Combine two dictionaries, concatenate string values?

Question

Related: Is there any pythonic way to combine two dicts (adding values for keys that appear in both)?

I\'d like to merge two string:string dictionaries, and concaten

Answer 1

Dict-comprehension:

>>> d = {'foo': 'bar', 'baz': 'bazbaz'}
>>> d1 = {'foo': 'baz'}
>>> keys = d.viewkeys() | d1.viewkeys()
>>> {k : d.get(k, '') + d1.get(k, '') for k in keys}
{'foo': 'barbaz', 'baz': 'bazbaz'}

For Python 2.6 and earlier:

>>> dict((k, d.get(k, '') + d1.get(k, '')) for k in keys)
{'foo': 'barbaz', 'baz': 'bazbaz'}

This will work for any number of dicts:

def func(*dicts):
    keys = set().union(*dicts)
    return {k: "".join(dic.get(k, '') for dic in dicts)  for k in keys}
... 
>>> d = {'foo': 'bar', 'baz': 'bazbaz'}
>>> d1 = {'foo': 'baz','spam': 'eggs'}
>>> d2 = {'foo': 'foofoo', 'spam': 'bar'}
>>> func(d, d1, d2)
{'foo': 'barbazfoofoo', 'baz': 'bazbaz', 'spam': 'eggsbar'}

Answer 2

One can use defaultdict to achieve this:

from collections import defaultdict

a = {'foo': 'bar', 'baz': 'bazbaz'}
b = {'foo': 'baz'}

new_dict = defaultdict(str)
for key, value in a.items():
    new_dict[key] += value
for key, value in b.items():
    new_dict[key] += value

print(new_dict)
# defaultdict(<class 'str'>, {'foo': 'barbaz', 'baz': 'bazbaz'})
print(dict(new_dict))
# {'foo': 'barbaz', 'baz': 'bazbaz'}

If there are many dicts to join, we could use itertools.chain.from_iterable:

from collections import defaultdict
from itertools import chain

a = {'foo': 'bar', 'baz': 'bazbaz'}
b = {'foo': 'baz'}
c = {'baz': '123'}
dicts = [a, b, c]

new_dict = defaultdict(str)
for key, value in chain.from_iterable(map(dict.items, dicts)):
    new_dict[key] += value

print(dict(new_dict))
# {'foo': 'barbaz', 'baz': 'bazbaz123'}

Answer 3

Can write a generic helper, such as:

a = {'foo':'bar', 'baz':'bazbaz'}
b = {'foo':'baz'}

def concatd(*dicts):
    if not dicts:
        return {} # or should this be None or an exception?
    fst = dicts[0]
    return {k: ''.join(d.get(k, '') for d in dicts) for k in fst}

print concatd(a, b)
# {'foo': 'barbaz', 'baz': 'bazbaz'}

c = {'foo': '**not more foo!**'}
print concatd(a, b, c)
# {'foo': 'barbaz**not more foo!**', 'baz': 'bazbaz'}