Get values from many dictionaries by hierarchy

Question

I have 4 dictionaries, let's call them:

dict1 , dict2 , dict3 , dict4

Example:

dict1 = {'A': 1, 'B':2}
dict2 = {'A': 3, 'C':4}
dict3 = {'B': 5, 'D':6}
dict4 = {'A': 7, 'B':8, 'C': 9, 'D':10, 'E':11}

Each dictionary level is "stronger" than those who come after it. As in, A found in dict1 will be 'stronger' than A found in dict2 in terms of precedence. Is there a short, elegant script to create a new dictionary, assembled from all four, where each key is taken from the "strongest" dictionary that contains that key?

The result should be: dict = {'A':1, 'B':2, 'C':4, 'D:6', 'E':11}

Some answers here can be adapted: https://stackoverflow.com/questions/38987/how-do-i-merge-two-dictionaries-in-a-single-expression — gorhawk, Nov 20 '19 at 15:07
Related: [How do I merge a list of dicts into a single dict?](https://stackoverflow.com/q/3494906/7851470) — Georgy, Nov 21 '19 at 15:13

score 5 · Accepted Answer · answered Nov 20 '19 at 15:09

I think the easiest/clearest approach here would be to create a new dictionary then use its update method, which overwrites existing keys. Something like this makes the precedence pretty obvious:

>>> x = {}
>>> x.update(dict4)
>>> x.update(dict3)
>>> x.update(dict2)
>>> x.update(dict1)
>>> x
{'A': 1, 'B': 2, 'C': 4, 'D': 6, 'E': 11}

Docs

You could of course make a utility of some sort for this, something like:

>>> def collapse(*dicts):
...     x = {}
...     for dict in dicts:
...         x.update(dict)
...     return x
...
>>>
>>> collapse(dict4, dict3, dict2, dict1)
{'A': 1, 'B': 2, 'C': 4, 'D': 6, 'E': 11}

(Though you'd need to remember to pass the dictionaries in the correct order.)

score 4 · Answer 2 · answered Nov 20 '19 at 15:10

4

You could do the following (works on python 3.5 and newer):

result = {**dict4, **dict3, **dict2, **dict1}

answered Nov 20 '19 at 15:10

Prebsus

695
9
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Primusa · Answer 3 · 2019-11-20T15:21:44.997

1

Here's a fairly simple way for an arbitrary number of dictionaries:

dict1 = {'A': 1, 'B':2}
dict2 = {'A': 3, 'C':4}
dict3 = {'B': 5, 'D':6}
dict4 = {'A': 7, 'B':8, 'C': 9, 'D':10, 'E':11}

# strongest dictionary last
dictionaries = [dict4, dict3, dict2, dict1]

dict(i for d in dictionaries for i in d.items())

Output:

{'A': 1, 'B': 2, 'C': 4, 'D': 6, 'E': 11}

edited Nov 20 '19 at 15:21

answered Nov 20 '19 at 15:13

Primusa

13,136
3
33
53

score -1 · Answer 4 · answered Nov 20 '19 at 23:54

You probably want a ChainMap, which is perfect for simulating scope.

>>> import collections
>>> cm = collections.ChainMap(dict1, dict2, dict3, dict4)
>>> dict(cm)
{'A': 1, 'B': 2, 'C': 4, 'D': 6, 'E': 11}
>>> cm['A'] = 'foo'
>>> cm
ChainMap({'A': 'foo', 'B': 2}, {'A': 3, 'C': 4}, {'B': 5, 'D': 6}, {'A': 7, 'B': 8, 'C': 9, 'D': 10, 'E': 11})
>>> dict1
{'A': 'foo', 'B': 2}

Get values from many dictionaries by hierarchy

4 Answers4