[Tutor] for _ in

[spir]

On Tue, 9 Mar 2010 11:25:19 +0800
Joson <zhuchunml at gmail.com> wrote:

>         if labels is None: labels = [OrderedSet() for _ in xrange(ndim)]
>         ......
> 
> It's a library named "divisi". "OrderedSet" is a string set defined in this
> lib. "ndim" is integer.  What's the meaning of "for _ in" ?
> 
> Joson

   [OrderedSet() for _ in xrange(n)]

builds a list of ndim empty ordered sets. '_' is the index, but is unused; '_' is commonly used as required, but unused, variable name (not only in python).
This is just a tricky python idiom, because there is no builtin syntax to do this.
   [OrderedSet()] * ndim
would build a list with ndim times the *same* set, so that if one item is changed, all are changed. The above syntax instread creates *distinct* sets.

Python 2.6.4 (r264:75706, Dec  7 2009, 18:45:15) 
[GCC 4.4.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.

>>> lists = [list()] *3
>>> lists
[[], [], []]
>>> list1 = lists[1]
>>> list1.append('*')
>>> lists
[['*'], ['*'], ['*']]

>>> lists = [list() for _ in range(3)]	# _ is unused
>>> lists
[[], [], []]
>>> list1 = lists[1]
>>> list1.append('*')
>>> lists
[[], ['*'], []]

>>> lists = [list()] * 3
>>> for i in range(len(lists)) : lists[i].append(i)
... 
>>> lists
[[0, 1, 2], [0, 1, 2], [0, 1, 2]]

>>> lists = [list() for i in range(3)]
>>> for i in range(len(lists)) : lists[i].append(i)
... 
>>> lists
[[0], [1], [2]]


Denis
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