[Tutor] slice lists and slicing syntax questions

[Dave Kuhlman]

On Sat, Oct 13, 2007 at 10:32:39AM -0400, Michael Langford wrote:
> On 10/12/07, gregg at lind-beil.net < gregg at lind-beil.net> wrote:
> >
> > I have been using Python for years now, in all kinds of environments, but
> > example:  x is vector of length 5, with value "a","b","c","d","e" , then:
> >
> > x[3,1,1,1,3,2] # gives [d, b, b, b, d, c]
> >
> > What is the python equivalent?
> 
>   a.  Am I understanding the situation correctly?
> 
> When you call [] on an object, it calls __getitem__The definition for
> getitem is __getitem__(self,key), where key can be an integer or a slice
> object. Slice objects are either a range or a range+step. You've got the
> right picture

__getitem__() suggests a Pythonic solution: subclass list and override
__getitem__():

    class subscriptlist(list):
        def __getitem__(self, subscripts):
            #print 'type(subscripts):', type(subscripts)
            if (isinstance(subscripts, tuple) or
                isinstance(subscripts, list)):
                vals = []
                for subscript in subscripts:
                    vals.append(list.__getitem__(self, subscript))
                return vals
            else:
                val = list.__getitem__(self, subscripts)
                return val

    def test():
        a = range(10)
        b = [x * 5 for x in a]
        c = subscriptlist(b)
        d = c[2,5,6,8]
        print 'd:', d
        e = c[2]
        print 'e:', e

    test()

Which prints out:

    d: [10, 25, 30, 40]
    e: 10

Was that what you wanted?

Notice that "c[2,5,6,8]" results in passing a tuple to __getitem__,
because it is the comma that marks a literal representation of a
tuple.

Dave


-- 
Dave Kuhlman
http://www.rexx.com/~dkuhlman