By kolergy

2013-02-04 18:04:35 8 Comments

I have a complex dictionary structure which I would like to access via a list of keys to address the correct item.

dataDict = {
        "r": 1,
        "s": 2,
        "t": 3
        "u": 1,
        "v": {
            "x": 1,
            "y": 2,
            "z": 3
        "w": 3

maplist = ["a", "r"]


maplist = ["b", "v", "y"]

I have made the following code which works but I'm sure there is a better and more efficient way to do this if anyone has an idea.

# Get a given data from a dictionary with position provided as a list
def getFromDict(dataDict, mapList):    
    for k in mapList: dataDict = dataDict[k]
    return dataDict

# Set a given data in a dictionary with position provided as a list
def setInDict(dataDict, mapList, value): 
    for k in mapList[:-1]: dataDict = dataDict[k]
    dataDict[mapList[-1]] = value


@alancalvitti 2019-05-01 19:00:07

Extending @DomTomCat and others' approach, these functional (ie, return modified data via deepcopy without affecting the input) setter and mapper works for nested dict and list.


def set_at_path(data0, keys, value):
    data = deepcopy(data0)
    if len(keys)>1:
        if isinstance(data,dict):
            return {k:(set_by_path(v,keys[1:],value) if k==keys[0] else v) for k,v in data.items()}
        if isinstance(data,list):
            return [set_by_path(x[1],keys[1:],value) if x[0]==keys[0] else x[1] for x in enumerate(data)]
        return data


def map_at_path(data0, keys, f):
    data = deepcopy(data0)
    if len(keys)>1:
        if isinstance(data,dict):
            return {k:(map_at_path(v,keys[1:],f) if k==keys[0] else v) for k,v in data.items()}
        if isinstance(data,list):
            return [map_at_path(x[1],keys[1:],f) if x[0]==keys[0] else x[1] for x in enumerate(data)]
        return data

@nehemiah 2018-12-03 00:56:24

It's satisfying to see these answers for having two static methods for setting & getting nested attributes. These solutions are way better than using nested trees

Here's my implementation.


To set nested attribute call sattr(my_dict, 1, 2, 3, 5) is equal to my_dict[1][2][3][4]=5

To get a nested attribute call gattr(my_dict, 1, 2)

def gattr(d, *attrs):
    This method receives a dict and list of attributes to return the innermost value of the give dict       
        for at in attrs:
            d = d[at]
        return d
    except(KeyError, TypeError):
        return None

def sattr(d, *attrs):
    Adds "val" to dict in the hierarchy mentioned via *attrs
    For ex:
    sattr(animals, "cat", "leg","fingers", 4) is equivalent to animals["cat"]["leg"]["fingers"]=4
    This method creates necessary objects until it reaches the final depth
    This behaviour is also known as autovivification and plenty of implementation are around
    This implementation addresses the corner case of replacing existing primitives
    for attr in attrs[:-2]:
        if type(d.get(attr)) is not dict:
            d[attr] = {}
        d = d[attr]
    d[attrs[-2]] = attrs[-1]

@lucas 2018-12-29 05:10:48

a method for concatenating strings:

def get_sub_object_from_path(dict_name, map_list):
    for i in map_list:
        _string = "['%s']" % i
        dict_name += _string
    value = eval(dict_name)
    return value
_dict = {'new': 'person', 'time': {'for': 'one'}}
map_list = ['time', 'for']
print get_sub_object_from_path("_dict",map_list)

@Jack Casey 2018-12-09 08:43:06

Very late to the party, but posting in case this may help someone in the future. For my use case, the following function worked the best. Works to pull any data type out of dictionary

dict is the dictionary containing our value

list is a list of "steps" towards our value

def getnestedvalue(dict, list):

    length = len(list)
        for depth, key in enumerate(list):
            if depth == length - 1:
                output = dict[key]
                return output
            dict = dict[key]
    except (KeyError, TypeError):
        return None

    return None

@And0k 2018-08-15 09:46:38

How about check and then set dict element without processing all indexes twice?


