On 9/28/07, Gabriel Zachmann <[EMAIL PROTECTED]> wrote: > Well, > > could some kind soul please explain to me why the following trivial code is > misbehaving? > > #!/usr/bin/python > > lst = [ 0, 1, 2 ] > > s = [] > > l = [ lst[0] ] > r = lst[1:] > while r: > x = (l,r) > print x > s.append( x ) > > l.append( r.pop(0) ) > > print s > > > > The output I get is: > > ([0], [1, 2]) > ([0, 1], [2]) > [([0, 1, 2], []), ([0, 1, 2], [])] > > and the error is in the last line: the two pairs in the outer list are > identical and they should be as the pairs on the first and the 2nd line, > respectively! > > I think I'm going nuts -- for the life of me I don't see what's going on ... > (I've been tracking down a bug in my larger python script, and the cause > seems to boil down to the above snippet.) > > Thanks a lot in advance for any insights, etc. > > Best regards, > Gabriel. > > -- > http://mail.python.org/mailman/listinfo/python-list >
If you're familiar with C or C++, think of s as holding a pointer to x which in turn holds a pointer to l and r, so when you change l or r, x (and s indirectly) is still pointing to the same lists which by the end of your loop have changed to r=[] and l=[0,1,2]. BTW: It's not really "misbehaving." It's doing exactly what you're telling it to do ;-) Jason -- http://mail.python.org/mailman/listinfo/python-list
