字典的Python列表 [英] Python list to dict

问题描述

我有一个庞大的清单，例如

I have a huge list like

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

我想遍历此内容，并获得不同的第一项的第二项的编号输出:

I want to walk through this and get an output of number of each second item for a distinct first item:

{a:[2,1,1] b:[3,1,3,1] c:[2,2]}

推荐答案

data = [['a','2'],['a','1'],['b','3'],['c','2'],['b','1'],['a','1'],['b','1'],['c','2'],['b','3'],['b','1']]
result = {}
for key, value in data:
    result.setdefault(key, []).append(value)

结果:

>>> result
{'a': ['2', '1', '1'], 'c': ['2', '2'], 'b': ['3', '1', '1', '3', '1']}

与defaultdict相比，我更喜欢 dict.setdefault()普通字典，尝试访问不存在的键会引发异常，而不是给出值(在这种情况下为空列表).

I prefer dict.setdefault() over defaultdict because you end up with a normal dictionary, where attempting to access a key that doesn't exist raises an exception instead of giving a value (in this case an empty list).

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