On 10/31/2013 5:29 PM, Ulrich Goebel wrote:
I'm locking for an "iterator" type with not only the .next() method, but with a .previous(), .first() and .last() method, so that I can through it from the beginning or from the end, and in both directions, even alternately (for example two steps forward, one backward, two steps forward).
You are free to write such a class, if it is appropriate for your actual use case.
If you have a concrete sequence object seq with random access, there is no reason to do so. First and last are seq[0] and seq[-1]. Given 'cursor' i, prev and next are 'i-=1;seq[i]' and 'i+=1;seq[i]'.
There *are* virtual sequences where first and last are known or relatively easy to compute and for which prev and next are much easier to compute than a random nth item. Note that if you start with first and mostly move forward, prev might best be implemented using a list of items already seen. The list and the prev function might be limited to the last N items seen. It you give up the last function, the underlying sequence does not even have to have a definite end.
A somewhat realistic (useful) example might be the following. You have a very long sequence of bytes that represent utf-8 encoded characters. You want to view the sequence as a sequence of sentences. The sequence is too long to simply create a list of (decoded) sentences in memory.
-- Terry Jan Reedy -- https://mail.python.org/mailman/listinfo/python-list
