Update checkstrformat.py

Author: BobTheBuidler

mypy/checkstrformat.py

@@ -1008,11 +1008,11 @@ def check_expr(expr: Expression) -> None:
                     self.msg.requires_int_or_single_byte(context)
                 else:
                     if isinstance(folded := constant_fold_expr(expr, "unused"), str):
-                        value = folded
+                        if len(folded) != 1:
+                            self.msg.requires_int_or_char(context)
                     elif isinstance(expr, BytesExpr):
-                        value = expr.value
-                    if len(value) != 1:
-                        self.msg.requires_int_or_char(context)
+                        if len(expr.value) != 1:
+                            self.msg.requires_int_or_char(context)
 
         return check_expr, check_type