Update checkexpr.py

Author: BobTheBuidler

mypy/checkexpr.py

@@ -18,6 +18,7 @@
 from mypy.checker_shared import ExpressionCheckerSharedApi
 from mypy.checkmember import analyze_member_access, has_operator
 from mypy.checkstrformat import StringFormatterChecker
+from mypy.constant_fold import constant_fold_expr
 from mypy.erasetype import erase_type, remove_instance_last_known_values, replace_meta_vars
 from mypy.errors import ErrorInfo, ErrorWatcher, report_internal_error
 from mypy.expandtype import (
@@ -659,7 +660,7 @@ def check_str_format_call(self, e: CallExpr) -> None:
         """More precise type checking for str.format() calls on literals."""
         assert isinstance(e.callee, MemberExpr)
         format_value = None
-        folded_callee_expr = constant_fold_expr(e.callee.expr, "unused")
+        folded_callee_expr = constant_fold_expr(e.callee.expr, "<unused>")
         if isinstance(folded_callee_expr, str):
             format_value = folded_callee_expr
         elif self.chk.has_type(e.callee.expr):