如何使用一个 for 循环遍历 2 个字典？

Question

I have two data structures like the following:

DATASET_0 = { 'key1': [ object1, object2, objectN ],
              'key2': [ object3, object4]
            }
DATASET_1 = { 'key1': [ object5, object6, objectN ],
              'key3': [ object7, object8],
              'key4': [ object9, object10],
            }

(This is a highly simplified version but it shows the two top level components - basically a dictionary of lists)

So the same key might appear in both dictionaries but pointing to different objects. Normally I could just merge the two dictionaries on the fly and iterate over the merge but in this case I can't do this. I can't make the top level item a list either.

If the top level item was a list I could do this:

for item in DATASET_0 + DATASET_1:
    blah

Is there anyway to do the same thing using dictionaries?

Answer 1

itertools.chain is a good friend in times like this.

from itertools import chain

d1 = {'foo': ['bar']}
d2 = {'foo': ['baz']}
for k, v in chain(d1.iteritems(), d2.iteritems()):
   print k, v

If you want to just iterate over the keys, that becomes even easier (and ports to python3.x with no modifications!)

for k in chain(d1, d2):
    print(k)

Answer 2

You can simply concatenate dict.items() lists:

for key, value in DATASET_0.items() + DATASET_1.items():

or chain the dict.iteritems() iterables (using itertools.chain() :

from itertools import chain

for key, value in chain(DATASET_0.iteritems(), DATASET_1.iteritems()):

Answer 3

Using itertools.chain

Make an iterator that returns elements from the first iterable until it is exhausted, then proceeds to the next iterable, until all of the iterables are exhausted. Used for treating consecutive sequences as a single sequence.

from itertools import chain

DATASET_0 = { 'key1': [ "object1", "object2", "objectN" ],
              'key2': [ "object3", "object4"]
            }
DATASET_1 = { 'key1': [ "object5", "object6", "objectN" ],
              'key3': [ "object7", "object8"],
              'key4': [ "object9", "object10"],
            }

for key, value in chain(DATASET_0.items(), DATASET_1.items()):
    print(key)  # Only printing key as proof of concept

>>> key1
>>> key2
>>> key1
>>> key3
>>> key4

Outdated in Python 3

• dict.items() no longer returns a list so it doesn't support the + operator
• dict.iteritems() has been omitted