Fixed Stack.__str__

Aufgabe03a.py

@@ -4,11 +4,13 @@ class Stack():
         self.stack = l
 
     def __str__(self):
-        max_length = len(str(max(self.stack, key=lambda x: len(str(x)))))
         result = ""
-        vert = "\u2502"
-        for ele in reversed(self.stack):
-            result += vert + str(ele).rjust(max_length) + vert + "\n"
+        max_length = 1
+        if not self.isEmpty():
+            max_length = len(str(max(self.stack, key=lambda x: len(str(x)))))
+            vert = "\u2502"
+            for ele in reversed(self.stack):
+                result += vert + str(ele).rjust(max_length) + vert + "\n"
         return result + "\u2514" + "\u2500"*max_length + "\u2518"
 
     def push(self, element):