enhancing 'list'

[samwyse]

Consider this a wish list.  I know I'm unlikely to get any of these in
time for for my birthday, but still I felt the need to toss it out and
see what happens.

Lately, I've slinging around a lot of lists, and there are some simple
things I'd like to do that just aren't there.

s.count(x[, cmp[, key]])
- return number of i‘s for which s[i] == x.  'cmp' specifies a custom
comparison function of two arguments, as in '.sort'.  'key' specifies
a custom key extraction function of one argument.
s.index(x[, i[, j[, cmp[, key]]]])
- return smallest k such that s[k] == x and i <= k < j.  'cmp' and
'key' are as above.
s.rindex(x[, i[, j[, cmp[, key]]]])
- return largest k such that s[k] == x and i <= k < j.  'cmp' and
'key' are as above.

There are two overlapping proposals here.  One is to add the .rindex
method, which strings already have.  The other is to extend the
optional arguments of .sort to all other methods that test for item
equality.

One last thing, the Python 2.6.2 spec says .count and .index only
apply to mutable sequence types.  I see no reason why they
(and .rindex) couldn't also apply to immutable sequences (tuples, in
particular).