def nested_yield(nested, keys_list):
    Get current nested data by send(None) method. Allows change it to Value by calling send(Value) next time
    :param nested: list or dict of lists or dicts
    :param keys_list: list of indexes/keys
    if not len(keys_list):  # assign to 1st level list
        if isinstance(nested, list):
            while True:
                nested[:] = yield nested
            raise IndexError('Only lists can take element without key')

    last_key = keys_list.pop()
    for key in keys_list:
        nested = nested[key]

    while True:
            nested[last_key] = yield nested[last_key]
        except IndexError as e:
            print('no index {} in {}'.format(last_key, nested))
            yield None

Example workflow:

ny = nested_yield(nested_dict, nested_address)
data_element = ny.send(None)
if data_element:
    # process element
    # extend/update nested data


>>> cfg= {'Options': [[1,[0]],[2,[4,[8,16]]],[3,[9]]]}
    ny = nested_yield(cfg, ['Options',1,1,1])
[8, 16]
>>> ny.send('Hello!')
>>> cfg
{'Options': [[1, [0]], [2, [4, 'Hello!']], [3, [9]]]}
>>> ny.close()

@Martijn Pieters 2013-02-04 18:07:38

Use reduce() to traverse the dictionary:

from functools import reduce  # forward compatibility for Python 3
import operator

def getFromDict(dataDict, mapList):
    return reduce(operator.getitem, mapList, dataDict)

and reuse getFromDict to find the location to store the value for setInDict():

def setInDict(dataDict, mapList, value):
    getFromDict(dataDict, mapList[:-1])[mapList[-1]] = value

All but the last element in mapList is needed to find the 'parent' dictionary to add the value to, then use the last element to set the value to the right key.


>>> getFromDict(dataDict, ["a", "r"])
>>> getFromDict(dataDict, ["b", "v", "y"])
>>> setInDict(dataDict, ["b", "v", "w"], 4)
>>> import pprint
>>> pprint.pprint(dataDict)
{'a': {'r': 1, 's': 2, 't': 3},
 'b': {'u': 1, 'v': {'w': 4, 'x': 1, 'y': 2, 'z': 3}, 'w': 3}}

Note that the Python PEP8 style guide prescribes snake_case names for functions. The above works equally well for lists or a mix of dictionaries and lists, so the names should really be get_by_path() and set_by_path():

from functools import reduce  # forward compatibility for Python 3
import operator

def get_by_path(root, items):
    """Access a nested object in root by item sequence."""
    return reduce(operator.getitem, items, root)

def set_by_path(root, items, value):
    """Set a value in a nested object in root by item sequence."""
    get_by_path(root, items[:-1])[items[-1]] = value

@Dmitriy Sintsov 2015-02-11 10:51:41

How much such traversing is reliable for arbitrary nested structures? Will it work for mixed dictionaries with nested lists as well? How do I modify getFromDict() to supply default_value and to have default default_value as None? I am novice in Python with many years of PHP development and before C development.

@Dmitriy Sintsov 2015-02-11 10:59:07

Also nested mapped set should create non-existing nodes, imo: lists for integer keys, dictionaries for string keys.

@Martijn Pieters 2015-02-11 12:04:24

@user1353510: as it happens, regular indexing syntax is used here, so it'll support lists inside dictionaries too. Just pass in integer indexes for those.

@Martijn Pieters 2015-02-11 12:04:43

@user1353510: different usecases call for different behaviour. The code here doesn't create intermediaries, no.

@Martijn Pieters 2015-02-11 12:05:55

@user1353510: for a default value, use try:, except (KeyError, IndexError): return default_value around the current return line.

@Martijn Pieters 2015-03-03 13:08:55

@user1353510: See List to nested dictionary in python for the other use-case; using dict.setdefault() rather than dict.__getitem__.

@antonavy 2016-08-09 13:58:34

Hack is that lambda also works for occasional list in your nested dict :) So you can use something like ['key1', 'key2', 0, 'key3'] where 0 is a list index.

