How do I create a nested dictionary under a key th

问题:

I have an empty array.

I want to assign a value like this: array[key][subkey] = 'value'

This produces a KeyError as array[key] does not exist yet.

What do I do? I tried the following...

array['key'] = None
array['key']['subkey'] = 'value'

TypeError: 'NoneType' object does not support item assignment

I tried:

array['key'] = []
array['key']['subkey'] = 'value'

TypeError: list indices must be integers, not str

I tried:

array['key'] = ['subkey']
array['key']['subkey'] = 'value'

TypeError: list indices must be integers, not str

So what do I do?

回答1:

You could use collections.defaultdict, passing the default factory as dict:

>>> from collections import defaultdict
>>> d = defaultdict(dict)
>>> d['key']['subkey'] = 'value'
>>> d
defaultdict(<type 'dict'>, {'key': {'subkey': 'value'}})

To apply further levels of nesting, you can create a defaultdict that returns defaultdicts to a n-th depth of nesting, using a function, preferably anonymous, to return the nested default dict(s):

>>> d = defaultdict(lambda: defaultdict(dict))
>>> d['key']['subkey']['subsubkey'] = 'value'
>>> d
defaultdict(<function <lambda> at 0x104082398>, {'key': defaultdict(<type 'dict'>, {'subkey': {'subsubkey': 'value'}})})

_{Example shows nesting up to depth n=1}

回答2:

You are using a [] list literal not a {} dict literal:

array['key'] = {}
array['key']['subkey'] = 'value'

But this isn't very useful in a loop.
In a loop you could test if 'key' is not in array - which is a cheap operation (O(1) lookup):

if 'key' not in array:
    array['key'] = {}
array['key']['subkey'] = 'value'

But you can use setdefault() to do the same thing and give key a default value if it doesn't already have a value, e.g.:

array.setdefault('key', {})['subkey'] = 'value'

And if this looks ugly, then you can always use collection.defaultdict.

回答3:

I have used this alternative way before (edit: although I preferred Moses Koledoye answer):

array = {}

try:
    array[key][subkey] = 'value'
except KeyError:
    array[key] = {}
    array[key][subkey] = 'value'

回答4:

You could use a nested defaultdict like this, goes arbitrarily deep automatically:

>>> from collections import defaultdict
>>> def ndd():
        return defaultdict(ndd)

>>> array = ndd()
>>> array['key']['subkey']['subsubkey'] = 'value'
>>> array
defaultdict(<function ndd at 0x041AC270>,
            {'key': defaultdict(<function ndd at 0x041AC270>,
                                {'subkey': defaultdict(<function ndd at 0x041AC270>,
                                                       {'subsubkey': 'value'})})})

(I indented the output for readability.)

Originally saw that from severb.

回答5:

Can the below codes be used as a different method?

>>> array = {}
>>> array["key"] = {"subkey": "value"}
>>> array
{'key': {'subkey': 'value'}}
>>>