[🐍PyTricks]: Merging two dictionaries in Python 3.5+ with a single expression

Question

# How to merge two dictionaries
# in Python 3.5+
>>> x = {'a': 1, 'b': 2}
>>> y = {'b': 3, 'c': 4}
>>> z = {**x, **y}
>>> z {'c': 4, 'a': 1, 'b': 3}
# In Python 2.x you could
# use this:
>>> z = dict(x, **y)
>>> z {'a': 1, 'c': 4, 'b': 3}
# In these examples, Python merges dictionary keys
# in the order listed in the expression, overwriting
# duplicates from left to right.
#

Answer 1

Hi! You can submit these as lessons in the lesson factory. Thanks.

Answer 2

I would like such an obfuscated approach if it was perl, not python ;)
what's wrong with a built-in update method?
>>> x = {'a': 1, 'b': 2}
>>> y = {'b': 3, 'c': 4}
>>> x.update(y)
>>> x
{'a': 1, 'b': 3, 'c': 4}

Answer 3

update has issues only if you want to only *copy* the dictionnaires and keep them unchanged. Then, the code is either inelegant, or not really efficient

Answer 4

rahul kumar LOL why you answering after 2 years.🤣😅😂😂

Answer 5

It is actually the most efficient and the most explicit method to merge dictionaries...
I have read an article (or watched a talk, I'm not sure which one) about precisely that, and there ARE problems with update...

Answer 6

@Amaras A
Could you please elaborate on his point? I will appreciate it.

Answer 7

@Maxim Zinyakov: well, there are some test, run them if you want to check https://gist.github.com/bnorick/adfe68808916621d369e
And it actually has issues only if you want both dictionaries left untouched and have an elegant solution.

Answer 8

@Amaras A
I still can't get your point. I see no "issues" with update method except it is a little bit slower then kwargs hack when merging _empty_ dictionaries. In other cases it is more effective.
https://code.sololearn.com/cBx0ayFF7Ps1/?ref=app
Note: this code gets time exceeded error on SoloLearn site, so try it locally.

Answer 9

Don’t use ** unpack. Even Guido doesn’t suggest this method to do dictionary addition.

Answer 10

why though? it's fast, correct and there is no reason not to use it. Has Guido said it should not be used @Chenghua Yang?

Answer 11

PEP 448 also expanded the abilities of ** by allowing this operator to be used for dumping key/value pairs from one dictionary into a new dictionary . Leveraging dictionary comprehension and the unpacking operator, you can merge the two dictionaries in a single expression .

>>>dict1 = {1:'one' , 2:'two'}
>>>dict2 = {3:'three', 4:'four'}
>>>fDict = {**dict1 , **dict2}
>>>print(fDict)
{1: 'one', 2: 'two', 3: 'three', 4: 'four'}

http://net-informations.com/JUMP_LINK__&&__python__&&__JUMP_LINK/ds/merge.htm