Robert P. J. Day wrote: > On Sat, 7 Nov 2009, Peng Yu wrote: > >> On Fri, Nov 6, 2009 at 5:57 PM, Dave Angel <da...@ieee.org> wrote: > >> > But if you have an expression you want to match each dir against, >> > the list comprehension is the best answer. And the trick to >> > stuffing that new list into the original list object is to use >> > slicing on the left side. The [:] notation is a default slice >> > that means the whole list. >> > >> > dirs[:] = [ item for item in dirs if bool_expression_on_item ] >> >> I suggest to add this example to the document of os.walk() to make >> other users' life easier. > > huh? why do you need the slice notation on the left? why can't you > just assign to "dirs" as opposed to "dirs[:]"? using the former seems > to work just fine. is this some kind of python optimization or idiom?
dirs = [...] rebinds the name "dirs" while dirs[:] = [...] updates the contents of the list currently bound to the "dirs" name. The latter is necessary in the context of os.walk() because it yields a list of subdirectories, gives the user a chance to update it and than uses this potentially updated list to decide which subdirectories to descend into. A simplified example: >>> def f(): ... items = ["a", "b", "c"] ... yield items ... print items ... >>> for items in f(): ... items = ["x", "y"] ... ['a', 'b', 'c'] >>> for items in f(): ... items[:] = ["x", "y"] ... ['x', 'y'] Peter -- http://mail.python.org/mailman/listinfo/python-list
