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Drop-in substitute for Py2.7's new collections.OrderedDict. The recipe has big-oh performance that matches regular dictionaries (amortized O(1) insertion/deletion/lookup and O(n) iteration/repr/copy/equality_testing).

Python, 258 lines
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# Backport of OrderedDict() class that runs on Python 2.4, 2.5, 2.6, 2.7 and pypy.
# Passes Python2.7's test suite and incorporates all the latest updates.

try:
    from thread import get_ident as _get_ident
except ImportError:
    from dummy_thread import get_ident as _get_ident

try:
    from _abcoll import KeysView, ValuesView, ItemsView
except ImportError:
    pass


class OrderedDict(dict):
    'Dictionary that remembers insertion order'
    # An inherited dict maps keys to values.
    # The inherited dict provides __getitem__, __len__, __contains__, and get.
    # The remaining methods are order-aware.
    # Big-O running times for all methods are the same as for regular dictionaries.

    # The internal self.__map dictionary maps keys to links in a doubly linked list.
    # The circular doubly linked list starts and ends with a sentinel element.
    # The sentinel element never gets deleted (this simplifies the algorithm).
    # Each link is stored as a list of length three:  [PREV, NEXT, KEY].

    def __init__(self, *args, **kwds):
        '''Initialize an ordered dictionary.  Signature is the same as for
        regular dictionaries, but keyword arguments are not recommended
        because their insertion order is arbitrary.

        '''
        if len(args) > 1:
            raise TypeError('expected at most 1 arguments, got %d' % len(args))
        try:
            self.__root
        except AttributeError:
            self.__root = root = []                     # sentinel node
            root[:] = [root, root, None]
            self.__map = {}
        self.__update(*args, **kwds)

    def __setitem__(self, key, value, dict_setitem=dict.__setitem__):
        'od.__setitem__(i, y) <==> od[i]=y'
        # Setting a new item creates a new link which goes at the end of the linked
        # list, and the inherited dictionary is updated with the new key/value pair.
        if key not in self:
            root = self.__root
            last = root[0]
            last[1] = root[0] = self.__map[key] = [last, root, key]
        dict_setitem(self, key, value)

    def __delitem__(self, key, dict_delitem=dict.__delitem__):
        'od.__delitem__(y) <==> del od[y]'
        # Deleting an existing item uses self.__map to find the link which is
        # then removed by updating the links in the predecessor and successor nodes.
        dict_delitem(self, key)
        link_prev, link_next, key = self.__map.pop(key)
        link_prev[1] = link_next
        link_next[0] = link_prev

    def __iter__(self):
        'od.__iter__() <==> iter(od)'
        root = self.__root
        curr = root[1]
        while curr is not root:
            yield curr[2]
            curr = curr[1]

    def __reversed__(self):
        'od.__reversed__() <==> reversed(od)'
        root = self.__root
        curr = root[0]
        while curr is not root:
            yield curr[2]
            curr = curr[0]

    def clear(self):
        'od.clear() -> None.  Remove all items from od.'
        try:
            for node in self.__map.itervalues():
                del node[:]
            root = self.__root
            root[:] = [root, root, None]
            self.__map.clear()
        except AttributeError:
            pass
        dict.clear(self)

    def popitem(self, last=True):
        '''od.popitem() -> (k, v), return and remove a (key, value) pair.
        Pairs are returned in LIFO order if last is true or FIFO order if false.

        '''
        if not self:
            raise KeyError('dictionary is empty')
        root = self.__root
        if last:
            link = root[0]
            link_prev = link[0]
            link_prev[1] = root
            root[0] = link_prev
        else:
            link = root[1]
            link_next = link[1]
            root[1] = link_next
            link_next[0] = root
        key = link[2]
        del self.__map[key]
        value = dict.pop(self, key)
        return key, value

    # -- the following methods do not depend on the internal structure --

    def keys(self):
        'od.keys() -> list of keys in od'
        return list(self)

    def values(self):
        'od.values() -> list of values in od'
        return [self[key] for key in self]

    def items(self):
        'od.items() -> list of (key, value) pairs in od'
        return [(key, self[key]) for key in self]

    def iterkeys(self):
        'od.iterkeys() -> an iterator over the keys in od'
        return iter(self)

    def itervalues(self):
        'od.itervalues -> an iterator over the values in od'
        for k in self:
            yield self[k]

    def iteritems(self):
        'od.iteritems -> an iterator over the (key, value) items in od'
        for k in self:
            yield (k, self[k])

    def update(*args, **kwds):
        '''od.update(E, **F) -> None.  Update od from dict/iterable E and F.

