如何合并列表中key相同的字典？

现有list：

list1 = [{a: 123}, {a: 456},{b: 789}]

合并成：

list2 = [{a: [123,456]},{b: [789]}]

如何实现，望大神指点，不胜感激！！

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用个map 键存在就append 不存在就赋值 最后转成list...

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In [1]: from collections import defaultdict

In [2]: list1 = [{'a': 123}, {'a': 456}, {'b': 789}]

In [3]: list2 = defaultdict(list)

In [4]: [list2[k].append(v) for i in list1 for k, v in i.items()]
Out[4]: [None, None, None]

In [5]: list2.items()
Out[5]: dict_items([('b', [789]), ('a', [123, 456])])

In [6]: dict(list2)
Out[6]: {'a': [123, 456], 'b': [789]}

In [7]:

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from collections import defaultdict
lst = [{'a': 123}, {'a': 456},{'b': 789}]

dic = {}
for _ in lst:
    for k, v in _.items():
        dic.setdefault(k, []).append(v)

print [{k:v} for k, v in dic.items()]