Clarification on python intro

[Alan Garrison]

For those at the meeting (or are currently learning python), there was 
a little confusion on my part about calling functions with mutable 
objects.  I wrote some test code to clarify what Tim brought up:

>>> # create three local lists (mutable objects)
>>> loc1 = [1,2,3]>>> loc2 = ["a","b","c"]>>> loc3 = ["d","e","f"]>>> # create function to test mutability>>> def Dinsdale(arg1,arg2,arg3):... 	arg1[0] = "99"                       # first, change indexed value... 	arg1 = ["spam", "SPAM", "SPAM!!!"]   # second, create new list instance... 	arg2 = [44,45,46]                    # first, create new list instance... 	arg2[0] = 55                         # second, change indexed value... 	arg3 = "Assigning the argument to be a new type"... 	>>> Dinsdale(loc1,loc2,loc3)   # call function with our locals>>> print loc1    # should have [0] index changed['99', 2, 3]>>> print loc2    # should not change['a', 'b', 'c']>>> print loc3    # should not change['d', 'e', 'f']>>> 

As you can see, function Dinsdale was passed three lists (mutable), 
but only loc1 was changed at all.  Note the order of the arg# 
changes in the funciton.  If I change an indexed part of the list 
first, it is changed outside the function.  If I give it a whole 
new list (arg1 = ["spam", "SPAM", "SPAM!!!"]), or any other new 
object type (arg3 = "Assigning the argument to be a new type"), 
a new object is created in the function and the reference to the 
loc# object is lost (within the function at least).  This is 
basically what Ben explained, so the above code should hopefully 
clarify things.