fixed the potential bug

src/simudator/core/module.py

@@ -81,12 +81,15 @@ class Module:
         """
         pass
 
-    def get_longest_line_len(self, ignore_keys=[]) -> int:
+    def get_longest_line_len(self, ignore_keys=None) -> int:
         """
         Helper function for pretty_print that returns the length of
         the longest line to print for a module.
         """
 
+        if ignore_keys == None:
+            ignore_keys = []
+
         longest_line_len = 0
         state = self.get_state()