Use array as index in python

Question

So I have an array like so ['test', 'testtwo'].

I wish to able to use that as an index for a dictionary like so myDict['test']['testtwo'].

Is this possible in python? Sorry for the short explanation.

EDIT:

exampleDict = {
  'test': {
    'testtwo': [
      '',
      ''
    ]
  }
}

And when doing some stuff in python I end up with the array ['test', 'testtwo'] and then need to use that to access exampleDict['test']['testtwo']. The keys change though and the number of keys in the array changes as well.

What is the bigger picture of what you want to achieve? a dictionary might be what you are looking for — Gilad Green
– Gilad Green, Commented Feb 17, 2017 at 14:58
do you mean: __list=['test', 'testtwo']; mydict = {__list: "foo" }? — felipsmartins
– felipsmartins, Commented Feb 17, 2017 at 15:01

tobias_k · Accepted Answer · 2017-02-17 15:41:00Z

2

You could either use a loop, iterating the indices in the list and updating the "current" dict as you go:

>>> exampleDict = {'test': {'testtwo': [ '', '']}}
>>> d = exampleDict                               
>>> for x in idx:
...     d = d[x]
>>> d
['', '']

Or you could even use reduce (functools.reduce in Python 3):

>>> reduce(lambda d, x: d[x], idx, exampleDict)
['', '']

You could use a similar approach to update the dict, but 1) you should use setdefault in case part of the index-list is not yet in the dict, and 2) you have to remove the last item from the list and use that as a regular index to the returned dictionary.

>>> idx2 = ['test', 'testthree', 'four']
>>> reduce(lambda d, x: d.setdefault(x, {}), idx2[:-1], exampleDict)[idx2[-1]] = "foo"
>>> exampleDict
{'test': {'testthree': {'four': 'foo'}, 'testtwo': ['', '']}}

In Python 3, you could make that line a bit easier to use using tuple-unpacking with *:

>>> *path, last = idx2
>>> reduce(lambda d, x: d.setdefault(x, {}), path, exampleDict)[last] = "foo"

edited Feb 17, 2017 at 15:41

answered Feb 17, 2017 at 15:12

tobias_k

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2 Comments

Matt Cowley Over a year ago

This works great, but how can I then update the original dictionary?

Matt Cowley Over a year ago

Thank you so much @tobias_k the last bit works like a charm!

synchronizer · Accepted Answer · 2017-02-17 15:06:16Z

2

You cannot use a list for a dictionary key because lists are mutable, and mutable keys aren't allowed, but I think that what you want is to use each element of the list as an index. Without more context, it's not easy to say whether this is a good idea without proper checks, but:

my_list = ['a', 'b']
my_dict[my_list[0]][my_list[1]] # access at ['a']['b']

I have a feeling that whichever problem you wish to solve might be solved in a different way.

edited Feb 17, 2017 at 15:06

answered Feb 17, 2017 at 15:02

synchronizer

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2 Comments

Matt Cowley Over a year ago

Like this, but the number of keys in my_list changes so I can't type it out manually as you did in the second line.

synchronizer Over a year ago

@MattCowley Right, but I hard-coded the list for the sake of example. You will need to pre-populate your dictionary while you build the list so key accesses to the dictionary are valid. I wrote my answer before you provided additional information thogh. tobias_k's answer looks good for the dictionary updates I described.

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