Adding test_isinstance_string_types, with asserts simply matching current behavior.

Author: rwgk

tests/test_pytypes.cpp

@@ -367,4 +367,11 @@ TEST_SUBMODULE(pytypes, m) {
             buf, static_cast<ssize_t>(strlen(buf)));
     });
 #endif
+
+    m.def("isinstance_pybind11_bytes", [](py::object o) { return py::isinstance<py::bytes>(o); });
+    m.def("isinstance_pybind11_unicode", [](py::object o) { return py::isinstance<py::str>(o); });
+
+    m.def("pass_to_pybind11_bytes", [](py::bytes b) { return py::len(b); });
+    m.def("pass_to_pybind11_unicode", [](py::str s) { return py::len(s); });
+    m.def("pass_to_std_string", [](std::string s) { return s.size(); });
 }

tests/test_pytypes.py

@@ -346,3 +346,41 @@ 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""
+    actual_unicode = u""
+    if str is bytes:
+        # Python 2: NOT same as native str, BUT same as pybind11::str
+        native_unicode_type = unicode  # noqa: F821
+    else:
+        # Python 3: same as pybind11::str
+        native_unicode_type = str
+
+    # Native isinstance, for comparison with below.
+    assert isinstance(actual_bytes, bytes)
+    assert not isinstance(actual_unicode, bytes)
+    assert not isinstance(actual_bytes, native_unicode_type)
+    assert isinstance(actual_unicode, native_unicode_type)
+
+    # pybind11 isinstance
+    assert m.isinstance_pybind11_bytes(actual_bytes)
+    assert not m.isinstance_pybind11_bytes(actual_unicode)
+    assert m.isinstance_pybind11_unicode(actual_bytes)  # NOT like native
+    assert m.isinstance_pybind11_unicode(actual_unicode)
+
+
+def test_pass_actual_bytes_or_unicode_to_string_types():
+    actual_bytes = b"Bytes"
+    actual_unicode = u"Str"
+
+    assert m.pass_to_pybind11_bytes(actual_bytes) == 5
+    with pytest.raises(TypeError):
+        m.pass_to_pybind11_bytes(actual_unicode)  # NO implicit encode
+
+    assert m.pass_to_pybind11_unicode(actual_bytes) == 5  # implicit decode
+    assert m.pass_to_pybind11_unicode(actual_unicode) == 3
+
+    assert m.pass_to_std_string(actual_bytes) == 5
+    assert m.pass_to_std_string(actual_unicode) == 3