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 -- http://mail.python.org/mailman/listinfo/python-list
