adding @jagerman's isinstance<str> specialization for Python 3 only

Author: rwgk

include/pybind11/pytypes.h

@@ -943,6 +943,10 @@ class str : public object {
 };
 /// @} pytypes
 
+#if PY_MAJOR_VERSION >= 3
+template <> inline bool isinstance<str>(handle obj) { return PyUnicode_Check(obj.ptr()); }
+#endif
+
 inline namespace literals {
 /** \rst
     String literal version of `str`

tests/test_pytypes.cpp

@@ -370,4 +370,7 @@ TEST_SUBMODULE(pytypes, m) {
 
     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); });
+
+    m.def("pass_to_std_string", [](std::string s) { return s.size(); });
+    m.def("pass_to_py_str", [](py::str s) { return py::len(s); });
 }

tests/test_pytypes.py

@@ -362,5 +362,23 @@ def test_isinstance_string_types():
     else:
         assert not isinstance(actual_bytes, str)
         assert isinstance(actual_unicode, str)
-        assert m.isinstance_str(actual_bytes)  # REALLY BAD for Python 3?
+        assert not m.isinstance_str(actual_bytes)
         assert m.isinstance_str(actual_unicode)
+
+
+def test_pass_str_or_bytes_to_std_string():
+    actual_bytes = b"bytes"
+    actual_unicode = u"str"
+    assert m.pass_to_std_string(actual_unicode) == 3
+    assert m.pass_to_std_string(actual_bytes) == 5
+    assert m.pass_to_py_str(actual_unicode) == 3
+    if str is bytes:  # Python 2
+        m.pass_to_py_str(actual_bytes) == 5
+    else:
+        with pytest.raises(TypeError) as excinfo:
+            m.pass_to_py_str(actual_bytes)
+        assert str(excinfo.value) == """\
+pass_to_py_str(): incompatible function arguments. The following argument types are supported:
+    1. (arg0: str) -> int
+
+Invoked with: b'bytes'"""