fix: conversion to string in python bindings

If provided `PyObject` can't be converted to string `TypeError` is
 reported instead of `SytemError` without any message.

modules/core/include/opencv2/core/bindings_utils.hpp

@@ -58,6 +58,12 @@ String dumpCString(const char* argument)
     return cv::format("String: %s", argument);
 }
 
+CV_WRAP static inline
+String dumpString(const String& argument)
+{
+    return cv::format("String: %s", argument.c_str());
+}
+
 CV_WRAP static inline
 AsyncArray testAsyncArray(InputArray argument)
 {

modules/python/src2/cv2.cpp

@@ -963,15 +963,31 @@ PyObject* pyopencv_from(const std::string& value)
 template<>
 bool pyopencv_to(PyObject* obj, String &value, const ArgInfo& info)
 {
-    CV_UNUSED(info);
     if(!obj || obj == Py_None)
+    {
         return true;
+    }
     std::string str;
     if (getUnicodeString(obj, str))
     {
         value = str;
         return true;
     }
+    else
+    {
+        // If error hasn't been already set by Python conversion functions
+        if (!PyErr_Occurred())
+        {
+            // Direct access to underlying slots of PyObjectType is not allowed
+            // when limited API is enabled
+#ifdef Py_LIMITED_API
+            failmsg("Can't convert object to 'str' for '%s'", info.name);
+#else
+            failmsg("Can't convert object of type '%s' to 'str' for '%s'",
+                    obj->ob_type->tp_name, info.name);
+#endif
+        }
+    }
     return false;
 }
 

modules/python/test/test_misc.py

@@ -314,7 +314,7 @@ class Arguments(NewOpenCVTests):
 
     def test_parse_to_cstring_convertible(self):
         try_to_convert = partial(self._try_to_convert, cv.utils.dumpCString)
-        for convertible in ('s', 'str', str(123), ('char'), np.str('test1'), np.str_('test2')):
+        for convertible in ('', 's', 'str', str(123), ('char'), np.str('test1'), np.str_('test2')):
             expected = 'string: ' + convertible
             actual = try_to_convert(convertible)
             self.assertEqual(expected, actual,
@@ -326,6 +326,20 @@ class Arguments(NewOpenCVTests):
             with self.assertRaises((TypeError), msg=get_no_exception_msg(not_convertible)):
                 _ = cv.utils.dumpCString(not_convertible)
 
+    def test_parse_to_string_convertible(self):
+        try_to_convert = partial(self._try_to_convert, cv.utils.dumpString)
+        for convertible in (None, '', 's', 'str', str(123), np.str('test1'), np.str_('test2')):
+            expected = 'string: ' + (convertible if convertible else '')
+            actual = try_to_convert(convertible)
+            self.assertEqual(expected, actual,
+                             msg=get_conversion_error_msg(convertible, expected, actual))
+
+    def test_parse_to_string_not_convertible(self):
+        for not_convertible in ((12,), ('t', 'e', 's', 't'), np.array(['123', ]),
+                                np.array(['t', 'e', 's', 't']), 1, True, False):
+            with self.assertRaises((TypeError), msg=get_no_exception_msg(not_convertible)):
+                _ = cv.utils.dumpString(not_convertible)
+
 
 class SamplesFindFile(NewOpenCVTests):