Prevent UnicodeEncodeError in Py2 when ``%s`` formatting is optimised for unicode strings.

CHANGES.rst

@@ -13,6 +13,9 @@ Bugs fixed
 * Overriding cpdef methods did not work in Python subclasses with slots.
   (Github issue #1771)
 
+* ``UnicodeEncodeError`` in Py2 when ``%s`` formatting is optimised for
+  unicode strings.
+
 
 0.28.2 (2018-04-13)
 ===================

Cython/Compiler/ExprNodes.py

@@ -3164,7 +3164,7 @@ class FormattedValueNode(ExprNode):
     c_format_spec = None
 
     find_conversion_func = {
-        's': 'PyObject_Str',
+        's': 'PyObject_Unicode',
         'r': 'PyObject_Repr',
         'a': 'PyObject_ASCII',  # NOTE: mapped to PyObject_Repr() in Py2
     }.get

Cython/Utility/ModuleSetupCode.c

@@ -558,6 +558,7 @@ static CYTHON_INLINE void * PyThread_tss_get(Py_tss_t *key) {
   #define PyString_Type                PyUnicode_Type
   #define PyString_Check               PyUnicode_Check
   #define PyString_CheckExact          PyUnicode_CheckExact
+  #define PyObject_Unicode             PyObject_Str
 #endif
 
 #if PY_MAJOR_VERSION >= 3

tests/run/fstring.pyx

@@ -500,3 +500,16 @@ def generated_fstring(int i, unicode u not None, o):
         u, u, u, u,
         o, o, o, o,
     )
+
+
+@cython.test_assert_path_exists(
+    "//FormattedValueNode",
+    "//AddNode",
+)
+def percent_s_unicode(u, int i):
+    u"""
+    >>> u = u'x\u0194z'
+    >>> print(percent_s_unicode(u, 12))
+    x\u0194z-12
+    """
+    return u"%s-%d" % (u, i)