How do I merge a list of dicts into a single dict?

Question

EDIT

i have:

[{'a':1},{'b':2},{'c':1},{'d':2}]

the output should be:

{'a':1,'b':2,'c':1,'d':2}

Answer 1

dict1.update( dict2 )

This is asymmetrical because you need to choose what to do with duplicate keys; in this case, dict2 will overwrite dict1. Exchange them for the other way.

EDIT: Ah, sorry, didn't see that.

It is possible to do this in a single expression:

>>> from itertools import chain
>>> dict( chain( *map( dict.items, theDicts ) ) )
{'a': 1, 'c': 1, 'b': 2, 'd': 2}

No credit to me for this last!

However, I'd argue that it might be more Pythonic (explicit > implicit, flat > nested ) to do this with a simple for loop. YMMV.

Answer 2

see this http://stackoverflow.com/questions/2365921/merging-python-dictionaries

Answer 3

>>> L=[{'a':1},{'b':2},{'c':1},{'d':2}]    
>>> dict(i.items()[0] for i in L)
{'a': 1, 'c': 1, 'b': 2, 'd': 2}

Note: the order of 'b' and 'c' doesn't match your output because dicts are unordered

if the dicts can have more than one key/value

>>> dict(j for i in L for j in i.items())

Answer 4

This works for dictionaries of any length:

>>> result = {}
>>> for d in L: result.update(d)
... 
{'a':1,'c':2,'b':1,'d':2}

And as generator-oneliner:

dict((k,v) for d in L for (k,v) in d.items())

Edit: As pointed out by katrielalex, dictionary comprehension:

{ k: v for d in L for k, v in d.items() }

is even better. OTOH they are Python3-only, so they may not be available to you.

Answer 5

This handles all the subdicts in your list, even if they have multiple items:

dict(sum((d.items() for d in L), []))

But I think update() is preferred:

>>> dd = {}
>>> for d in L: dd.update(d)

Answer 6

>>> dictlist = [{'a':1},{'b':2},{'c':1},{'d':2, 'e':3}]
>>> dict(kv for d in dictlist for kv in d.iteritems())
{'a': 1, 'c': 1, 'b': 2, 'e': 3, 'd': 2}
>>>

Note I added a second key/value pair to the last dictionary to show it works with multiple entries. Also keys from dicts later in the list will overwrite the same key from an earlier dict.