How do I extract all the values of a specific key from a list of dictionaries?

Question

I have a list of dictionaries that all have the same structure within the list. For example:

test_data = [{\'id\':1, \'value\':\'one\'}, {\'id\':2, \'value\         


        

Answer 1

You can do this:

result = map (lambda x:x['value'],test_data)

Answer 2

If you just need to iterate over the values once, use the generator expression:

generator = ( item['value'] for item in test_data )

...

for i in generator:
    do_something(i)

Another (esoteric) option might be to use map with itemgetter - it could be slightly faster than the generator expression, or not, depending on circumstances:

from operator import itemgetter

generator = map(itemgetter('value'), test_data)

And if you absolutely need a list, a list comprehension is faster than iterated list.append, thus:

results = [ item['value'] for item in test_data ]

Answer 3

If your data is truly large, a generator will be more efficient:

list((object['value'] for object in test_data))

ex:

>>> list((object['value'] for object in test_data))
['one', 'two', 'three']

The generator portion is this:

(object['value'] for object in test_data)

By wrapping that in a list(), you exhaust the generator and return its values nicely in an array.