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python-peps / pep-0270.txt

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PEP: 270
Title: uniq method for list objects
Version: $Revision$
Last-Modified: $Date$
Author: jp@demonseed.net (Jason Petrone)
Status: Rejected
Type: Standards Track
Created: 21-Aug-2001
Python-Version: 2.2
Post-History:


Notice

    This PEP is withdrawn by the author.  He writes:

        Removing duplicate elements from a list is a common task, but
        there are only two reasons I can see for making it a built-in.
        The first is if it could be done much faster, which isn't the
        case.  The second is if it makes it significantly easier to
        write code.  The introduction of sets.py eliminates this
        situation since creating a sequence without duplicates is just
        a matter of choosing a different data structure: a set instead
        of a list.

    As described in PEP 218, sets are being added to the standard
    library for Python 2.3.


Abstract

    This PEP proposes adding a method for removing duplicate elements to
    the list object.


Rationale

    Removing duplicates from a list is a common task.  I think it is
    useful and general enough to belong as a method in list objects.
    It also has potential for faster execution when implemented in C,
    especially if optimization using hashing or sorted cannot be used.

    On comp.lang.python there are many, many, posts[1] asking about
    the best way to do this task.  Its a little tricky to implement
    optimally and it would be nice to save people the trouble of
    figuring it out themselves.


Considerations

    Tim Peters suggests trying to use a hash table, then trying to
    sort, and finally falling back on brute force[2].  Should uniq
    maintain list order at the expense of speed?

    Is it spelled 'uniq' or 'unique'? 


Reference Implementation

    I've written the brute force version.  Its about 20 lines of code
    in listobject.c.  Adding support for hash table and sorted
    duplicate removal would only take another hour or so.


References

    [1] http://groups.google.com/groups?as_q=duplicates&as_ugroup=comp.lang.python  

    [2] Tim Peters unique() entry in the Python cookbook:
        http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52560/index_txt


Copyright

    This document has been placed in the public domain.



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