Fix [ with Python objects in : slice args

NEWS.md

@@ -5,6 +5,9 @@
 
 - Updates for Positron to fix issues with `repl_python()` and Variables Pane (#1755).
 
+- Fixed an issue where `[` received Python objects as slice arguments.
+  e.g., `x[start:end]` when `start` or `end` were Python objects (#1731).
+
 - Reticulate-managed `uv` can now resolve system-installed Pythons,
   supporting platforms where pre-built binaries are unavailable, such as
   musl-based Alpine Linux (#1751, #1752).

src/python.cpp

@@ -4619,22 +4619,34 @@ PyObjectRef py_capsule(SEXP x) {
 PyObjectRef py_slice(SEXP start = R_NilValue, SEXP stop = R_NilValue, SEXP step = R_NilValue) {
   GILScope _gil;
 
-  PyObjectPtr start_, stop_, step_;
+  auto coerce_slice_arg = [](SEXP x) -> PyObject* {
+    if (x == R_NilValue) {
+      return NULL;
+    }
+    if (TYPEOF(x) == INTSXP || TYPEOF(x) == REALSXP) {
+      return PyLong_FromLong(Rf_asInteger(x));
+    }
+    if (is_py_object(x)) {
+      PyObject* pyobj = PyObjectRef(x, false).get();
+      Py_IncRef(pyobj);
+      return pyobj;
+    }
+    return r_to_py(x, false);
+  };
 
-  if (start != R_NilValue)
-    start_.assign(PyLong_FromLong(Rf_asInteger(start)));
-  if (stop != R_NilValue)
-    stop_.assign(PyLong_FromLong(Rf_asInteger(stop)));
-  if (step != R_NilValue)
-    step_.assign(PyLong_FromLong(Rf_asInteger(step)));
+  PyObjectPtr start_(coerce_slice_arg(start));
+  PyObjectPtr  stop_(coerce_slice_arg(stop));
+  PyObjectPtr  step_(coerce_slice_arg(step));
 
-  PyObject* out(PySlice_New(start_, stop_, step_));
+  PyObject* out = PySlice_New(start_, stop_, step_);
   if (out == NULL)
     throw PythonException(py_fetch_error());
+
   return py_ref(out, false);
 }
 
 
+
 //' @rdname iterate
 //' @export
 // [[Rcpp::export]]

tests/testthat/test-python-base-r-generics.R

@@ -169,3 +169,24 @@ test_that("[ can infer slices, multiple args", {
   expect_identical(py_eval("x"), py_to_r(x))
 
 })
+
+
+test_that("[ passes through python objects", {
+
+  skip_if_no_numpy()
+
+  np <- import("numpy", convert = FALSE)
+
+  x <- np$arange(10L)
+  ir <- 3L
+  ip <- np$array(3L)
+
+  expect_equal(py_to_r(x[ir]), 3L)
+  expect_equal(py_to_r(x[ip]), 3L)
+
+  expect_equal(py_to_r(x[ir:NA]), array(3:9))
+  expect_equal(py_to_r(x[ip:NA]), array(3:9))
+  expect_equal(py_to_r(x[NA:ir]), array(0:2))
+  expect_equal(py_to_r(x[NA:ip]), array(0:2))
+
+})