Creating a defaultdict with empty numpy array

Question

I'm wondering if there's a more clever way to create a default dict from collections. The dict should have an empty numpy ndarray as default value.

My best result is so far:

import collections
d = collections.defaultdict(lambda: numpy.ndarray(0))

However, i'm wondering if there's a possibility to skip the lambda term and create the dict in a more direct way. Like:

d = collections.defaultdict(numpy.ndarray(0))  # <- Nice and short - but not callable

Why do you want to use ndarray with defaultdict? You'll have to create a new array each time to insert an item to an existing key, better use list which supports append operation. — Ashwini Chaudhary
– Ashwini Chaudhary, Commented Jul 29, 2014 at 11:38
np.concat() can be used to accumulate arrays. If the arrays are large and few, this may be more efficient. So, I like the proposed solution above. — Hephaestus
– Hephaestus, Commented Jun 6, 2018 at 5:39
Accumulation: base = defaultdict(lambda : np.ndarray(0)); base = np.concatenate((base, new_array) — Hephaestus
– Hephaestus, Commented Jun 6, 2018 at 6:03

Martijn Pieters · Accepted Answer · 2014-07-29 11:34:31Z

24

You can use functools.partial() instead of a lambda:

from collections import defaultdict
from functools import partial

defaultdict(partial(numpy.ndarray, 0))

You always need a callable for defaultdict(), and numpy.ndarray() always needs at least one argument, so you cannot just pass in numpy.ndarray here.

answered Jul 29, 2014 at 11:34

Martijn Pieters

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2 Comments

ballade4op52 Over a year ago

I'm new to Numpy. Is Ashwini correct when he says 'You'll have to create a new array each time to insert an item to an existing key. Better use list which supports append operation'?

Martijn Pieters Over a year ago

@Phillip yes, they are correct. Numpy arrays have a fixed size; see How to extend an array in-place in Numpy?.

smttsp · Accepted Answer · 2021-08-17 19:22:55Z

4

Another way is the following if you know the array size beforehand:

new_dict = defaultdict(lambda: numpy.zeros(array_size))

answered Aug 17, 2021 at 19:22

smttsp

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