relax speedups str check

CHANGES.rst

@@ -3,6 +3,8 @@ Version 3.1.0
 
 Unreleased
 
+-   Fix compatibility when ``__str__`` returns a ``str`` subclass. :issue:`472`
+
 
 Version 3.0.1
 -------------

src/markupsafe/_speedups.c

@@ -151,7 +151,7 @@ escape_unicode_kind4(PyUnicodeObject *in)
 static PyObject*
 escape_unicode(PyObject *self, PyObject *s)
 {
-	if (!PyUnicode_CheckExact(s))
+	if (!PyUnicode_Check(s))
 		return NULL;
 
     // This check is no longer needed in Python 3.12.

tests/test_escape.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import typing as t
+
 import pytest
 
 from markupsafe import escape
@@ -30,3 +32,37 @@
 )
 def test_escape(value: str, expect: str) -> None:
     assert escape(value) == Markup(expect)
+
+
+class Proxy:
+    def __init__(self, value: t.Any) -> None:
+        self.__value = value
+
+    @property  # type: ignore[misc]
+    def __class__(self) -> type[t.Any]:
+        # Make o.__class__ and isinstance(o, str) see the proxied object.
+        return self.__value.__class__  # type: ignore[no-any-return]
+
+    def __str__(self) -> str:
+        return str(self.__value)
+
+
+def test_proxy() -> None:
+    """Handle a proxy object that pretends its __class__ is str."""
+    p = Proxy("test")
+    assert p.__class__ is str
+    assert isinstance(p, str)
+    assert escape(p) == Markup("test")
+
+
+class ReferenceStr(str):
+    def __str__(self) -> str:
+        # This should return a str, but it returns the subclass instead.
+        return self
+
+
+def test_subclass() -> None:
+    """Handle if str(o) does not return a plain str."""
+    s = ReferenceStr("test")
+    assert isinstance(s, str)
+    assert escape(s) == Markup("test")