[Python-Dev] Problem withthe API for str.rpartition()

[Guido van Rossum]

+1

On 9/1/06, Raymond Hettinger <rhettinger at ewtllc.com> wrote:
> Currently, both the partition() and rpartition() methods return a (head,
> sep, tail) tuple and the only difference between the two is whether the
> partition element search starts from the beginning or end of the
> string.  When no separator is found, both methods return the string S
> and two empty strings so that 'a'.partition('x') == 'a'.rpartition('x')
> == ('a', '', '').
>
> For rpartition() the notion of head and tail are backwards -- you
> repeatedly search the tail, not the head.  The distinction is vital
> because the use cases for rpartition() are a mirror image of those for
> partition().  Accordingly, rpartition()'s result should be interpreted
> as (tail, sep, head) and the partition-not-found endcase needs change so
> that 'a'.rpartition('x') == ('', '', 'a') .
>
> The test invariant should be:
>     For every s and p:    s.partition(p) == s[::-1].rpartition(p)[::-1]
>
> The following code demonstrates why the current choice is problematic:
>
>     line = 'a.b.c.d'
>     while line:
>         field, sep, line = line.partition('.')
>         print field
>
>     line = 'a.b.c.d'
>     while line:
>         line, sep, field = line.rpartition('.')
>         print field
>
> The second fragment never terminates.
>
> Since this is a critical API flaw rather than a implementation bug, I
> think it should get fixed right away rather than waiting for Py2.5.1.
>
>
>
> Raymond
> _______________________________________________
> Python-Dev mailing list
> Python-Dev at python.org
> http://mail.python.org/mailman/listinfo/python-dev
> Unsubscribe: http://mail.python.org/mailman/options/python-dev/guido%40python.org
>


-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)