good solution ,thanks~! 2009/9/15 Steven D'Aprano <ste...@remove.this.cybersource.com.au>
> On Mon, 14 Sep 2009 18:55:13 -0700, Chris Rebert wrote: > > > On Mon, Sep 14, 2009 at 6:49 PM, Helvin <helvin...@gmail.com> wrote: > ... > > > I have looked at documentation, and how strings and lists work, but I > > > cannot understand the behaviour of the following: > ... > > > for item in list: > > > if item is ' ': > > > print 'discard these: ',item > > > index = list.index(item) > > > del list[index] > > ... > > > Moral: Don't modify a list while iterating over it. Use the loop to > > create a separate, new list from the old one instead. > > > This doesn't just apply to Python, it is good advice in every language > I'm familiar with. At the very least, if you have to modify over a list > in place and you are deleting or inserting items, work *backwards*: > > for i in xrange(len(alist), -1, -1): > item = alist[i] > if item == 'delete me': > del alist[i] > > > This is almost never the right solution in Python, but as a general > technique, it works in all sorts of situations. (E.g. when varnishing a > floor, don't start at the doorway and varnish towards the end of the > room, because you'll be walking all over the fresh varnish. Do it the > other way, starting at the end of the room, and work backwards towards > the door.) > > In Python, the right solution is almost always to make a new copy of the > list. Here are three ways to do that: > > > newlist = [] > for item in alist: > if item != 'delete me': > newlist.append(item) > > > newlist = [item for item in alist if item != 'delete me'] > > newlist = filter(lambda item: item != 'delete me', alist) > > > > Once you have newlist, you can then rebind it to alist: > > alist = newlist > > or you can replace the contents of alist with the contents of newlist: > > alist[:] = newlist > > > The two have a subtle difference in behavior that may not be apparent > unless you have multiple names bound to alist. > > > > -- > Steven > -- > http://mail.python.org/mailman/listinfo/python-list >
-- http://mail.python.org/mailman/listinfo/python-list
