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I agree that there are problems of beginners misunderstanding 
Python's object model.  However, the proposed fix is not exactly 
correct.  Python *always* calls functions by binding local 
parameter names to argument objects or lists or dicts thereof.  
Whenever a name is rebound to a new object, it is *always* 
unbound from the previous object, as it must be.   Mutability is 
irrelevant.  So is localness induced by a function call.

Changing locality and mutability, your example is equivalent to

a = 1
val = a
val = val + 1

a = [1]
val = a
val = val + [2]
# or
def f(val):
  val = val + [2]
f(a)

*all* of which leave 'a' unchanged, but all of which a beginner 
might think change 'a'.  Perhaps you can suggest a different 
rewording.