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':'two'}, {'id':3, 'value':'three'}]
I want to get each of the value
items from each dictionary in the list:
['one', 'two', 'three']
I can of course iterate through the list and extract each value using a for loop:
results = []
for item in test_data:
results.append(item['value'])
however my data set is quite large. I’m wondering if there’s a faster way to this.
Answers:
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 ]
You can do this:
result = map (lambda x:x['value'],test_data)
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.
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':'two'}, {'id':3, 'value':'three'}]
I want to get each of the value
items from each dictionary in the list:
['one', 'two', 'three']
I can of course iterate through the list and extract each value using a for loop:
results = []
for item in test_data:
results.append(item['value'])
however my data set is quite large. I’m wondering if there’s a faster way to this.
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 ]
You can do this:
result = map (lambda x:x['value'],test_data)
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.