python-peps/peps/pep-0372.rst

PEP: 372
Title: Adding an ordered dictionary to collections
Author: Armin Ronacher <armin.ronacher@active-4.com>,
        Raymond Hettinger <python@rcn.com>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 15-Jun-2008
Python-Version: 2.7, 3.1
Post-History:


Abstract
========

This PEP proposes an ordered dictionary as a new data structure for
the ``collections`` module, called "OrderedDict" in this PEP.  The
proposed API incorporates the experiences gained from working with
similar implementations that exist in various real-world applications
and other programming languages.


Patch
=====

A working Py3.1 patch including tests and documentation is at:

    `OrderedDict patch <https://github.com/python/cpython/issues/49647>`_

The check-in was in revisions: 70101 and 70102

Rationale
=========

In current Python versions, the widely used built-in dict type does
not specify an order for the key/value pairs stored.  This makes it
hard to use dictionaries as data storage for some specific use cases.

Some dynamic programming languages like PHP and Ruby 1.9 guarantee a
certain order on iteration.  In those languages, and existing Python
ordered-dict implementations, the ordering of items is defined by the
time of insertion of the key.  New keys are appended at the end, but
keys that are overwritten are not moved to the end.

The following example shows the behavior for simple assignments:

    >>> d = OrderedDict()
    >>> d['parrot'] = 'dead'
    >>> d['penguin'] = 'exploded'
    >>> d.items()
    [('parrot', 'dead'), ('penguin', 'exploded')]

That the ordering is preserved makes an OrderedDict useful for a couple of
situations:

- XML/HTML processing libraries currently drop the ordering of
  attributes, use a list instead of a dict which makes filtering
  cumbersome, or implement their own ordered dictionary.  This affects
  ElementTree, html5lib, Genshi and many more libraries.

- There are many ordered dict implementations in various libraries
  and applications, most of them subtly incompatible with each other.
  Furthermore, subclassing dict is a non-trivial task and many
  implementations don't override all the methods properly which can
  lead to unexpected results.

  Additionally, many ordered dicts are implemented in an inefficient
  way, making many operations more complex then they have to be.

- :pep:`3115` allows metaclasses to change the mapping object used for
  the class body.  An ordered dict could be used to create ordered
  member declarations similar to C structs.  This could be useful, for
  example, for future ``ctypes`` releases as well as ORMs that define
  database tables as classes, like the one the Django framework ships.
  Django currently uses an ugly hack to restore the ordering of
  members in database models.

- The RawConfigParser class accepts a ``dict_type`` argument that
  allows an application to set the type of dictionary used internally.
  The motivation for this addition was expressly to allow users to
  provide an ordered dictionary. [1]_

- Code ported from other programming languages such as PHP often
  depends on an ordered dict.  Having an implementation of an
  ordering-preserving dictionary in the standard library could ease
  the transition and improve the compatibility of different libraries.


Ordered Dict API
================

The ordered dict API would be mostly compatible with dict and existing
ordered dicts.  Note: this PEP refers to the 2.7 and 3.0 dictionary
API as described in collections.Mapping abstract base class.

The constructor and ``update()`` both accept iterables of tuples as
well as mappings like a dict does.  Unlike a regular dictionary,
the insertion order is preserved.

    >>> d = OrderedDict([('a', 'b'), ('c', 'd')])
    >>> d.update({'foo': 'bar'})
    >>> d
    collections.OrderedDict([('a', 'b'), ('c', 'd'), ('foo', 'bar')])

If ordered dicts are updated from regular dicts, the ordering of new
keys is of course undefined.

All iteration methods as well as ``keys()``, ``values()`` and
``items()`` return the values ordered by the time the key was
first inserted:

    >>> d['spam'] = 'eggs'
    >>> d.keys()
    ['a', 'c', 'foo', 'spam']
    >>> d.values()
    ['b', 'd', 'bar', 'eggs']
    >>> d.items()
    [('a', 'b'), ('c', 'd'), ('foo', 'bar'), ('spam', 'eggs')]

New methods not available on dict:

``OrderedDict.__reversed__()``
    Supports reverse iteration by key.


Questions and Answers
=====================

What happens if an existing key is reassigned?

    The key is not moved but assigned a new value in place.  This is
    consistent with existing implementations.

What happens if keys appear multiple times in the list passed to the
constructor?

    The same as for regular dicts -- the latter item overrides the
    former.  This has the side-effect that the position of the first
    key is used because only the value is actually overwritten::

        >>> OrderedDict([('a', 1), ('b', 2), ('a', 3)])
        collections.OrderedDict([('a', 3), ('b', 2)])

    This behavior is consistent with existing implementations in
    Python, the PHP array and the hashmap in Ruby 1.9.

Is the ordered dict a dict subclass?  Why?

    Yes.  Like ``defaultdict``, an ordered dictionary subclasses ``dict``.
    Being a dict subclass make some of the methods faster (like
    ``__getitem__`` and ``__len__``).  More importantly, being a dict
    subclass lets ordered dictionaries be usable with tools like json that
    insist on having dict inputs by testing isinstance(d, dict).

Do any limitations arise from subclassing dict?

    Yes.  Since the API for dicts is different in Py2.x and Py3.x, the
    OrderedDict API must also be different.  So, the Py2.7 version will need
    to override iterkeys, itervalues, and iteritems.

Does ``OrderedDict.popitem()`` return a particular key/value pair?

    Yes.  It pops-off the most recently inserted new key and its
    corresponding value.  This corresponds to the usual LIFO behavior
    exhibited by traditional push/pop pairs.  It is semantically
    equivalent to ``k=list(od)[-1]; v=od[k]; del od[k]; return (k,v)``.
    The actual implementation is more efficient and pops directly
    from a sorted list of keys.

