merging Python dictionaries

Question

I am trying to merge the following python dictionaries as follow:

dict1= {'paul':100, 'john':80, 'ted':34, 'herve':10}
dict2 = {'paul':'a', 'john':'b', 'ted':'c', 'peter':'d'}

output = {'paul':[100,'a'],
          'john':[80, 'b'],
          'ted':[34,'c'],
          'peter':[None, 'd'],
          'herve':[10, None]}

I wish to keep all keys from both dictionaries.
Is there an efficient way to do this?

A little formatting would go a long way here. Try editing your question: select the code and click the "101 010" button in the edit window. Also, separate dict1 & dict2 into two separate lines. — Pete, Mar 02 '10 at 19:19
[@rcreswick's answer](http://stackoverflow.com/questions/38987/how-can-i-merge-two-python-dictionaries-as-a-single-expression/44512#44512) to this question should suit your needs. — Seth, Mar 02 '10 at 19:15
**See also:** [similar question](https://stackoverflow.com/questions/9415785/merging-several-python-dictionaries) — dreftymac, Dec 11 '17 at 21:07

score 22 · Answer 1 · edited Sep 02 '18 at 16:09

22

output = {k: [dict1[k], dict2.get(k)] for k in dict1}
output.update({k: [None, dict2[k]] for k in dict2 if k not in dict1})

edited Sep 02 '18 at 16:09

yugr

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answered Mar 02 '10 at 19:16

Alex Martelli

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Could you elaborate on why this works? What is the object being passed to `dict()` and `update()`? Is it a generator? – Heisenberg Feb 09 '15 at 21:57
1

@Heisenberg, they're **generator expressions** (AKA "genexp"s), the generator equivalent of "list comprehensions" (AKA "listcomp"s) -- a handy way to make a generator in-line (saves transient use of memory compared to the equivalent listcomp, if the list is not needed later -- as it wouldn't be here since `dict` and `update` both just need some kind of iterator yielding 2-item tuples taken as `(key, value)`). – Alex Martelli Feb 10 '15 at 00:48

score 17 · Answer 2 · edited Sep 02 '18 at 16:07

17

This will work:

{k: [dict1.get(k), dict2.get(k)] for k in set(dict1.keys() + dict2.keys())}

Output:

{'john': [80, 'b'], 'paul': [100, 'a'], 'peter': [None, 'd'], 'ted': [34, 'c'], 'herve': [10, None]}

edited Sep 02 '18 at 16:07

yugr

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answered Mar 02 '10 at 19:18

Nadia Alramli

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John La Rooy · Answer 3 · 2011-06-12T11:26:52.113

In Python2.7 or Python3.1 you can easily generalise to work with any number of dictionaries using a combination of list, set and dict comprehensions!

>>> dict1 = {'paul':100, 'john':80, 'ted':34, 'herve':10}
>>> dict2 = {'paul':'a', 'john':'b', 'ted':'c', 'peter':'d'}
>>> dicts = dict1,dict2
>>> {k:[d.get(k) for d in dicts] for k in {k for d in dicts for k in d}}
{'john': [80, 'b'], 'paul': [100, 'a'], 'peter': [None, 'd'], 'ted': [34, 'c'], 'herve': [10, None]}

Python2.6 doesn't have set comprehensions or dict comprehensions

>>> dict1 = {'paul':100, 'john':80, 'ted':34, 'herve':10}
>>> dict2 = {'paul':'a', 'john':'b', 'ted':'c', 'peter':'d'}
>>> dicts = dict1,dict2
>>> dict((k,[d.get(k) for d in dicts]) for k in set(k for d in dicts for k in d))
{'john': [80, 'b'], 'paul': [100, 'a'], 'peter': [None, 'd'], 'ted': [34, 'c'], 'herve': [10, None]}

@SilentGhost, I was just thinking that too. Imagine my surprise when I saw your comment :) It's quite succinct in 3.1 now — John La Rooy, Mar 02 '10 at 21:11

asdfg · Answer 4 · 2010-03-09T11:57:30.840

2

In Python3.1,

output = {k:[dict1.get(k),dict2.get(k)] for k in dict1.keys() | dict2.keys()}

In Python2.6,

output = dict((k,[dict1.get(k),dict2.get(k)]) for k in set(dict1.keys() + dict2.keys()))

edited Mar 09 '10 at 11:57

answered Mar 02 '10 at 19:44

asdfg

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score 1 · Answer 5 · answered Jul 10 '22 at 09:31

Using chain.from_iterable (from itertools) you can avoid the list/dict/set comprehension with:

dict(chain.from_iterable(map(lambda d: d.items(), list_of_dicts])))

It can be more or less convenient and readable than double comprehension, depending on your personal preference.

merging Python dictionaries

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