        If E is a dict instance, does:           for k in E: od[k] = E[k]
        If E has a .keys() method, does:         for k in E.keys(): od[k] = E[k]
        Or if E is an iterable of items, does:   for k, v in E: od[k] = v
        In either case, this is followed by:     for k, v in F.items(): od[k] = v

        '''
        if len(args) > 2:
            raise TypeError('update() takes at most 2 positional '
                            'arguments (%d given)' % (len(args),))
        elif not args:
            raise TypeError('update() takes at least 1 argument (0 given)')
        self = args[0]
        # Make progressively weaker assumptions about "other"
        other = ()
        if len(args) == 2:
            other = args[1]
        if isinstance(other, dict):
            for key in other:
                self[key] = other[key]
        elif hasattr(other, 'keys'):
            for key in other.keys():
                self[key] = other[key]
        else:
            for key, value in other:
                self[key] = value
        for key, value in kwds.items():
            self[key] = value

    __update = update  # let subclasses override update without breaking __init__

    __marker = object()

    def pop(self, key, default=__marker):
        '''od.pop(k[,d]) -> v, remove specified key and return the corresponding value.
        If key is not found, d is returned if given, otherwise KeyError is raised.

        '''
        if key in self:
            result = self[key]
            del self[key]
            return result
        if default is self.__marker:
            raise KeyError(key)
        return default

    def setdefault(self, key, default=None):
        'od.setdefault(k[,d]) -> od.get(k,d), also set od[k]=d if k not in od'
        if key in self:
            return self[key]
        self[key] = default
        return default

    def __repr__(self, _repr_running={}):
        'od.__repr__() <==> repr(od)'
        call_key = id(self), _get_ident()
        if call_key in _repr_running:
            return '...'
        _repr_running[call_key] = 1
        try:
            if not self:
                return '%s()' % (self.__class__.__name__,)
            return '%s(%r)' % (self.__class__.__name__, self.items())
        finally:
            del _repr_running[call_key]

    def __reduce__(self):
        'Return state information for pickling'
        items = [[k, self[k]] for k in self]
        inst_dict = vars(self).copy()
        for k in vars(OrderedDict()):
            inst_dict.pop(k, None)
        if inst_dict:
            return (self.__class__, (items,), inst_dict)
        return self.__class__, (items,)

    def copy(self):
        'od.copy() -> a shallow copy of od'
        return self.__class__(self)

    @classmethod
    def fromkeys(cls, iterable, value=None):
        '''OD.fromkeys(S[, v]) -> New ordered dictionary with keys from S
        and values equal to v (which defaults to None).

        '''
        d = cls()
        for key in iterable:
            d[key] = value
        return d

    def __eq__(self, other):
        '''od.__eq__(y) <==> od==y.  Comparison to another OD is order-sensitive
        while comparison to a regular mapping is order-insensitive.

        '''
        if isinstance(other, OrderedDict):
            return len(self)==len(other) and self.items() == other.items()
        return dict.__eq__(self, other)

    def __ne__(self, other):
        return not self == other

    # -- the following methods are only used in Python 2.7 --

    def viewkeys(self):
        "od.viewkeys() -> a set-like object providing a view on od's keys"
        return KeysView(self)

    def viewvalues(self):
        "od.viewvalues() -> an object providing a view on od's values"
        return ValuesView(self)

    def viewitems(self):
        "od.viewitems() -> a set-like object providing a view on od's items"
        return ItemsView(self)