@Martijn Pieters 2016-08-09 14:07:57

@antonavy: you could have just as easily tried that; yes, all the lambda does is use subscription, so if d['key1']['key2'] is a list, then it'll all work.

@nehemiah 2018-12-03 00:59:32

@MartijnPieters You might want to initialize the dict in set_by_path. Fails for this case

@Martijn Pieters 2018-12-03 13:10:45

@nehemiah: that's a different usecase. You want get_by_path to materialise dictionaries that don't exist yet, that's a fair usecase but not one that my functions here cover. That's what the comments to user1353510 above already address.

@DomTomCat 2016-06-08 13:47:04

  1. The accepted solution won't work directly for python3 - it will need an from functools import reduce.
  2. Also it seems more pythonic to use a for loop. See the quote from What’s New In Python 3.0.

    Removed reduce(). Use functools.reduce() if you really need it; however, 99 percent of the time an explicit for loop is more readable.

  3. Next, the accepted solution doesn't set non-existing nested keys (it returns a KeyError) - see @eafit's answer for a solution

So why not use the suggested method from kolergy's question for getting a value:

def getFromDict(dataDict, mapList):    
    for k in mapList: dataDict = dataDict[k]
    return dataDict

And the code from @eafit's answer for setting a value:

def nested_set(dic, keys, value):
    for key in keys[:-1]:
        dic = dic.setdefault(key, {})
    dic[keys[-1]] = value

Both work straight in python 2 and 3

@Dylan F 2018-06-13 03:33:15

I prefer this solution - but be careful. If I'm not mistaken, since Python dictionaries are not immutable getFromDict has the potential to destroy the caller's dataDict. I would copy.deepcopy(dataDict) first. Of course, (as written) this behavior is desired in the second function.

@Pulkit 2018-03-16 01:30:53

An alternative way if you don't want to raise errors if one of the keys is absent (so that your main code can run without interruption):

def get_value(self,your_dict,*keys):
    curr_dict_ = your_dict
    for k in keys:
        v = curr_dict.get(k,None)
        if v is None:
        if isinstance(v,dict):
            curr_dict = v
    return v

In this case, if any of the input keys is not present, None is returned, which can be used as a check in your main code to perform an alternative task.

@Grant Palmer 2018-03-06 21:30:01

If you also want the ability to work with arbitrary json including nested lists and dicts, and nicely handle invalid lookup paths, here's my solution:

from functools import reduce

def get_furthest(s, path):
    Gets the furthest value along a given key path in a subscriptable structure.

    subscriptable, list -> any
    :param s: the subscriptable structure to examine
    :param path: the lookup path to follow
    :return: a tuple of the value at the furthest valid key, and whether the full path is valid

    def step_key(acc, key):
        s = acc[0]
        if isinstance(s, str):
            return (s, False)
            return (s[key], acc[1])
        except LookupError:
            return (s, False)

    return reduce(step_key, path, (s, True))

def get_val(s, path):
    val, successful = get_furthest(s, path)
    if successful:
        return val
        raise LookupError('Invalid lookup path: {}'.format(path))

def set_val(s, path, value):
    get_val(s, path[:-1])[path[-1]] = value

@Poh Zi How 2018-03-05 13:38:11

Solved this with recursion:

def get(d,l):
    if len(l)==1: return d[l[0]]
    return get(d[l[0]],l[1:])

Using your example:

dataDict = {
        "r": 1,
        "s": 2,
        "t": 3
        "u": 1,
        "v": {
            "x": 1,
            "y": 2,
            "z": 3
        "w": 3
maplist1 = ["a", "r"]
maplist2 = ["b", "v", "y"]
print(get(dataDict, maplist1)) # 1
print(get(dataDict, maplist2)) # 2

@Arount 2018-02-16 14:00:54

Pure Python style, without any import:

def nested_set(element, value, *keys):
    if type(element) is not dict:
        raise AttributeError('nested_set() expects dict as first argument.')
    if len(keys) < 2:
        raise AttributeError('nested_set() expects at least three arguments, not enough given.')

