[Python] Fix Array.length/.Length to use len() for plain list interop (#4342)

Author: dbrattli

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 `Array.length`, `.Length`, `Array.isEmpty`, and `ResizeArray.Count` to use `len()` instead of `.length` property for plain Python list interop (by @dbrattli)
 * [Python] Fix `Task<T>` pass-through returns not being awaited in if/else and try/with branches (by @dbrattli)
 * [Python] Fix `:? T as x` type test pattern in closures causing `UnboundLocalError` due to `cast()` shadowing outer variable (by @dbrattli)
 

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 `Array.length`, `.Length`, `Array.isEmpty`, and `ResizeArray.Count` to use `len()` instead of `.length` property for plain Python list interop (by @dbrattli)
 * [Python] Fix `Task<T>` pass-through returns not being awaited in if/else and try/with branches (by @dbrattli)
 * [Python] Fix `:? T as x` type test pattern in closures causing `UnboundLocalError` due to `cast()` shadowing outer variable (by @dbrattli)
 

src/Fable.Transforms/Python/Replacements.fs

@@ -1760,12 +1760,9 @@ let resizeArrays (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (this
     | "GetEnumerator", Some ar, _ -> getEnumerator com r t ar |> Some
     | "get_Count", Some(MaybeCasted(ar)), _ ->
         match ar.Type with
-        // ResizeArray is Python list - use len() wrapped in int32()
-        | Array(_, ResizeArray) ->
+        | Array _ ->
             let lenExpr = Helper.GlobalCall("len", Int32.Number, [ ar ], ?loc = r)
             Helper.LibCall(com, "core", "int32", t, [ lenExpr ], ?loc = r) |> Some
-        // MutableArray/ImmutableArray are FSharpArray (Rust) with .length property returning Int32
-        | Array _ -> getFieldWith r t ar "length" |> Some
         | _ -> Helper.LibCall(com, "util", "count", t, [ ar ], ?loc = r) |> Some
     | "Clear", Some ar, _ -> Helper.LibCall(com, "Util", "clear", t, [ ar ], ?loc = r) |> Some
     | "Find", Some ar, [ arg ] ->
@@ -1894,8 +1891,8 @@ let arrays (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (thisArg: E
     // printfn "arrays: %A" i.CompiledName
     match i.CompiledName, thisArg, args with
     | "get_Length", Some arg, _ ->
-        // All arrays in Python are FSharpArray (Rust) which has .length property returning Int32
-        getFieldWith r t arg "length" |> Some
+        let lenExpr = Helper.GlobalCall("len", Int32.Number, [ arg ], ?loc = r)
+        Helper.LibCall(com, "core", "int32", t, [ lenExpr ], ?loc = r) |> Some
     | "get_Item", Some arg, [ idx ] -> getExpr r t arg idx |> Some
     | "set_Item", Some arg, [ idx; value ] -> setExpr r arg idx value |> Some
     | "Copy", None, [ _source; _sourceIndex; _target; _targetIndex; _count ] -> copyToArray com r t i args
@@ -1946,13 +1943,8 @@ let arrayModule (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (_: Ex
         Helper.LibCall(com, "list", "of_array", t, args, i.SignatureArgTypes, ?loc = r)
         |> Some
     | ("Length" | "Count"), [ arg ] ->
-        match arg.Type with
-        // ResizeArray is Python list - use len() wrapped in int32()
-        | Array(_, ResizeArray) ->
-            let lenExpr = Helper.GlobalCall("len", Int32.Number, [ arg ], ?loc = r)
-            Helper.LibCall(com, "core", "int32", t, [ lenExpr ], ?loc = r) |> Some
-        // MutableArray/ImmutableArray are FSharpArray (Rust) with .length property returning Int32
-        | _ -> getFieldWith r t arg "length" |> Some
+        let lenExpr = Helper.GlobalCall("len", Int32.Number, [ arg ], ?loc = r)
+        Helper.LibCall(com, "core", "int32", t, [ lenExpr ], ?loc = r) |> Some
     | "Item", [ idx; ar ] -> getExpr r t ar idx |> Some
     | "Get", [ ar; idx ] -> getExpr r t ar idx |> Some
     | "Set", [ ar; idx; value ] -> setExpr r ar idx value |> Some
@@ -1969,8 +1961,8 @@ let arrayModule (com: ICompiler) (ctx: Context) r (t: Type) (i: CallInfo) (_: Ex
         // Use library function to create empty FSharpArray (Rust) instead of raw Python list
         Helper.LibCall(com, "array", "empty", t, [], ?loc = r) |> Some
     | "IsEmpty", [ ar ] ->
-        // Use .length property (Int32) instead of len() which returns Python int
-        eq (getFieldWith r Int32.Number ar "length") (makeIntConst 0) |> Some
+        eq (Helper.GlobalCall("len", Int32.Number, [ ar ], ?loc = r)) (makeIntConst 0)
+        |> Some
     | "Concat", [ ar1; ar2 ] -> makeBinOp r t ar1 ar2 BinaryPlus |> Some
     | Patterns.DicContains nativeArrayFunctions meth, _ ->
         let args, thisArg = List.splitLast args

tests/Python/TestPyInterop.fs

@@ -902,4 +902,24 @@ let ``test Pydantic model_dump_json with float array`` () =
     json.Contains("1.5") |> equal true
     json.Contains("2.5") |> equal true
 
+// Regression tests: Array.length/.Length/Array.isEmpty must use len() so they
+// work on plain Python lists (e.g. from Emit, unbox, native APIs), not just FSharpArray.
+
+[<Fact>]
+let ``test Array.length works on plain Python list`` () =
+    let xs: int[] = emitPyExpr () "[1, 2, 3]"
+    Array.length xs |> equal 3
+
+[<Fact>]
+let ``test .Length works on plain Python list`` () =
+    let xs: int[] = emitPyExpr () "[10, 20]"
+    xs.Length |> equal 2
+
+[<Fact>]
+let ``test Array.isEmpty works on plain Python list`` () =
+    let xs: int[] = emitPyExpr () "[1]"
+    Array.isEmpty xs |> equal false
+    let ys: int[] = emitPyExpr () "[]"
+    Array.isEmpty ys |> equal true
+
 #endif