15 comments

Eric Talevich 14 years, 5 months ago  # | flag

The function all was introduced in Py2.5, so to make this work in Py2.4, the method __eq__ should be changed to something like:

def __eq__(self, other):
    if isinstance(other, OrderedDict):
        if len(self) != len(other):
            return False
        for p, q in  zip(self.items(), other.items()):
            if p != q:
                return False
        return True
    return dict.__eq__(self, other)
Steven Hazel 14 years, 3 months ago  # | flag

Python 2.4 doesn't support the conditional expression "key = reversed(self).next() if last else iter(self).next()" used on line 50. Instead, you'll need to do this:

if last:
    key = reversed(self).next()
else:
    key = iter(self).next()
Steven Hazel 14 years, 3 months ago  # | flag

I've put this recipe in PyPI as ordereddict.

Nguyễn Hồng Quân 13 years ago  # | flag

Thanks. To make the string represent easier to read, the line 82 should be changed to return '%s{%s}' % (self.__class__.__name__, ','.join(['%r: %r' % (k, v) for k, v in self.items()])) It will produce, for example: OrderedDict{'hi': (89, None)}

Nguyễn Hồng Quân 13 years ago  # | flag

return '%s{%s}' % (self.__class__.__name__, ','.join(['%r: %r' % (k, v) for k, v in self.items()]))

jia 12 years, 8 months ago  # | flag

def update(args, *kwds): should have "self" as it's first parameter

Martin Miller 12 years, 5 months ago  # | flag

@jia: See the self = args[0] on line 155.

Sharaf Ali 11 years, 3 months ago  # | flag

Will Python 2.7 be able to Open This Dict with using Its Default OrderedDict? as Its seems working fine but there could be situations where it may fail?

Mark Pundurs 10 years, 12 months ago  # | flag

@Martin Miller, the line you note doesn't address jia's issue but highlights it. I haven't tested jia's solution; what worked for me was to add self as the first argument to the __update call on line 38:

self.__update(self, *args, **kwds)
Martin Miller 10 years, 11 months ago  # | flag

@Mark Pundurs: I don't understand what you're trying to convey by saying "highlights it".

The self = args[0] means the first argument is self even if it's not named that in the usual way via the method definition.

Martin Miller 10 years, 11 months ago  # | flag

@Mark Pundurs: Regarding line 38. In the self.__update(*args, **kwds) call, self will automatically be added as the first argument since it's a call to a bound method.

Juan Pablo 10 years, 4 months ago  # | flag

On Python 2.6.6 it doesn't keep the order.

>>> from ordereddict import OrderedDict
>>> for key, item in OrderedDict({'a': 1, 'b': 2, 'c': 3, 'd': 4}).iteritems(): print key, item
... 
a 1
c 3
b 2
d 4



>>> for key, item in OrderedDict({'a': 1, 'b': 2, 'c': 3, 'd': 4}).items(): print key, item
... 
a 1
c 3
b 2
d 4
Matt "Moggers" Molyneaux 10 years, 4 months ago  # | flag

Juan, you're passing OrderedDict a normal unordered dict - the exact same behaviour can be observed with Python 2.7 and collections.OrderedDict. Using kwargs does the same thing (again, just a normal dict), however passing OrderedDict a list of key/value pairs works as expected:

>>> for key, item in OrderedDict([('a', 1), ('b', 2), ('c', 3), ('d', 4)]).iteritems(): print key, item
... 
a 1
b 2
c 3
d 4

Compared to a dict:

>>> for key, item in dict([('a', 1), ('b', 2), ('c', 3), ('d', 4)]).iteritems(): print key, item
... 
a 1
c 3
b 2
d 4
Charles Greenberg 8 years, 2 months ago  # | flag

Is it possible to adapt this to also work as a defaultdict? I'm working in python 2.6 so I can use the defaultdict module.

Martin Miller 7 years, 6 months ago  # | flag

@Charles Greenberg: There's a relatively simple OrderedDefaultdict implementation at the beginning of my answer to a question on stackoverflow. Since it uses OrderedDict as its base class and that class wasn't added until Python 2.7, you could use the recipe for one on this site that works with Python 2.4+.