[英]Set nested dict value and create intermediate keys
I feel like I saw a way to do this recently. 我觉得最近我看到了一种方法。 Say I've got an empty dict and I want to set a value in a nested dict inside that empty dict, but obviously that nested dict hasn't been created yet.
假设我有一个空字典,我想在该空字典中的嵌套字典中设置一个值,但显然嵌套字典尚未创建。 Is there a 1-line way to create the intermediate keys?
是否有一种创建中间键的方法? This is what I want to do:
这就是我想要做的:
mydict = {}
mydict['foo']['bar']['foobar'] = 25
If you execute this code you'll get a KeyError exception for 'foo'. 如果执行此代码,您将获得'foo'的KeyError异常。 Is there a function to create intermediate keys?
是否有创建中间键的功能?
Thanks. 谢谢。
from collections import defaultdict
recursivedict = lambda: defaultdict(recursivedict)
mydict = recursivedict()
When you access mydict['foo']
, it sets mydict['foo']
to another recursivedict
. 当你访问
mydict['foo']
,它mydict['foo']
为另一个recursivedict
。 It'll actually construct a recursivedict
for mydict['foo']['bar']['foobar']
as well, but then it'll get thrown out by assigning that to 25
. 它实际上也会为
mydict['foo']['bar']['foobar']
构建一个recursivedict
,但是它会被分配给25
来抛出它。
An alternative option - depending on your uses, is to use tuples as keys instead of nested dictionaries: 另一种选择 - 取决于您的用途,是使用元组作为键而不是嵌套字典:
mydict = {}
mydict['foo', 'bar', 'foobar'] = 25
This will work perfectly well unless you want to get a branch of the tree at any point (you can't get mydict['foo'] in this case). 这将非常有效,除非你想在任何一点获得树的分支(在这种情况下你不能得到mydict ['foo'])。
If you knew how many layers of nesting you want, you could also use functools.partial
instead of lambda. 如果你知道你想要多少层嵌套,你也可以使用
functools.partial
而不是lambda。
from functools import partial
from collections import defaultdict
tripledict = partial(defaultdict, partial(defaultdict, dict))
mydict = tripledict()
mydict['foo']['bar']['foobar'] = 25
Which some people find more readable, and is faster to create instances of than the equivalent lambda-based solution: 有些人发现更具可读性,并且创建实例比基于lambda的等效解决方案更快:
python -m timeit -s "from functools import partial" -s "from collections import defaultdict" -s "tripledefaultdict = partial(defaultdict, partial(defaultdict, dict))" "tripledefaultdict()"
1000000 loops, best of 3: 0.281 usec per loop
python -m timeit -s "from collections import defaultdict" -s "recursivedict = lambda: defaultdict(recursivedict)" "recursivedict()"
1000000 loops, best of 3: 0.446 usec per loop
Although, as always, there is no point optimising until you know there is a bottleneck, so pick what is most useful and readable before what is fastest. 虽然一如既往,在知道存在瓶颈之前没有必要进行优化,因此在最快之前选择最有用和可读的内容。
Not sure why you'd want to, but: 不知道为什么你想要,但是:
>>> from collections import defaultdict as dd
>>> mydict = dd(lambda: dd(lambda: {}))
>>> mydict['foo']['bar']['foobar'] = 25
>>> mydict
defaultdict(<function <lambda> at 0x021B8978>, {'foo': defaultdict(<function <lambda> at 0x021B8618>, {'bar': {'foobar': 25}})})
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