PyAPI: support evaluating an expression as a string or None

Utility functions will be used as part of !120283.

source/blender/python/BPY_extern_run.h

@@ -183,6 +183,31 @@ bool BPY_run_string_as_string(struct bContext *C,
                               struct BPy_RunErrInfo *err_info,
                               char **r_value) ATTR_NONNULL(1, 3, 5);
 
+/**
+ * Evaluate `expr` as a string or None.
+ * Where a success return value with `*r_value == nullptr` indicates a value of None.
+ *
+ * \param C: See \ref common_args.
+ * \param imports: See \ref common_args.
+ * \param expr: The expression to evaluate.
+ * \param err_info: See \ref common_args.
+ * \param r_value: The resulting value.
+ * \return Success.
+ */
+bool BPY_run_string_as_string_and_len_or_none(struct bContext *C,
+                                              const char *imports[],
+                                              const char *expr,
+                                              struct BPy_RunErrInfo *err_info,
+                                              char **r_value,
+                                              size_t *r_value_len) ATTR_NONNULL(1, 3, 5, 6);
+
+/** See #BPY_run_string_as_string_and_len */
+bool BPY_run_string_as_string_or_none(struct bContext *C,
+                                      const char *imports[],
+                                      const char *expr,
+                                      struct BPy_RunErrInfo *err_info,
+                                      char **r_value) ATTR_NONNULL(1, 3, 5);
+
 /** \} */
 
 #ifdef __cplusplus

source/blender/python/generic/py_capi_utils.cc

@@ -1624,6 +1624,64 @@ bool PyC_RunString_AsString(const char *imports[],
   return PyC_RunString_AsStringAndSize(imports, expr, filename, r_value, &value_size);
 }
 
+bool PyC_RunString_AsStringAndSizeOrNone(const char *imports[],
+                                         const char *expr,
+                                         const char *filename,
+                                         char **r_value,
+                                         size_t *r_value_size)
+{
+  PyObject *py_dict, *retval;
+  bool ok = true;
+  PyObject *main_mod = nullptr;
+
+  PyC_MainModule_Backup(&main_mod);
+
+  py_dict = PyC_DefaultNameSpace(filename);
+
+  if (imports && !PyC_NameSpace_ImportArray(py_dict, imports)) {
+    ok = false;
+  }
+  else if ((retval = PyRun_String(expr, Py_eval_input, py_dict, py_dict)) == nullptr) {
+    ok = false;
+  }
+  else {
+    if (retval == Py_None) {
+      *r_value = nullptr;
+      *r_value_size = 0;
+    }
+    else {
+      const char *val;
+      Py_ssize_t val_len;
+
+      val = PyUnicode_AsUTF8AndSize(retval, &val_len);
+      if (val == nullptr && PyErr_Occurred()) {
+        ok = false;
+      }
+      else {
+        char *val_alloc = static_cast<char *>(MEM_mallocN(val_len + 1, __func__));
+        memcpy(val_alloc, val, val_len + 1);
+        *r_value = val_alloc;
+        *r_value_size = val_len;
+      }
+    }
+
+    Py_DECREF(retval);
+  }
+
+  PyC_MainModule_Restore(main_mod);
+
+  return ok;
+}
+
+bool PyC_RunString_AsStringOrNone(const char *imports[],
+                                  const char *expr,
+                                  const char *filename,
+                                  char **r_value)
+{
+  size_t value_size;
+  return PyC_RunString_AsStringAndSizeOrNone(imports, expr, filename, r_value, &value_size);
+}
+
 /** \} */
 
 #endif /* #ifndef MATH_STANDALONE */

source/blender/python/generic/py_capi_utils.h

@@ -241,6 +241,19 @@ bool PyC_RunString_AsString(const char **imports,
                             const char *filename,
                             char **r_value);
 
+/**
+ * \param r_value_size: The length of the string assigned: `strlen(*r_value)`.
+ */
+bool PyC_RunString_AsStringAndSizeOrNone(const char **imports,
+                                         const char *expr,
+                                         const char *filename,
+                                         char **r_value,
+                                         size_t *r_value_size);
+bool PyC_RunString_AsStringOrNone(const char **imports,
+                                  const char *expr,
+                                  const char *filename,
+                                  char **r_value);
+
 /**
  * Use with PyArg_ParseTuple's "O&" formatting.
  *

source/blender/python/intern/bpy_interface_run.cc

@@ -445,6 +445,43 @@ bool BPY_run_string_as_string(
   return BPY_run_string_as_string_and_len(C, imports, expr, err_info, r_value, &value_dummy_len);
 }
 
+bool BPY_run_string_as_string_and_len_or_none(bContext *C,
+                                              const char *imports[],
+                                              const char *expr,
+                                              BPy_RunErrInfo *err_info,
+                                              char **r_value,
+                                              size_t *r_value_len)
+{
+  PyGILState_STATE gilstate;
+  bool ok = true;
+
+  if (expr[0] == '\0') {
+    *r_value = nullptr;
+    return ok;
+  }
+
+  bpy_context_set(C, &gilstate);
+
+  ok = PyC_RunString_AsStringAndSizeOrNone(
+      imports, expr, "<expr as str or none>", r_value, r_value_len);
+
+  if (ok == false) {
+    run_string_handle_error(err_info);
+  }
+
+  bpy_context_clear(C, &gilstate);
+
+  return ok;
+}
+
+bool BPY_run_string_as_string_or_none(
+    bContext *C, const char *imports[], const char *expr, BPy_RunErrInfo *err_info, char **r_value)
+{
+  size_t value_dummy_len;
+  return BPY_run_string_as_string_and_len_or_none(
+      C, imports, expr, err_info, r_value, &value_dummy_len);
+}
+
 bool BPY_run_string_as_intptr(bContext *C,
                               const char *imports[],
                               const char *expr,