An oddity in list comparison and element assignment

[michael.f.ellis at gmail.com]

The following script puzzles me.  It creates two nested lists that
compare identically.  After identical element assignments, the lists
are different.  In one case, a single element is replaced. In the
other, an entire column is replaced.

---------------------------------------------------------------------------------------

'''
An oddity in the behavior of lists of lists. Occurs under
Python 2.4.3 (#69, Mar 29 2006, 17:35:34) [MSC v.1310 32 bit (Intel)]
on win32.
Not tested on other platforms or builds.
'''
a =[[[1,2],[1,2]],[[1,2],[1,2]]]
b = [[range(1,3)]*2]*2
assert(a==b)
print "Initially, python reports that the lists are equal"
a[1][1]=[5]
b[1][1]=[5]
try:
  assert(a==b)
except AssertionError:
  print "After identical element assignments, the lists are not equal"
  print "a is now ", a
  print "b is now ", b
-------------------------------------------------------------------------------------

Here's the output on my system.

------------------------------------------------------------------------------------
Initially, python reports that the lists are equal
After identical element assignments, the lists are not equal
a is now  [[[1, 2], [1, 2]], [[1, 2], [5]]]
b is now  [[[1, 2], [5]], [[1, 2], [5]]]
------------------------------------------------------------------------------------

This seems contrary to one of my fundamental expectations, namely that
objects which compare equally must remain equal after identical
operations.  I think what must be going on is that the 'b' list
contains replicated references instead of copies of [range(1,3)]*2 .
IMO, python's == operator should detect this difference in list
structure since it leads to different behavior under element
assignments.

Mike Ellis