> The list nlist inside of function xx is not the same as the variable u > outside of the function: nlist and u refer to two separate list objects. > When you modify nlist, you are not modifying u. > <http://mail.python.org/mailman/listinfo/python-list>
Well - that's not quite true. Before calling the function, u is [1, 2, 3, 4] - but after calling the function, u is [1, 2, 3, 4, 999]. This is a result of using 'nlist += [999]' - the same thing doesn't happen if you use 'nlist = nlist+[999]' instead. I'm not completely aware of what's going on behind the scenes here, but I think the problem is that 'nlist' is actually a reference to a list object - it points to the same place as u. When you assign to it within the function, then it becomes separate from u - which is why nlist = nlist+[999] and nlist = nlist[:-1] don't modify u - but if you modify nlist in place before doing that, such as by using +=, then it's still pointing to u, and so u gets modified as well. So, for example: >>> def blank_list(nlist): ... nlist[:] = [] # modifies the list in-place ... >>> u [1, 2, 3, 4] >>> blank_list(u) >>> u [] # u has been changed by our change to nlist! But if we change it so that it assigns to nlist rather than modifying it: >>> def dont_blank_list(nlist): ... nlist = [] # creates a new list object rather than modifying the current one ... >>> u = [1, 2, 3, 4] >>> u [1, 2, 3, 4] >>> dont_blank_list(u) >>> u [1, 2, 3, 4] # u is completely unaffected! -- Robert K. Day robert....@merton.oxon.org -- Robert K. Day robert....@merton.oxon.org
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