How to get all keys from a value for a dict of lists?

Question

I have a dictionary in a format like this

d = {
    "Fruit_1" : ["mango", "apple"],
    "Fruit_2" : ["apple"],
    "Fruit_3" : ["mango", "banana", "apple", "kiwi", "orange"]
}

I'm passing a value as "mango" and I want to get all corresponding keys where only mango occurs. I am not able to get corresponding keys where ever value occurs.

score 9 · Answer 1 · answered Oct 04 '18 at 09:14

9

Iterate in d.items and check mango existence in value.

In [21]: [key for key,value in d.items() if 'mango' in value]
Out[21]: ['Fruit_1', 'Fruit_3']

answered Oct 04 '18 at 09:14

Rahul K P

15,740
4
35
52

BlackBeard · Accepted Answer · 2018-10-04T09:17:36.963

2

You can do this perhaps:

d = {
    "Fruit_1" : ["mango", "apple"],
    "Fruit_2" : ["apple"],
    "Fruit_3" : ["mango", "banana", "apple", "kiwi", "orange"]
}

# list comprehension
mango_keys = [fruit for fruit in d.keys() if "mango" in d[fruit]]
print(mango_keys)       


# ['Fruit_1', 'Fruit_3']         


# or more traditional for-loop (but non pythonic)

for fruit in d.keys():
    if "mango" in d[fruit]:
        print(fruit)

edited Oct 04 '18 at 09:17

answered Oct 04 '18 at 09:12

BlackBeard

10,246
7
52
62

I would suggest (as per 3 other solutions) it's better to iterate `dict.items` if you are going to use both key and value in your logic. – jpp Oct 04 '18 at 09:33
You can just use `for fruit in d` (you don't need the `.keys()`). – Alexander Oct 04 '18 at 09:36
@jpp but the OP has mentioned **"want to get all corresponding keys"** in the questing. Hence, `dict.keys()` – BlackBeard Oct 04 '18 at 09:36
@Alexander I thought "Explicit is better than implicit." – BlackBeard Oct 04 '18 at 09:37
@BlackBeard, Extracting keys via `dict.items` is not implicit / non-standard, if you are using the values too! – jpp Oct 04 '18 at 09:38
1

it's better to use items so you don't have to use the key to access the value (since you're iterating on the dict) – Jean-François Fabre Oct 04 '18 at 09:38
@jpp but if I want **only** keys why should I use `dict.items`? – BlackBeard Oct 04 '18 at 09:39
@BlackBeard, I think you misunderstand. It's *not* what you **want**, it's what you **use** that matters. – jpp Oct 04 '18 at 09:40
https://stackoverflow.com/questions/3294889/iterating-over-dictionaries-using-for-loops – Alexander Oct 04 '18 at 09:42
you need the values, or you would not do `d[fruit]` – Jean-François Fabre Oct 04 '18 at 09:42
1

@jpp if you mean [this](https://stackoverflow.com/questions/52643037/how-to-get-all-keys-from-dictionary-from-a-value/52643105?noredirect=1#comment92219291_52643105) then yes, I got the point. Thank you :) – BlackBeard Oct 04 '18 at 09:42

score 2 · Answer 3 · answered Oct 04 '18 at 09:17

The naive approaches (looping through all items and looking for the fruit) work but have a high complexity, mostly if you have to perform a lot of requests. You could slightly improve it by replacing your list values by a set (for faster in lookup), but that would still be slow (O(n**2) => O(n) but room for improvement).

If you want to be able to perform those queries a lot of times, it would be better to rebuild the dictionary so lookup is very fast once built, using collections.defaultdict

d = {
    "Fruit_1" : ["mango", "apple"],
    "Fruit_2" : ["apple"],
    "Fruit_3" : ["mango", "banana", "apple", "kiwi", "orange"]
}

import collections

newd = collections.defaultdict(list)

for k,vl in d.items():
    for v in vl:
        newd[v].append(k)

print(newd)
print(newd["mango"])

this is the rebuilt dict:

defaultdict(<class 'list'>, {'apple': ['Fruit_2', 'Fruit_3', 'Fruit_1'], 'orange': ['Fruit_3'], 'banana': ['Fruit_3'], 'kiwi': ['Fruit_3'], 'mango': ['Fruit_3', 'Fruit_1']})

this is the query for "mango":

['Fruit_3', 'Fruit_1']

score 1 · Answer 4 · answered Oct 04 '18 at 09:10

Like this?

>>> d = {
...     "Fruit_1" : ["mango", "apple"],
...     "Fruit_2" : ["apple"],
...     "Fruit_3" : ["mango", "banana", "apple", "kiwi", "orange"]
... }
>>> 
>>> [key for key, value in d.items() if 'mango' in value]
['Fruit_1', 'Fruit_3']

The idea is to iterate over the (key, value) itempairs and check each value for the existence of 'mango'. If yes, keep the key.

Since you are new to Python here's the tradidtional for-loop logic:

>>> result = []
>>> for key, value in d.items():
...     if 'mango' in value:
...         result.append(key)
... 
>>> result
['Fruit_1', 'Fruit_3']

score 1 · Answer 5 · answered Oct 04 '18 at 09:13

For a single query, you can use a list comprehension. This will has O(n) time complexity each time you search a value:

res = [k for k, v in d.items() if 'mango' in v]

For multiple queries, you can use a defaultdict of set objects via a one-off O(n) cost:

from collections import defaultdict

dd = defaultdict(set)

for k, v in d.items():
    for fruit in v:
        dd[fruit].add(k)

print(dd)

defaultdict({'mango': {'Fruit_1', 'Fruit_3'},
             'apple': {'Fruit_1', 'Fruit_2', 'Fruit_3'},
             'banana': {'Fruit_3'},
             'kiwi': {'Fruit_3'},
             'orange': {'Fruit_3'}})

You can then use dd['mango'] to extract relevant keys.

How to get all keys from a value for a dict of lists?

5 Answers5

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