Adding dictionaries together [duplicate]

Question

I have two dictionaries and I'd like to be able to make them one:

Something like this pseudo-Python would be nice:

dic0 = {'dic0': 0}
dic1 = {'dic1': 1}

ndic = dic0 + dic1
# ndic would equal {'dic0': 0, 'dic1': 1}

@palsch {'dic0':0} + {'dic0':1} should be equivalent to d={'dic0':0}; d.update({'dic0':1}) ? — Sam
– Sam, Commented Apr 24, 2016 at 13:56
Your syntax with the + operator is supported since Python 3.9, as per PEP 584 — MartijnVanAttekum
– MartijnVanAttekum, Commented Jan 4, 2021 at 16:45

Georg Plaz · Accepted Answer · 2021-08-27 15:21:46Z

214

If you're interested in creating a new dict without using intermediary storage: (this is faster, and in my opinion, cleaner than using dict.items())

dic2 = dict(dic0, **dic1)

Or if you're happy to use one of the existing dicts:

dic0.update(dic1)

edited Aug 27, 2021 at 15:21

Georg Plaz

6,1445 gold badges45 silver badges66 bronze badges

answered May 14, 2011 at 22:33

bluepnume

17.2k8 gold badges42 silver badges48 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Flimm Over a year ago

The first line fails if the keys are anything other than strings.

Nikana Reklawyks Over a year ago

@BerryTsakala The problem with + is what happens in case of conflicts ? .update() is properly asymmetric.

Hi-Angel Over a year ago

You can make it terser by replacing 1st example with dic2 = {dic0, **dic1}

ArtOfWarfare Over a year ago

@NikanaReklawyks - Could either throw an exception (like 1/0 does) or just accept that it's asymmetric. Other implemented additions of non-numbers are already asymmetric - [1] + [2] is different from [2] + [1] and '1' + '2' is different from '2' + '1'.

Nate Glenn Over a year ago

@BerryTsakala you got your wish with python 3.9 :) But the operator is |, not +. docs.python.org/3/whatsnew/3.9.html

ideasman42 · Accepted Answer · 2023-01-10 03:14:15Z

Here are quite a few ways to add dictionaries.

You can use Python3's dictionary unpacking feature:

ndic = {**dic0, **dic1}

Note that in the case of duplicates, values from later arguments are used. This is also the case for the other examples listed here.

Or create a new dict by adding both items.

ndic = dict(tuple(dic0.items()) + tuple(dic1.items()))

If modifying dic0 is OK:

dic0.update(dic1)

If modifying dic0 is NOT OK:

ndic = dic0.copy()
ndic.update(dic1)

If all the keys in one dict are ensured to be strings (dic1 in this case, of course args can be swapped)

ndic = dict(dic0, **dic1)

In some cases it may be handy to use dict comprehensions (Python 2.7 or newer),
Especially if you want to filter out or transform some keys/values at the same time.

ndic = {k: v for d in (dic0, dic1) for k, v in d.items()}

ideasman42 · Accepted Answer · 2023-01-10 03:17:20Z

19

>>> dic0 = {'dic0':0}
>>> dic1 = {'dic1':1}
>>> ndic = dict(list(dic0.items()) + list(dic1.items()))
>>> ndic
{'dic0': 0, 'dic1': 1}
>>>

edited Jan 10, 2023 at 3:17

ideasman42

49.7k54 gold badges229 silver badges368 bronze badges

answered May 14, 2011 at 22:14

Vijay

9447 silver badges15 bronze badges

3 Comments

Bryce Guinta Over a year ago

Note that the equivalent syntax for this in Python 3.x is ndic = list(dict(dic0.items()) + list(dic1.items())) since .items() not longer returns a list, but a (iterable)view

dimo414 Over a year ago

@BryceGuinta I assume you meant ndic = dict(list(dic0.items()) + list(dic1.items()))

Bryce Guinta Over a year ago

@dimo414 Yes, my bad. I can't change it now however. I use ChainMap from collections to achieve this functionality now via dict(ChainMap(dic1, dic0)). However I have had to grab the source code from the chainmap pypi package for Python2.7. Notice how I switched the order of the dicts. In the Vijay's example the rightmost keys' values overwrite the leftmost while ChainMap gets it right and the leftmost keys' values have precedence over the right.

mmmmmm · Accepted Answer · 2011-05-14 22:10:59Z

13

You are looking for the update method

dic0.update( dic1 )
print( dic0 )

gives

{'dic0': 0, 'dic1': 1}

answered May 14, 2011 at 22:10

mmmmmm

32.8k28 gold badges92 silver badges124 bronze badges

1 Comment

mseery Over a year ago

Nice, although it modifies dic0. I'm not sure if that is acceptable to the original poster.

Daniel Roseman · Accepted Answer · 2011-05-14 22:11:12Z

8

dic0.update(dic1)

Note this doesn't actually return the combined dictionary, it just mutates dic0.

answered May 14, 2011 at 22:11

Daniel Roseman

602k68 gold badges913 silver badges923 bronze badges

Comments

wovano · Accepted Answer · 2021-11-08 10:21:26Z

5

The easiest way to do it is to simply use your example code, but using the items() member of each dictionary. So, the code would be:

dic0 = {'dic0': 0}
dic1 = {'dic1': 1}
dic2 = dict(dic0.items() + dic1.items())

I tested this in IDLE and it works fine. However, the previous question on this topic states that this method is slow and chews up memory. There are several other ways recommended there, so please see that if memory usage is important.

edited Nov 8, 2021 at 10:21

wovano

5,1855 gold badges33 silver badges58 bronze badges

answered May 14, 2011 at 22:15

LukeFitz

1663 bronze badges

1 Comment

bluepnume Over a year ago

FYI, this creates a list of tuples, you'd need to run dict(dic0.items() + dic1.items())

Collectives™ on Stack Overflow

Adding dictionaries together [duplicate]

6 Answers 6

5 Comments

Comments

3 Comments

1 Comment

Comments

1 Comment

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

5 Comments

Comments

3 Comments

1 Comment

Comments

1 Comment

Linked

Related