Does OrderedDict support indexing, slicing, and whatnot?

    As a matter of fact, ``OrderedDict`` does not implement the ``Sequence``
    interface.  Rather, it is a ``MutableMapping`` that remembers
    the order of key insertion.  The only sequence-like addition is
    support for ``reversed``.

    A further advantage of not allowing indexing is that it leaves open
    the possibility of a fast C implementation using linked lists.

Does OrderedDict support alternate sort orders such as alphabetical?

   No.  Those wanting different sort orders really need to be using another
   technique.  The OrderedDict is all about recording insertion order.   If any
   other order is of interest, then another structure (like an in-memory
   dbm) is likely a better fit.

How well does OrderedDict work with the json module, PyYAML, and ConfigParser?

   For json, the good news is that json's encoder respects OrderedDict's iteration order::

        >>> items = [('one', 1), ('two', 2), ('three',3), ('four',4), ('five',5)]
        >>> json.dumps(OrderedDict(items))
        '{"one": 1, "two": 2, "three": 3, "four": 4, "five": 5}'

   In Py2.6, the object_hook for json decoders passes-in an already built
   dictionary so order is lost before the object hook sees it.  This
   problem is being fixed for Python 2.7/3.1 by adding a new hook that
   preserves order (see https://github.com/python/cpython/issues/49631 ).
   With the new hook, order can be preserved::

        >>> jtext = '{"one": 1, "two": 2, "three": 3, "four": 4, "five": 5}'
        >>> json.loads(jtext, object_pairs_hook=OrderedDict)
        OrderedDict({'one': 1, 'two': 2, 'three': 3, 'four': 4, 'five': 5})

   For PyYAML, a full round-trip is problem free::

        >>> ytext = yaml.dump(OrderedDict(items))
        >>> print ytext
        !!python/object/apply:collections.OrderedDict
        - - [one, 1]
          - [two, 2]
          - [three, 3]
          - [four, 4]
          - [five, 5]

        >>> yaml.load(ytext)
        OrderedDict({'one': 1, 'two': 2, 'three': 3, 'four': 4, 'five': 5})

   For the ConfigParser module, round-tripping is also problem free.  Custom
   dicts were added in Py2.6 specifically to support ordered dictionaries::

        >>> config = ConfigParser(dict_type=OrderedDict)
        >>> config.read('myconfig.ini')
        >>> config.remove_option('Log', 'error')
        >>> config.write(open('myconfig.ini', 'w'))

How does OrderedDict handle equality testing?

    Comparing two ordered dictionaries implies that the test will be
    order-sensitive so that list ``(od1.items())==list(od2.items())``.

    When ordered dicts are compared with other Mappings, their order
    insensitive comparison is used.  This allows ordered dictionaries
    to be substituted anywhere regular dictionaries are used.

How __repr__ format will maintain order during a repr/eval round-trip?

    OrderedDict([('a', 1), ('b', 2)])

What are the trade-offs of the possible underlying data structures?

   * Keeping a sorted list of keys is fast for all operations except
     __delitem__() which becomes an O(n) exercise.  This data structure leads
     to very simple code and little wasted space.

   * Keeping a separate dictionary to record insertion sequence numbers makes
     the code a little bit more complex.  All of the basic operations are O(1)
     but the constant factor is increased for __setitem__() and __delitem__()
     meaning that every use case will have to pay for this speedup (since all
     buildup go through __setitem__). Also, the first traversal incurs a
     one-time ``O(n log n)`` sorting cost.  The storage costs are double that
     for the sorted-list-of-keys approach.

   * A version written in C could use a linked list.  The code would be more
     complex than the other two approaches but it would conserve space and
     would keep the same big-oh performance as regular dictionaries.  It is
     the fastest and most space efficient.


Reference Implementation
========================

An implementation with tests and documentation is at:

    `OrderedDict patch <https://github.com/python/cpython/issues/49647>`_

The proposed version has several merits:

* Strict compliance with the MutableMapping API and no new methods
  so that the learning curve is near zero.  It is simply a dictionary
  that remembers insertion order.

* Generally good performance.  The big-oh times are the same as regular
  dictionaries except that key deletion is O(n).

Other implementations of ordered dicts in various Python projects or
standalone libraries, that inspired the API proposed here, are:

- `odict in Python`_
- `odict in Babel`_
- `OrderedDict in Django`_
- `The odict module`_
- `ordereddict`_ (a C implementation of the odict module)
- `StableDict`_
- `Armin Rigo's OrderedDict`_

.. _odict in Python: http://dev.pocoo.org/hg/sandbox/raw-file/tip/odict.py
.. _odict in Babel: http://babel.edgewall.org/browser/trunk/babel/util.py?rev=374#L178
.. _OrderedDict in Django:
   http://code.djangoproject.com/browser/django/trunk/django/utils/datastructures.py?rev=7140#L53
.. _The odict module: http://www.voidspace.org.uk/python/odict.html
.. _ordereddict: http://www.xs4all.nl/~anthon/Python/ordereddict/
.. _StableDict: http://pypi.python.org/pypi/StableDict/0.2
.. _Armin Rigo's OrderedDict: http://codespeak.net/svn/user/arigo/hack/pyfuse/OrderedDict.py


Future Directions
=================

With the availability of an ordered dict in the standard library,
other libraries may take advantage of that.  For example, ElementTree
could return odicts in the future that retain the attribute ordering
of the source file.


References
==========

.. [1] https://github.com/python/cpython/issues/42649


Copyright
=========

This document has been placed in the public domain.