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A simple implementation of a dictionary which always (when applicable) returns keys, values, items (key-value pairs) sorted by keys (inserting/removing order doesn't matter and only keys are important; so please note that it is something different than OrderedDict in Python 3.1/2.7 or Django's SortedDict).

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#!/usr/bin/env python
#
# sorteddict.py
# Sorted dictionary (implementation for Python 2.x)
#
# Copyright (c) 2010 Jan Kaliszewski (zuo)
#
# The MIT License:
#  
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.

from bisect import bisect_left, insort
from itertools import izip, repeat


def dictdoc(method):
    "A decorator making reuse of the ordinary dict's docstrings more concise."
    dict_method = getattr(dict, method.__name__)
    if hasattr(dict_method, '__doc__'):
        method.__doc__ = dict_method.__doc__
    return method


class SortedDict(dict):
    '''Dictionary with sorted keys.

    The interface is similar to the ordinary dict's one, but:
    * methods: __repr__(), __str__(), __iter__(), iterkeys(), itervalues(),
      iteritems(), keys(), values(), items() and popitem() -- return results
      taking into consideration sorted keys order;
    * new methods: largest_key(), largest_item(), smallest_key(),
      smallest_item() added.
    '''

    def __init__(self, *args, **kwargs):
        '''Like with the ordinary dict: from a mapping, from an iterable
        of (key, value) pairs, or from keyword arguments.'''
        dict.__init__(self, *args, **kwargs)
        self._sorted_keys = sorted(dict.iterkeys(self))

    @dictdoc
    def __repr__(self):
        return 'SortedDict({%s})' % ', '.join('%r: %r' % item
                                              for item in self.iteritems())
                                              
    @dictdoc
    def __str__(self):
        return repr(self)

    @dictdoc
    def __setitem__(self, key, value):
        key_is_new = key not in self
        dict.__setitem__(self, key, value)
        if key_is_new:
            insort(self._sorted_keys, key)

    @dictdoc
    def __delitem__(self, key):
        dict.__delitem__(self, key)
        del self._sorted_keys[bisect_left(self._sorted_keys, key)]

    def __iter__(self, reverse=False):
        '''D.__iter__() <==> iter(D) <==> D.iterkeys() -> an iterator over
        sorted keys (add reverse=True for reverse ordering).'''
        if reverse:
            return reversed(self._sorted_keys)
        else:
            return iter(self._sorted_keys)

    iterkeys = __iter__

    def itervalues(self, reverse=False):
        '''D.itervalues() -> an iterator over values sorted by keys
        (add reverse=True for reverse ordering).'''
        return (self[key] for key in self.iterkeys(reverse))

    def iteritems(self, reverse=False):
        '''D.iteritems() -> an iterator over (key, value) pairs sorted by keys
        (add reverse=True for reverse ordering).'''
        return ((key, self[key]) for key in self.iterkeys(reverse))

    def keys(self, reverse=False):
        '''D.keys() -> a sorted list of keys
        (add reverse=True for reverse ordering).'''
        return list(self.iterkeys(reverse))

    def values(self, reverse=False):
        '''D.values() -> a list of values sorted by keys
        (add reverse=True for reverse ordering).'''
        return list(self.itervalues(reverse))

    def items(self, reverse=False):
        '''D.items() -> a list of (key, value) pairs sorted by keys
        (add reverse=True for reverse ordering).'''
        return list(self.iteritems(reverse))

    @dictdoc
    def clear(self):
        dict.clear(self)
        del self._sorted_keys[:]

    def copy(self):
        '''D.copy() -> a shallow copy of D (still as a SortedDict).'''
        return self.__class__(self)

    @classmethod
    @dictdoc
    def fromkeys(cls, seq, value=None):
        return cls(izip(seq, repeat(value)))

    @dictdoc
    def pop(self, key, *args, **kwargs):
        if key in self:
            del self._sorted_keys[bisect_left(self._sorted_keys, key)]
        return dict.pop(self, key, *args, **kwargs)

    def popitem(self):
        '''D.popitem() -> (k, v). Remove and return a (key, value) pair with
        the largest key; raise KeyError if D is empty.'''
        try:
            key = self._sorted_keys.pop()
        except IndexError:
            raise KeyError('popitem(): dictionary is empty')
        else:
            return key, dict.pop(self, key)

    @dictdoc
    def setdefault(self, key, default=None):
        if key not in self:
            insort(self._sorted_keys, key)
        return dict.setdefault(self, key, default)

    @dictdoc
    def update(self, other=()):
        if hasattr(other, 'keys') and hasattr(other, 'values'):
            # mapping
            newkeys = [key for key in other if key not in self]
        else:
            # iterator/seqence of pairs
            other = list(other)
            newkeys = [key for key, _ in other if key not in self]
        dict.update(self, other)
        for key in newkeys:
            insort(self._sorted_keys, key)

    def largest_key(self):
        '''D.largest_key() -> the largest key; raise KeyError if D is empty.'''
        try:
            return self._sorted_keys[-1]
        except IndexError:
            raise KeyError('largest_key(): dictionary is empty')

    def largest_item(self):
        '''D.largest_item() -> a (key, value) pair with the largest key;
        raise KeyError if D is empty.'''
        key = self.largest_key()
        return key, self[key]

    def smallest_key(self):
        '''D.smallest_key() -> the smallest key; raise KeyError if D is empty.'''
        try:
            return self._sorted_keys[0]
        except IndexError:
            raise KeyError('smallest_key(): dictionary is empty')

    def smallest_item(self):
        '''D.smallest_item() -> a (key, value) pair with the smallest key;
        raise KeyError if D is empty.'''
        key = self.smallest_key()
        return key, self[key]

Implemented using an additional "synchronized" list of keys, maintained with functions from bisect module. Maintaining parallely two containers uses a bit more memory than the original dict and (what's more important) slows down modifying operations (setting, deleting, updating...) -- but keeps "getting" lookups (d[x], get(x), x in d...) as fast as when using original dict. (Another approach could be e.g. to get rid of the original dict's mechanism, basing only on bisect functions -- it'd maybe make modifying operations faster but would distincly slow down "getting" lookups; other approaches -- see links below...).

UPDATED:

  • reverse keyword argument for [iter]keys|values|items() methods added.
  • Methods: largest_key(), largest_item(), smallest_key(), smallest_item() added.
  • Docstrings and auxiliary dictdoc decorator added.

TODO:

  • key option (like in sorted(), min(), max() etc.).

See also (found @PyPI):

2 comments

Daniel Stutzbach 11 years, 4 months ago  # | flag

You might also be interested in the "sorteddict" type in my blist package. It solves the same problem, but it efficiently keeps the keys in sorted order, yielding different performance characteristics. For example, d.keys()[0] is O(1).

Links:

Grant Jenks 7 years, 2 months ago  # | flag

Like the blist module, the sortedcontainers module provides a SortedDict data type that maintains the keys in sorted order. It's written in pure-Python, very fast (fast-as-C implementations, faster than blist), fully tested (100% coverage and stress) and documented (with performance comparison). Learn more about sortedcontainers, available on PyPI and github.