Fix error when converting empty numpy string arrays

NEWS.md

@@ -1,10 +1,12 @@
 # reticulate (development version)
 
+- Fixed error when converting an empty NumPy char array to R (#1662).
+
 - Fixed error when using reticulate with radian (#1668, #1670).
 
 - Fixed segfault encountered when running Python finalizer (#1663, #1664)
 
-- Fixed segfault encountered in RStudio when rapidly switching 
+- Fixed segfault encountered in RStudio when rapidly switching
   between R and Python chunks in a Quarto document (#1665).
 
 # reticulate 1.39.0

src/python.cpp

@@ -1550,6 +1550,14 @@ SEXP py_to_r_cpp(PyObject* x, bool convert, bool simple) {
       case NPY_VSTRING:
       case NPY_UNICODE: {
 
+        rArray = Rf_allocArray(STRSXP, dimsVector);
+        RObject protectArray(rArray);
+
+        // special case 0-size vectors, because np.nditer() throws:
+        // ValueError: Iteration of zero-sized operands is not enabled
+        if (Rf_length(rArray) == 0)
+          break;
+
         static PyObject* nditerArgs = []() {
           PyObject* flags = PyTuple_New(1);
           // iterating over a StringDType requires us to pass 'refs_ok' flag,
@@ -1559,7 +1567,7 @@ SEXP py_to_r_cpp(PyObject* x, bool convert, bool simple) {
           PyTuple_SetItem(args, 1, flags); // steals ref
           return args;
         }();
-        // PyTuple_SetItem steals reference the array, but it's already wraped
+        // PyTuple_SetItem steals reference the array, but it's already wrapped
         // into PyObjectPtr earlier (so it gets deleted after the scope of this function)
         // To avoid trying to delete it twice, we need to increase its ref count here.
         PyTuple_SetItem(nditerArgs, 0, (PyObject*)array);
@@ -1572,10 +1580,6 @@ SEXP py_to_r_cpp(PyObject* x, bool convert, bool simple) {
           throw PythonException(py_fetch_error());
         }
 
-        rArray = Rf_allocArray(STRSXP, dimsVector);
-        RObject protectArray(rArray);
-
-
         for (int i=0; i<len; i++) {
           PyObjectPtr el(PyIter_Next(iter)); // returns an scalar array.
           PyObjectPtr pyStr(PyObject_CallMethod(el, "item", NULL));

tests/testthat/test-python-numpy.R

@@ -190,3 +190,15 @@ test_that("numpy non-simple arrays work", {
                               c(3, 4)))
 
 })
+
+test_that("0-length character arrays are handled correctly", {
+  # https://github.com/rstudio/reticulate/issues/1662
+  skip_if_no_numpy()
+  np <- import("numpy")
+
+  for (dtype in c("U", "U1", "U10", "O", "S", "S1", "S10")) {
+    expect_equal(np$array(character(), dtype = dtype),
+                 array(character()))
+  }
+})
+