Update python.md

Be more explicit to assign default lists to a new python instance.
Defaults are already mentioned, but this code is safer to avoid having different
instances pointing to the very same list without intention.

python.md

@@ -856,12 +856,15 @@ class Superhero(Human):
     # This constructor inherits the "name" argument from the "Human" class and
     # adds the "superpower" and "movie" arguments:
     def __init__(self, name, movie=False,
-                 superpowers=["super strength", "bulletproofing"]):
-
+                 superpowers=None):
         # add additional class attributes:
         self.fictional = True
         self.movie = movie
         # be aware of mutable default values, since defaults are shared
+        if superpowers is None:
+            # assigns default superpowers, but this way new list is created on each initialization.
+            self.superpowers = ["super strength", "bulletproofing"] 
+        else:
             self.superpowers = superpowers
 
         # The "super" function lets you access the parent class's methods