Make py::isinstance<py::str> less permissive (#1463)

Author: wjakob

include/pybind11/pytypes.h

@@ -943,6 +943,9 @@ class str : public object {
 };
 /// @} pytypes
 
+// str::check_ is too permissive for `isinstance` (in particular it allows a `bytes` object)
+template <> inline bool isinstance<str>(handle obj) { return PyUnicode_Check(obj.ptr()); }
+
 inline namespace literals {
 /** \rst
     String literal version of `str`

tests/test_pytypes.cpp

@@ -290,6 +290,9 @@ TEST_SUBMODULE(pytypes, m) {
         return l;
     });
 
+    // test_str_isinstance
+    m.def("is_str_instance", [](py::object o) { return py::isinstance<py::str>(o); });
+
     m.def("test_list_slicing", [](py::list a) {
         return a[py::slice(0, -1, 2)];
     });

tests/test_pytypes.py

@@ -251,3 +251,8 @@ def test_number_protocol():
 def test_list_slicing():
     li = list(range(100))
     assert li[::2] == m.test_list_slicing(li)
+
+
+def test_str_isinstance():
+    assert m.is_str_instance(u"abc")
+    assert not m.is_str_instance(b"abc")