Update for_helpers.py

Author: BobTheBuidler

mypyc/irbuild/for_helpers.py

@@ -446,21 +446,22 @@ def make_for_loop_generator(
         if isinstance(expr, IndexExpr) and isinstance(expr.index, SliceExpr):
             # TODO: maybe we must not apply this optimization to list type specifically
             # because the need to check length changes at each iteration?
-            start = expr.index.start
-            stop = expr.index.stop
-            step = expr.index.step
-
-            if all(
-                s is None or isinstance(constant_fold_expr(builder, s), int)
-                for s in (start, stop, step)
-            ):
+            
+            def constant_fold_or_none(expr: Expression | None) -> Any:
+                return None if expr is None else constant_fold_expr(builder, expr)
+
+            start = constant_fold_or_none(expr.index.start)
+            stop = constant_fold_or_none(expr.index.stop)
+            step = constant_fold_or_none(expr.index.step)
+
+            if all(s is None or isinstance(s, int) for s in (start, stop, step)):
                 for_list.init(
                     builder.accept(expr.base),
                     target_type,
                     reverse=False,
-                    start=constant_fold_expr(builder, start),
-                    stop=constant_fold_expr(builder, stop),
-                    step=constant_fold_expr(builder, step),
+                    start=start,
+                    stop=stop,
+                    step=step,
                 )
                 return for_list