[Python] Fix String.Concat spread operator issue in Python transpilation (#4298)

* [Python] Fix String.Concat spread operator issue in Python transpilation

Author: dbrattli

pyproject.toml

@@ -47,24 +47,29 @@ reportUnusedVariable = false
 reportUnnecessaryIsInstance = true
 reportUnnecessaryComparison = true
 reportUnnecessaryCast = true
-reportPrivateUsage = true
+reportPrivateUsage = false # Getters/setters reference private members from outside the class
 reportImportCycles = true
 reportDuplicateImport = true
 reportConstantRedefinition = true
 reportOverlappingOverload = true
 reportInconsistentConstructor = true
 reportImplicitStringConcatenation = true
+
 pythonVersion = "3.12"
 typeCheckingMode = "standard"
 
 [tool.ruff]
-# Keep in sync with .pre-commit-config.yaml
 line-length = 120
-lint.ignore = []
-lint.select = ["E", "W",  "F", "I", "T", "RUF", "TID", "UP"]
 target-version = "py312"
+respect-gitignore = false
 include =["*.py"]
 
+[tool.ruff.lint]
+select = ["E", "W",  "F", "I", "T", "RUF", "TID", "UP"]
+ignore = [
+    "F841" # Ignore unused variable warnings
+]
+
 [tool.ruff.lint.flake8-bugbear]
 # These Rust extension module types are immutable (frozen)
 extend-immutable-calls = [
@@ -91,4 +96,4 @@ minversion = "8.0"
 pythonpath = ["temp/tests/Python", "."]
 testpaths = [
     "temp/tests/Python",
-]
+]

src/Fable.Cli/CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [Python] Fix String.Concat spread operator issue in Python transpilation (by @dbrattli)
 * [Python] Fix regression `[<Erase>]` on class types not preventing them from being emitted to Python (by @dbrattli)
 * [Python] Fix regression `%A` format specifier to output booleans as lowercase `true`/`false` (by @dbrattli)
 * [Python] Fix various bugs in fable-library numeric types and string operations (by @dbrattli)

src/Fable.Compiler/CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [Python] Fix String.Concat spread operator issue in Python transpilation (by @dbrattli)
 * [Python] Fix regression `[<Erase>]` on class types not preventing them from being emitted to Python (by @dbrattli)
 * [Python] Fix regression `%A` format specifier to output booleans as lowercase `true`/`false` (by @dbrattli)
 * [Python] Fix various bugs in fable-library numeric types and string operations (by @dbrattli)

src/Fable.Transforms/Python/Replacements.fs

@@ -1486,11 +1486,9 @@ let strings (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Expr opt
     | "Concat", None, _ ->
         match i.SignatureArgTypes with
         | [ MaybeNullable(Array _) | MaybeNullable(IEnumerable) ] ->
-            Helper.LibCall(com, "string", "join", t, ((makeStrConst "") :: args), ?loc = r)
-            |> Some
-        | _ ->
-            Helper.LibCall(com, "string", "concat", t, args, hasSpread = true, ?loc = r)
+            Helper.LibCall(com, "string", "join", t, makeStrConst "" :: args, ?loc = r)
             |> Some
+        | _ -> Helper.LibCall(com, "string", "concat", t, args, ?loc = r) |> Some
     | "CompareOrdinal", None, _ -> Helper.LibCall(com, "string", "compare_ordinal", t, args, ?loc = r) |> Some
     | Patterns.SetContains implementedStringFunctions, thisArg, args ->
         Helper.LibCall(

src/fable-library-py/fable_library/core/strings.pyi

@@ -47,7 +47,7 @@ def initialize(n: int, f: Callable[[int], str]) -> str: ...
 def insert(string: str, start_index: int, value: str) -> str: ...
 def is_null_or_empty(string: str | None) -> bool: ...
 def is_null_or_white_space(string: str | None) -> bool: ...
-def concat(strings: Any) -> str: ...
+def concat(*strings: str) -> str: ...
 def join(delimiter: str, strings: Any) -> str: ...
 def pad_left(string: str, length: int, ch: str | None = None, is_right: bool = False) -> str: ...
 def pad_right(string: str, length: int, ch: str | None = None) -> str: ...

src/fable-library-py/src/strings.rs

@@ -1221,39 +1221,11 @@ mod formatting {
         string.is_none_or(|s| s.trim().is_empty())
     }
 
-    /// Concatenate strings - supports both F# varargs and iterable patterns
+    /// Concatenate strings - varargs of strings
     #[pyfunction]
-    #[pyo3(signature = (*args))]
-    pub fn concat(args: &Bound<'_, pyo3::types::PyTuple>) -> PyResult<String> {
-        match args.len() {
-            0 => Ok(String::new()),
-            1 => {
-                // Single argument - check if it's iterable
-                if let Ok(iter) = args.get_item(0)?.try_iter() {
-                    // Zero-cost abstraction: iterator chain with collect
-                    Ok(iter
-                        .map(|item| -> PyResult<String> {
-                            let item = item?;
-                            Ok(item.str()?.to_string())
-                        })
-                        .collect::<PyResult<Vec<_>>>()?
-                        .join(""))
-                } else {
-                    // Single non-iterable argument
-                    Ok(args.get_item(0)?.str()?.to_string())
-                }
-            }
-            _ => {
-                // Multiple arguments - zero-cost iterator chain
-                Ok((0..args.len())
-                    .map(|i| -> PyResult<String> {
-                        let item = args.get_item(i)?;
-                        Ok(item.str()?.to_string())
-                    })
-                    .collect::<PyResult<Vec<_>>>()?
-                    .join(""))
-            }
-        }
+    #[pyo3(signature = (*strings))]
+    pub fn concat(strings: Vec<String>) -> String {
+        strings.join("")
     }
 
     /// Join strings with delimiter