Understanding While Loop Execution

[Brad]

Hi folks,

I'm still fairly new to programming in python and programming in
general. A friend of mine is in a CompSci 101 course and was working
on a slider game when he encountered a problem. We eventually figured
out what the problem was and built a test case to help solve it, but I
can't for the life of me figure out the why behind it. I tried
googling it and searching the list but didn't find anything that
really explained it. I'm sure it's probably just both of us
misunderstanding what the "while" statement does. So I hoped to ask
for some clarification here. So here is what we worked out was going
on with his code.

from random import shuffle

mylist=[2,1,3]
baselist=[1,2,3]
newlist=[]
count=0
while mylist!=baselist:
    count+=1
    shuffle(mylist)
    newlist.append(mylist)
    print count, mylist, newlist

Output:
>>>
1 [3, 1, 2] [[3, 1, 2]]
2 [1, 2, 3] [[1, 2, 3], [1, 2, 3]]
>>>

What he wanted was a list of lists to use later as a replay. What we
expected newlist.append(mylist) to do was to save a copy of mylist
into the collection for each iteration of the while statement.
However, what struck us as odd is that for each time the while loop
executes it changes all the lists in the list. What I found even
exasperating was if I created yet another list.

from random import shuffle

mylist=[2,1,3]
baselist=[1,2,3]
newlist=[]
saved_shufs=[]
count=0

while mylist!=baselist:
    count+=1
    shuffle(mylist)
    newlist.append(mylist)
    saved_shufs.append(newlist[0])
    print count, mylist, newlist[0], saved_shufs

Output:
>>>
1 [1, 3, 2] [1, 3, 2] [[1, 3, 2]]
2 [3, 2, 1] [3, 2, 1] [[3, 2, 1], [3, 2, 1]]
3 [1, 2, 3] [1, 2, 3] [[1, 2, 3], [1, 2, 3], [1, 2, 3]]
>>>

newlist[0] printed out correctly but when appending it into
saved_shufs it still overwrote everything.

Eventually, after plinking about I remembered that tuples were
immutable and wound up creating a variable like
tuple_of_mylist=tuple(mylist) then appending that into newlist. That
kept the list of tuples fine. I'm still wondering though what I'm not
grasping about "while" that made it do that to the lists? Or is it not
even while, is it something else I'm not getting?

Thanks in advance,
B