Setup element getter for dictionaries

Question

This works, according to the documentation:

>>> inventory = [('apple', 3), ('banana', 2), ('pear', 5), ('orange', 1)]
>>> list(map(operator.itemgetter(1), inventory))
[3, 2, 5, 1]

Now I want to do something similar, for a list of dicts:

>>> data = [{'xxx': 'x'}, {'xxx': 'y'}]
>>> list(map(operator.attrgetter('xxx'), data))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no attribute 'xxx'

I was expecting:

['x', 'y']

But it seems attrgetter does not work for dictionaries (only for classes?). How can I setup an "element getter" for dictionaries?

`xxx` is not an attribute of the dictionaries you are showing. — Anthon, May 01 '17 at 09:13

score 2 · Answer 1 · answered May 01 '17 at 09:14

2

Well, the item you want to get is 'xxx'. (In both cases, you want to call the object's __getitem__ method for each object in the list.)

>>> from operator import itemgetter
>>> data = [{'xxx': 'x'}, {'xxx': 'y'}]
>>> map(itemgetter('xxx'), data)
['x', 'y']

Think of attributes as anything you access via the dot notation, i.e. mydict.xxx in your case. You should see why that fails.

answered May 01 '17 at 09:14

timgeb

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Strange that the documentation for itemgetter only shows examples for access of list or tuples elements, that is, for integer indexes – blueFast May 01 '17 at 09:22

score 2 · Answer 2 · edited May 23 '17 at 12:26

As mentioned in timgeb's answer, you should be calling itemgetter with the key (because attrgetter are used to get the class attribute which are accessible by . like class_obj.<attribute>).

In order to achieve your desired result, usage of list comprehension is a easier approach:

>>> data = [{'xxx': 'x'}, {'xxx': 'y'}]
>>> [d['xxx'] for d in data]
['x', 'y']

Also, please note that map since python 3.x returns a generator object instead of a list object.

Setup element getter for dictionaries

2 Answers2

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