writable iterators?

[Chris Torek]

In article <iu00fs1dhg at news3.newsguy.com> I wrote, in part:
>Another possible syntax:
>
>    for item in container with key:
>
>which translates roughly to "bind both key and item to the value
>for lists, but bind key to the key and value for the value for
>dictionary-ish items".  Then ... the OP would write, e.g.:
>
>    for elem in sequence with index:
>        ...
>        sequence[index] = newvalue
>
>which of course calls the usual container.__setitem__.  In this
>case the "new protocol" is to have iterators define a function
>that returns not just the next value in the sequence, but also
>an appropriate "key" argument to __setitem__.  For lists, this
>is just the index; for dictionaries, it is the key; for other
>containers, it is whatever they use for their keys.

I note I seem to have switched halfway through thinking about
this from "value" to "index" for lists, and not written that. :-)

Here's a sample of a simple generator that does the trick for
list, buffer, and dict:

    def indexed_seq(seq):
        """
        produce a pair
            <key_or_index> <value>
        such that seq[key_or_index] is <value> initially; you can
        write on seq[key_or_index] to set a new value while this
        operates.  Note that we don't allow tuple and string here
        since they are not writeable.
        """
        if isinstance(seq, (list, buffer)):
            for i, v in enumerate(seq):
                yield i, v
        elif isinstance(seq, dict):
            for k in seq:
                yield k, seq[k]
        else:
            raise TypeError("don't know how to index %s" % type(seq))

which shows that there is no need for a new syntax.  (Turning the
above into an iterator, and handling container classes that have
an __iter__ callable that produces an iterator that defines an
appropriate index-and-value-getter, is left as an exercise. :-) )
-- 
In-Real-Life: Chris Torek, Wind River Systems
Intel require I note that my opinions are not those of WRS or Intel
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W)  +1 801 277 2603
email: gmail (figure it out)      http://web.torek.net/torek/index.html