A strange list concatenation result

[Mok-Kong Shen]

def test(list1,list2):
   list1+=[4,5,6]
   list2=list2+[4,5,6]
   print("inside ",list1,list2)
   return

# With

list1=list2=[1,2,3]
test(list1,list2)
print("outside",list1,list2)

# I got the following:
# inside  [1, 2, 3, 4, 5, 6] [1, 2, 3, 4, 5, 6, 4, 5, 6]
# outside [1, 2, 3, 4, 5, 6] [1, 2, 3, 4, 5, 6]

# With

list1=[1,2,3]
list2=[1,2,3]
test(list1,list2)
print("outside",list1,list2)

# I got the following:
# inside  [1, 2, 3, 4, 5, 6] [1, 2, 3, 4, 5, 6]
# outside [1, 2, 3, 4, 5, 6] [1, 2, 3]