"ustring!s" (i.e. str(ustring)) is actually not safe to optimise away for...

"ustring!s" (i.e. str(ustring)) is actually not safe to optimise away for f-string formatting in Py2 as it can fail to encode the Unicode string to a byte string. And it's unlikely to appear in practice.

Cython/Compiler/ExprNodes.py

@@ -3045,8 +3045,8 @@ class FormattedValueNode(ExprNode):
             self.format_spec = self.format_spec.analyse_types(env).coerce_to_pyobject(env)
         if self.c_format_spec is None:
             self.value = self.value.coerce_to_pyobject(env)
-            if not self.format_spec and self.value.type is unicode_type and not self.value.may_be_none():
-                if self.conversion_char in (None, 's'):
+            if not self.format_spec and not self.conversion_char:
+                if self.value.type is unicode_type and not self.value.may_be_none():
                     # value is definitely a unicode string and we don't format it any special
                     return self.value
         return self