (NameError) (#4382)

Author: dbrattli

src/Fable.Cli/CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 * [Python] Fix `nonlocal`/`global` declarations generated inside `match/case` bodies causing `SyntaxError` (by @dbrattli)
+* [Python] Fix exception variable captured in deferred closures causing `NameError` (PEP 3110 scoping) (by @dbrattli)
 
 ## 5.0.0-rc.2 - 2026-03-03
 

src/Fable.Compiler/CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 * [Python] Fix `nonlocal`/`global` declarations generated inside `match/case` bodies causing `SyntaxError` (by @dbrattli)
+* [Python] Fix exception variable captured in deferred closures causing `NameError` (PEP 3110 scoping) (by @dbrattli)
 
 ## 5.0.0-rc.2 - 2026-03-03
 

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

@@ -1210,9 +1210,29 @@ let transformTryCatch com (ctx: Context) r returnStrategy (body, catch: (Fable.I
     // try .. catch statements cannot be tail call optimized
     let ctx = { ctx with TailCallOpportunity = None }
 
-    let makeHandler exnType handlerBody identifier =
-        let transformedBody = transformBlock com ctx returnStrategy handlerBody
-        ExceptHandler.exceptHandler (``type`` = Some exnType, name = identifier, body = transformedBody)
+    let makeHandler exnType handlerBody (param: Fable.Ident) identifier =
+        let (Identifier identName) = identifier
+
+        // Python's `except E as name:` deletes `name` at the end of the except block.
+        // If the exception variable is used (e.g., captured in a deferred closure), copy
+        // it to a safe local with a unique name that Python won't delete, and rename all
+        // body references to use the safe copy.
+        if isIdentUsed param.Name handlerBody then
+            let safeIdentName = getUniqueNameInDeclarationScope ctx (identName + "_")
+            let safeParam = { param with Name = safeIdentName }
+
+            let renamedBody =
+                FableTransforms.replaceValues (Map [ param.Name, Fable.IdentExpr safeParam ]) handlerBody
+
+            let transformedBody = transformBlock com ctx returnStrategy renamedBody
+            // Annotated assignment: ex_: ExceptionType = ex
+            let saveStmt =
+                Statement.assign (identAsExpr com ctx safeParam, exnType, value = Expression.name identName)
+
+            ExceptHandler.exceptHandler (``type`` = Some exnType, name = identifier, body = saveStmt :: transformedBody)
+        else
+            let transformedBody = transformBlock com ctx returnStrategy handlerBody
+            ExceptHandler.exceptHandler (``type`` = Some exnType, name = identifier, body = transformedBody)
 
     let handlers, handlerStmts =
         match catch with
@@ -1228,7 +1248,8 @@ let transformTryCatch com (ctx: Context) r returnStrategy (body, catch: (Fable.I
                 // No type tests found, use Exception to match F#/.NET semantics.
                 // Users can explicitly catch KeyboardInterrupt, SystemExit, GeneratorExit
                 // using type tests if needed.
-                let handler = makeHandler (Expression.identifier "Exception") catchBody identifier
+                let handler =
+                    makeHandler (Expression.identifier "Exception") catchBody param identifier
 
                 Some [ handler ], []
 
@@ -1239,7 +1260,7 @@ let transformTryCatch com (ctx: Context) r returnStrategy (body, catch: (Fable.I
                     |> List.choose (fun (typ, handlerBody) ->
                         getExceptionTypeExpr com ctx typ
                         |> Option.map (fun (exnTypeExpr, stmts) ->
-                            makeHandler exnTypeExpr handlerBody identifier, stmts
+                            makeHandler exnTypeExpr handlerBody param identifier, stmts
                         )
                     )
                     |> List.unzip
@@ -1249,7 +1270,9 @@ let transformTryCatch com (ctx: Context) r returnStrategy (body, catch: (Fable.I
                 let fallbackHandlers =
                     match fallback with
                     | Some fallbackExpr when not (ExceptionHandling.isReraise fallbackExpr) ->
-                        [ makeHandler (Expression.identifier "Exception") fallbackExpr identifier ]
+                        [
+                            makeHandler (Expression.identifier "Exception") fallbackExpr param identifier
+                        ]
                     | _ -> []
 
                 Some(handlers @ fallbackHandlers), List.concat stmts

tests/Python/TestMisc.fs

@@ -1501,3 +1501,28 @@ let ``test static properties work correctly`` () =
 
     // Test read-only explicit property
     StaticPropertiesTestClass.ReadOnlyProperty |> equal "ReadOnlyValue"
+
+[<Fact>]
+let ``test exception variable captured in deferred zero-arg closure`` () =
+    // Regression: Python deletes the `except ... as name` variable at the end of the
+    // except block. A closure that captures it and is called *after* the block would
+    // get a NameError. Fix: copy to `name_` inside the block and rename references.
+    let getMsg =
+        try
+            failwith "boom"
+            fun () -> "no error"
+        with ex ->
+            fun () -> ex.Message  // captured, called after block exits
+    getMsg () |> equal "boom"
+
+[<Fact>]
+let ``test exception variable captured in deferred single-arg closure`` () =
+    // Same fix, but the closure takes an argument (mirrors the AsyncRx `defer` pattern:
+    // `with ex -> ofAsyncWorker (fun obv _ -> obv.OnErrorAsync ex)`).
+    let sub =
+        try
+            failwith "boom"
+            (fun (_: int) -> "no error")
+        with ex ->
+            (fun (_: int) -> ex.Message)  // lambda-with-arg captures ex
+    sub 42 |> equal "boom"