> Maybe. I need a volunteer to write the PEP!
PEP: XXX
Title: Mutable Iterations
Version: $Revision$
Last-Modified: $Date$
Author: <placeholder>, Adam DePrince <[EMAIL PROTECTED]>
Status: Draft
Type: Standards
Content-Type: text/plain
Created: 25-March-2006
Post-History:
Abstract:
This PEP proposes an extension to the iteration protocol to
support deletion. Invocation of the delete method would result in
the deletion of the corresponding object from the iterations
underlying data store.
This PEP further proposes that dict.iter{keys,items,values} be
removed and the functions dict.{keys,items,values} return iters of
this new deletable variation, and that the iter prefixed function
variations be deprecated.
Support for delete would become an optional component of the iter
protocol.
Motivation
The current dictionary API has separate functions to return lists
or iters for each of keys, values and items. This is cumbersome
and annoying, yet is tolerated because neither alone possesses the
full functionality desired by the python community.
The iter variation provides all of the performance advantages
normally associated with iters; primarily minimal in-flight memory
consumption. Its use, however, denies the user the ability to
mutate the underlying data structure. Modification of the
underlying dict results in in a RuntimeError upon the subsequent
call of iter.next
The non-iter variation returns a snapshot of the current
dictionary state stored within a list. This has the advantage of
permitting in-situ mutation of the underlying dictionary without
upsetting the current loop. The disadvantage is that of
performance and resource consumption; the portions requested of
the underlying dictionary could easily exceed marginal memory.
In many situation, such as with dictionaries that consume
substantial portion of memory, or dictionary implementations that
operate out of core, the list variant of these operations is
effectively unavailable.
A common programming pattern is to iterate over the elements of a
dictionary selecting those for removal. Somewhat less common is
the insertion of elements during the traversal. It is the former
that we seek to address at this time.
This PEP attempts to merge the benefits of the list and iter
variants of keys/values/items under a single implementation.
SPECIFICATION:
Example of desired operation:
>>> d = {'foo':1,'bar':2 }
>>> i = d.values()
>>> print i.next()
2
>>> i.delete()
>>> print i.next()
1
>>> print d
{'foo':1}
>>> print i.next()
StopIteration
SPECIAL CONSIDERATION
This would require the implementation of a non-rehashing variant
of dict.__del__.
It may not be possible to prevent rehashing upon the insertion of
an element into a dict as it is for a delete, therefore element
insertion is not being considered at this time.
ALSO CONSIDERED
Implementation is the style of the JAVA views proposal. One
concrete example of a list view superimposed upon a dict.
Concerns were expressed about the record keeping and computational
overhead of addressing holes that would appear within the list
upon insertion and deletion.
IMPLEMENTATION
TBD
REFERENCES
TBD
COPYRIGHT
This document has been placed in the public domain.
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