Adding test_isinstance_string_types, with asserts simply matching current behavior.

Basis for discussing actual desired behavior.

Background:
* pybind@77059f0
* pybind#2198

Author: rwgk

tests/test_pytypes.cpp

@@ -367,4 +367,7 @@ TEST_SUBMODULE(pytypes, m) {
             buf, static_cast<ssize_t>(strlen(buf)));
     });
 #endif
+
+    m.def("isinstance_str", [](py::object o) { return py::isinstance<py::str>(o); });
+    m.def("isinstance_bytes", [](py::object o) { return py::isinstance<py::bytes>(o); });
 }

tests/test_pytypes.py

@@ -346,3 +346,21 @@ def test_memoryview_from_memory():
     assert isinstance(view, memoryview)
     assert view.format == 'B'
     assert bytes(view) == b'\xff\xe1\xab\x37'
+
+
+def test_isinstance_string_types():
+    actual_bytes = b""
+    # Requires Python 2 or >= 3.3 (https://www.python.org/dev/peps/pep-0414/):
+    actual_unicode = u""
+    assert m.isinstance_bytes(actual_bytes)
+    assert not m.isinstance_bytes(actual_unicode)
+    if str is bytes:  # Python 2
+        assert isinstance(actual_bytes, str)
+        assert not isinstance(actual_unicode, str)
+        assert m.isinstance_str(actual_bytes)  # BAD? (but matches native isinstance for Python 2)
+        assert m.isinstance_str(actual_unicode)  # DESIRED for Python 2? (does not match native isinstance for Python 2)
+    else:
+        assert not isinstance(actual_bytes, str)
+        assert isinstance(actual_unicode, str)
+        assert m.isinstance_str(actual_bytes)  # REALLY BAD for Python 3?
+        assert m.isinstance_str(actual_unicode)