    _keys = keys[:-1]
    _element = element
    for key in _keys:
        _element = _element[key]
    _element[keys[-1]] = value

example = {"foo": { "bar": { "baz": "ok" } } }
keys = ['foo', 'bar']
nested_set(example, "yay", *keys)


{'foo': {'bar': 'yay'}}

@xyres 2017-12-08 22:56:27

How about using recursive functions?

To get a value:

def getFromDict(dataDict, maplist):
    first, rest = maplist[0], maplist[1:]

    if rest: 
        # if `rest` is not empty, run the function recursively
        return getFromDict(dataDict[first], rest)
        return dataDict[first]

And to set a value:

def setInDict(dataDict, maplist, value):
    first, rest = maplist[0], maplist[1:]

    if rest:
            if not isinstance(dataDict[first], dict):
                # if the key is not a dict, then make it a dict
                dataDict[first] = {}
        except KeyError:
            # if key doesn't exist, create one
            dataDict[first] = {}

        setInDict(dataDict[first], rest, value)
        dataDict[first] = value

@josifoski 2018-06-25 14:21:52

Great solution!

@OkezieE 2017-03-01 02:07:58

Instead of taking a performance hit each time you want to look up a value, how about you flatten the dictionary once then simply look up the key like b:v:y

def flatten(mydict):
  new_dict = {}
  for key,value in mydict.items():
    if type(value) == dict:
      _dict = {':'.join([key, _key]):_value for _key, _value in flatten(value).items()}
  return new_dict

dataDict = {
    "r": 1,
    "s": 2,
    "t": 3
    "u": 1,
    "v": {
        "x": 1,
        "y": 2,
        "z": 3
    "w": 3

flat_dict = flatten(dataDict)
print flat_dict
{'b:w': 3, 'b:u': 1, 'b:v:y': 2, 'b:v:x': 1, 'b:v:z': 3, 'a:r': 1, 'a:s': 2, 'a:t': 3}

This way you can simply look up items using flat_dict['b:v:y'] which will give you 1.

And instead of traversing the dictionary on each lookup, you may be able to speed this up by flattening the dictionary and saving the output so that a lookup from cold start would mean loading up the flattened dictionary and simply performing a key/value lookup with no traversal.

@eafit 2016-04-29 04:38:54

Using reduce is clever, but the OP's set method may have issues if the parent keys do not pre-exist in the nested dictionary. Since this is the first SO post I saw for this subject in my google search, I would like to make it slightly better.

The set method in ( Setting a value in a nested python dictionary given a list of indices and value ) seems more robust to missing parental keys. To copy it over:

def nested_set(dic, keys, value):
    for key in keys[:-1]:
        dic = dic.setdefault(key, {})
    dic[keys[-1]] = value

Also, it can be convenient to have a method that traverses the key tree and get all the absolute key paths, for which I have created:

def keysInDict(dataDict, parent=[]):
    if not isinstance(dataDict, dict):
        return [tuple(parent)]
        return reduce(list.__add__, 
            [keysInDict(v,parent+[k]) for k,v in dataDict.items()], [])

One use of it is to convert the nested tree to a pandas DataFrame, using the following code (assuming that all leafs in the nested dictionary have the same depth).

def dict_to_df(dataDict):
    ret = []
    for k in keysInDict(dataDict):
        v = np.array( getFromDict(dataDict, k), )
        v = pd.DataFrame(v)
        v.columns = pd.MultiIndex.from_product(list(k) + [v.columns])
    return reduce(pd.DataFrame.join, ret)

@alancalvitti 2019-05-01 17:19:59

why arbitrarily limit the 'keys' argument length to 2 or more in nested_set?

@DMfll 2015-04-29 06:02:00

This library may be helpful:

A python library for accessing and searching dictionaries via /slashed/paths ala xpath

Basically it lets you glob over a dictionary as if it were a filesystem.

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