Q:Pythonic way to create list of lists

[Arnaud Delobelle]

grkuntzmd at gmail.com writes:

> I am just learning Python.
>
> I am trying to create a list of empty lists: [[], [], [], ...] (10
> items total).
>
> What is the most Pythonic way to do this?
>
> If I use a list comprehension (as in myList = [[] for item in xrange
> (0, 10)]), Netbeans warns me that 'item' is never used.

IMHO this is the best way.

> If I use a for-loop (as in for item in myList = []; for item in xrange
> (0, 10): myList.append([])), Netbeans still warns me of the same
> thing.
>
> If I use '*' (as myList = [[]] * 10), all of the empty lists refer to
> the same object; changing one changes them all.
>
> Do I have to live with the warning, or is there a "better" way?

You could do this:

>>> map(list, [()]*10)
[[], [], [], [], [], [], [], [], [], []]

But it is wasteful as it creates two lists.

There are some contrived ways to go around this using using itertools,
e.g.

>>> from itertools import *
>>> map(list, repeat((), 10))
[[], [], [], [], [], [], [], [], [], []]

Note that if you use Python 3 you would get an iterator so it would need
to be wrapped in a list() call. Or you could take advantage of
iter(function, sentinel) which is little used:

>>> list(islice(iter(list, None), 10))
[[], [], [], [], [], [], [], [], [], []]

But none of these are very compelling.

-- 
Arnaud