Grouping dictionary items by attribute

Question

I am trying to group the items in a dictionary by a particular key, let's say we have the following dictionary:

[{'type': 'animal', 'name': 'rabbit'}, {'type': 'animal', 'name': 'cow'}, {'type': 'plant', 'name': 'orange tree'}, {'type': 'animal', 'name': 'coyote'}]

And I wanted to group these items by type. According to this answer this can be done using defaultdict() because it doesn't raise a KeyError, as follows:

grouped = defaultdict()
for item in items:
    grouped[item.type].append(item)

However, I am actually getting a KeyError.

So what's wrong? All the information I see says defaultdict() is supposed to create an empty list if it's not existing, and the insert the element.

I am using Python 2.7.

Answer 1

use defaultdict(list), not defaultdict(). Read the docs for defaultdict to find out more.

Also, you cannot use some_dict.a_key (in your case: .type) - you must use some_dict['a_key'] (in your case ['type'])

Answer 2

You can also use a normal dictionary and its setdefault method:

l = [{'type': 'animal', 'name': 'rabbit'}, {'type': 'animal', 'name': 'cow'}, {'type': 'plant', 'name': 'orange tree'}, {'type': 'animal', 'name': 'coyote'}]
d = {}
for x in l:
    d.setdefault(x["type"], []).append(x)
print d

The result would be this (nicely formatted):

{'plant' : [{'type': 'plant', 'name': 'orange tree'}], 
 'animal': [{'type': 'animal', 'name': 'rabbit'}, 
            {'type': 'animal', 'name': 'cow'}, 
            {'type': 'animal', 'name': 'coyote'}]}

Answer 3

from collections import defaultdict

animals=[{'type': 'animal', 'name': 'rabbit'}, {'type': 'animal', 'name': 'cow'}, 
{'type': 'plant', 'name': 'orange tree'}, {'type': 'animal', 'name': 'coyote'}]

animal_by_type = defaultdict(list)
for animal in animals:
    animal_by_type[animal['type']].append(animal['name'])
print animal_by_type