如何在OrderedDict的开头添加元素? [英] How to add an element to the beginning of an OrderedDict?

问题描述

我有这个:

d1 = OrderedDict([('a', '1'), ('b', '2')])

如果我这样做:

d1.update({'c':'3'})

然后我得到了:

OrderedDict([('a', '1'), ('b', '2'), ('c', '3')])

但是我想要这个:

[('c', '3'), ('a', '1'), ('b', '2')]

无需创建新词典.

推荐答案

在Python 2中没有内置方法可以执行此操作. OrderedDict内部结构复杂度为O(1).

There's no built-in method for doing this in Python 2. If you need this, you need to write a prepend() method/function that operates on the OrderedDict internals with O(1) complexity.

对于Python 3.2和更高版本，您应使用 move_to_end 方法.该方法接受last参数，该参数指示元素将被移动到OrderedDict的底部(last=True)还是顶部(last=False).

For Python 3.2 and later, you should use the move_to_end method. The method accepts a last argument which indicates whether the element will be moved to the bottom (last=True) or the top (last=False) of the OrderedDict.

最后，如果您想要一种快速，肮脏且缓慢的解决方案，则可以从头开始创建一个新的OrderedDict.

Finally, if you want a quick, dirty and slow solution, you can just create a new OrderedDict from scratch.

四种不同解决方案的详细信息:

Details for the four different solutions:

from collections import OrderedDict

class MyOrderedDict(OrderedDict):

    def prepend(self, key, value, dict_setitem=dict.__setitem__):

        root = self._OrderedDict__root
        first = root[1]

        if key in self:
            link = self._OrderedDict__map[key]
            link_prev, link_next, _ = link
            link_prev[1] = link_next
            link_next[0] = link_prev
            link[0] = root
            link[1] = first
            root[1] = first[0] = link
        else:
            root[1] = first[0] = self._OrderedDict__map[key] = [root, first, key]
            dict_setitem(self, key, value)

演示:

>>> d = MyOrderedDict([('a', '1'), ('b', '2')])
>>> d
MyOrderedDict([('a', '1'), ('b', '2')])
>>> d.prepend('c', 100)
>>> d
MyOrderedDict([('c', 100), ('a', '1'), ('b', '2')])
>>> d.prepend('a', d['a'])
>>> d
MyOrderedDict([('a', '1'), ('c', 100), ('b', '2')])
>>> d.prepend('d', 200)
>>> d
MyOrderedDict([('d', 200), ('a', '1'), ('c', 100), ('b', '2')])

---

操作OrderedDict对象的独立函数

此函数通过接受dict对象，键和值来执行相同的操作.我个人更喜欢上课:

---

Standalone function that manipulates OrderedDict objects

This function does the same thing by accepting the dict object, key and value. I personally prefer the class:

from collections import OrderedDict

def ordered_dict_prepend(dct, key, value, dict_setitem=dict.__setitem__):
    root = dct._OrderedDict__root
    first = root[1]

    if key in dct:
        link = dct._OrderedDict__map[key]
        link_prev, link_next, _ = link
        link_prev[1] = link_next
        link_next[0] = link_prev
        link[0] = root
        link[1] = first
        root[1] = first[0] = link
    else:
        root[1] = first[0] = dct._OrderedDict__map[key] = [root, first, key]
        dict_setitem(dct, key, value)

演示:

>>> d = OrderedDict([('a', '1'), ('b', '2')])
>>> ordered_dict_prepend(d, 'c', 100)
>>> d
OrderedDict([('c', 100), ('a', '1'), ('b', '2')])
>>> ordered_dict_prepend(d, 'a', d['a'])
>>> d
OrderedDict([('a', '1'), ('c', 100), ('b', '2')])
>>> ordered_dict_prepend(d, 'd', 500)
>>> d
OrderedDict([('d', 500), ('a', '1'), ('c', 100), ('b', '2')])

---

使用OrderedDict.move_to_end()(Python> = 3.2)

引入了Python 3.2

---

Use OrderedDict.move_to_end() (Python >= 3.2)

Python 3.2 introduced the OrderedDict.move_to_end() method. Using it, we can move an existing key to either end of the dictionary in O(1) time.

>>> d1 = OrderedDict([('a', '1'), ('b', '2')])
>>> d1.update({'c':'3'})
>>> d1.move_to_end('c', last=False)
>>> d1
OrderedDict([('c', '3'), ('a', '1'), ('b', '2')])

如果我们需要插入一个元素并将其移到顶部，只需一步即可，我们可以直接使用它创建一个prepend()包装器(此处未显示).

If we need to insert an element and move it to the top, all in one step, we can directly use it to create a prepend() wrapper (not presented here).

如果您不想这样做，并且性能不是问题，那么最简单的方法是创建一个新的字典:

If you don't want to do that and performance is not an issue then easiest way is to create a new dict:

from itertools import chain, ifilterfalse
from collections import OrderedDict


def unique_everseen(iterable, key=None):
    "List unique elements, preserving order. Remember all elements ever seen."
    # unique_everseen('AAAABBBCCDAABBB') --> A B C D
    # unique_everseen('ABBCcAD', str.lower) --> A B C D
    seen = set()
    seen_add = seen.add
    if key is None:
        for element in ifilterfalse(seen.__contains__, iterable):
            seen_add(element)
            yield element
    else:
        for element in iterable:
            k = key(element)
            if k not in seen:
                seen_add(k)
                yield element

d1 = OrderedDict([('a', '1'), ('b', '2'),('c', 4)])
d2 = OrderedDict([('c', 3), ('e', 5)])   #dict containing items to be added at the front
new_dic = OrderedDict((k, d2.get(k, d1.get(k))) for k in \
                                           unique_everseen(chain(d2, d1)))
print new_dic

输出:

OrderedDict([('c', 3), ('e', 5), ('a', '1'), ('b', '2')])

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