How to set specific attributes of a dictionary to []

Question

Let us imagine the following dictionary

dictionary = {
    "key1": {
         "value": [1, 3, 5],
     },

    "key2": {
         "value": [1, 2, -1],
     },
}

Is it possible to set all the "values" to [] without iterating over the dictionary keys? I want something like dictionary[]["value"]=[] such that all "value" attributes are set to []. But that doesn't work.

You don't need to iterate over the keys. Iterating over the values is enough to get the job done. — Aran-Fey, Sep 07 '18 at 16:13
It might be helpful to give some *context* - why do you think you want this? As well as being fragile, the answer you've accepted is going to be pretty inefficient. — jonrsharpe, Sep 07 '18 at 16:25

Austin · Accepted Answer · 2018-09-07T16:08:33.513

5

Because you need to avoid iteration, here is a little hacky way of solving the case.

Convert dictionary to string, replace and then back to dictionary:

import re, ast

dictionary = {
    "key1": {
         "value": [1, 3, 5],
     },

    "key2": {
         "value": [1, 2, -1],
     },
}

print(ast.literal_eval(re.sub(r'\[.*?\]', '[]', str(dictionary))))
# {'key1': {'value': []}, 'key2': {'value': []}}

edited Sep 07 '18 at 16:08

answered Sep 07 '18 at 16:05

Austin

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"A little". Right :) +1 – Mad Physicist Sep 07 '18 at 16:07
2

How does this avoid iteration? You might not be writing a for loop, but how do you think the repr gets generated? Also it's pretty fragile. – jonrsharpe Sep 07 '18 at 16:09
@jonrsharpe: I suppose there is no iteration. Seriously, I'm not sure how repr is generated. Could you shed some light on it? – Austin Sep 07 '18 at 16:14
1

Well, there are two ways to implement repr: 1) It iterates over the keys and values of the dict and converts them to strings, or 2) It's magic. – Aran-Fey Sep 07 '18 at 16:15
1

We thank you for this witchcraft you have shared with us – yorodm Sep 07 '18 at 16:55

jpp · Answer 2 · 2018-09-07T16:59:43.917

I'm going to take a different tack here. Your question is a little misinformed. The implication is that it's "better" to avoid iterating dictionary keys. As mentioned, you can iterate over dictionary values. But, since internally Python stores dictionaries via two arrays, iteration is unavoidable.

Returning to your core question:

I want something like dictionary[]["value"]=[] such that all "value" attributes are set to [].

Just use collections.defaultdict:

from collections import defaultdict

d = {k: defaultdict(list) for k in dictionary}

print(d['key1']['value'])  # []
print(d['key2']['value'])  # []

For the dictionary structure you have defined, this will certainly be more efficient than string conversion via repr + regex substitution.

If you insist on explicitly setting keys, you can avoid defaultdict at the cost of an inner dictionary comprehension:

d = {k: {i: [] for i in v} for k, v in dictionary.items()}

{'key1': {'value': []}, 'key2': {'value': []}}

How to set specific attributes of a dictionary to []

2 Answers2