A demonstration of shallow vs deep copying.
This was the source of many student bugs in tippy's apply_move method.
Make sure you understand this!

3.4.1 (v3.4.1:c0e311e010fc, May 18 2014, 00:54:21) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)]
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# A list that we want to keep intact.
# We're going to try several ways to make a copy of it, and see which
# succeed in keeping L1 intact.
L1 = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

# This definitely won't work.  L1 and L2 are references to the same list
# object!  This didn't likely trip up many of you.
L2 = L1
id(L1)
4316207304
id(L2)
4316207304

# Here we copy L1 instead.
L2 = [r for r in L1]
L2
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
L1
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
# See that L2 is a different list than L1.  Everything should be fine, right?
id(L2)
4314697672
# Let's change L2 and find out.
L2[1][1] = 999999999
L2
[[1, 2, 3], [4, 999999999, 6], [7, 8, 9]]
L1
[[1, 2, 3], [4, 999999999, 6], [7, 8, 9]]
# Oh no!  L1 is different too.
# We did not protect ourselves against changes within the sublists:
# Although L1 and L2 are two different list objects, at each index
# they both refer to the same list object (for the sub-lists that
# represent rows).
id(L1)
4316207304
id(L2)
4314697672
# See -- same memory addresses:
id(L1[1])
4316207688
id(L2[1])
4316207688
# (But we did protect L1 against changes to L2 at the top level, because
# they are two different list objects at that level.):
L2[0] = "hello"
L2
['hello', [4, 999999999, 6], [7, 8, 9]]
L1
[[1, 2, 3], [4, 999999999, 6], [7, 8, 9]]

# Okay, L1 is been changed.  Let's make another attempt to copy it, and
# try not to change L1 any further.
L1
[[1, 2, 3], [4, 999999999, 6], [7, 8, 9]]
# Here we make copies at both levels of the list:
L3 = [[item for item in r] for r in L1]
id(L3)
4314177608
id(L1)
4316207304
# Not only are L1 and L3 different list objects, but their elements are too:
id(L3[1])
4316009608
id(L1[1])
4316207688
# So when we change L3 in a sublist, L1 does not change.
L3[2][1] = -42
L3
[[1, 2, 3], [4, 999999999, 6], [7, -42, 9]]
L1
[[1, 2, 3], [4, 999999999, 6], [7, 8, 9]]

# L3 was a successful "deep copy" of L1.