字典合并中的字典合并 [英] Dict merge in a dict comprehension

问题描述

在python 3.5中，我们可以使用double-splat解包来合并字典

In python 3.5, we can merge dicts by using double-splat unpacking

>>> d1 = {1: 'one', 2: 'two'}
>>> d2 = {3: 'three'}
>>> {**d1, **d2}
{1: 'one', 2: 'two', 3: 'three'}

很酷.不过，它似乎并没有推广到动态用例:

Cool. It doesn't seem to generalise to dynamic use cases, though:

>>> ds = [d1, d2]
>>> {**d for d in ds}
SyntaxError: dict unpacking cannot be used in dict comprehension

相反，我们必须执行reduce(lambda x,y: {**x, **y}, ds, {})，这看起来很丑陋.当该表达式似乎没有任何歧义时，为什么解析器不允许一种明显的方法"?

Instead we have to do reduce(lambda x,y: {**x, **y}, ds, {}), which seems a lot uglier. Why the "one obvious way to do it" is not allowed by the parser, when there doesn't seem to be any ambiguity in that expression?

推荐答案

这并不完全是您的问题的答案，但我会考虑使用

It's not exactly an answer to your question but I'd consider using ChainMap to be an idiomatic and elegant way to do what you propose (merging dictionaries in-line):

>>> from collections import ChainMap
>>> d1 = {1: 'one', 2: 'two'}
>>> d2 = {3: 'three'}
>>> ds = [d1, d2]
>>> dict(ChainMap(*ds))
{1: 'one', 2: 'two', 3: 'three'}

尽管这不是一个特别透明的解决方案，但是由于许多程序员可能不完全了解ChainMap的工作方式.请注意(如@AnttiHaapala所指出的)，首先找到被使用"是这样，因此，根据您的意图，您可能需要先调用reversed，然后再将dict传递到ChainMap.

Although it's not a particularly transparent solution, since many programmers might not know exactly how a ChainMap works. Note that (as @AnttiHaapala points out) "first found is used" so, depending on your intentions you might need to make a call to reversed before passing your dicts into ChainMap.

>>> d2 = {3: 'three', 2:'LOL'}
>>> dict(ChainMap(*ds))
{1: 'one', 2: 'two', 3: 'three'}

>>> dict(ChainMap(*reversed(ds)))
{1: 'one', 2: 'LOL', 3: 'three'}

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