Get value from dictionary for first key that exists

Question

TL;DR

Is there a Python dictionary method or simple expression that returns a value for the first key (from a list of possible keys) that exists in a dictionary?

Details

Lets say I have a Python dictionary with a number of key-value pairs. The existence of any particular key not guaranteed.

d = {'A':1, 'B':2, 'D':4}

If I want to get a value for a given key, and return some other default value (e.g. None) if that key doesn't exist, I simply do:

my_value = d.get('C', None) # Returns None

But what if I want to check a number of possible keys before defaulting to a final default value? One way would be:

my_value = d.get('C', d.get('E', d.get('B', None))) # Returns 2

but this gets rather convoluted as the number of alternate keys increases.

Is there a Python function that exists for this scenario? I imagine something like:

d.get_from_first_key_that_exists(('C', 'E', 'B'), None) # Should return 2

If such a method doesn't exist, is there a simple expression that is commonly used in such a scenario?

write your expected output – sachin dubey Nov 28 '17 at 05:37 — sachin dubey, Nov 28 '17 at 05:37
@sachindubey Added my expected output. – Andrew Guy Nov 28 '17 at 05:39 — Andrew Guy, Nov 28 '17 at 05:39

score 6 · Accepted Answer · answered Nov 28 '17 at 05:41

6

Using a plain old for loop, the flow control is clearest:

for k in 'CEB':
    try:
        v = d[k]
    except KeyError:
        pass
    else:
        break
else:
    v = None

If you want to one-liner it, that's possible with a comprehension-like syntax:

v = next((d[k] for k in 'CEB' if k in d), None)

answered Nov 28 '17 at 05:41

wim

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Nice! The `for` loop is definitely more explicit, but I really like the comprehension example, especially with the use of `next`. – Andrew Guy Nov 28 '17 at 05:49

Mitchell Currie · Answer 2 · 2017-11-28T06:45:08.883

2

Nothing exists.

x = ["1", "3"]
d = {'1': 1, '2': 2, '3': 3}
d[[k for k in x if x in d][0]]

Just use a list comprehension.

Edit: Bonus Points:

all_matches = [d[k] for k in x if x in d]
all_matches[0]

if you'd like all possible keys (poor efficiency if you don't)

edited Nov 28 '17 at 06:45

answered Nov 28 '17 at 05:41

Mitchell Currie

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pushpendra chauhan · Answer 3 · 2017-11-28T05:49:54.417

1

You can do something like this

dict_keys=yourdict.keys()
expected_keys=['a','b','c']

result= list(set(dict_keys).intersection(set(expected_keys)))
if result:
    for i in expected_keys:
        if i in result:
            yourdict.get(i)

edited Nov 28 '17 at 05:49

answered Nov 28 '17 at 05:40

pushpendra chauhan

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edited the code – pushpendra chauhan Nov 28 '17 at 05:41
This is probably better than mine, you beat me to it. Ideally we could do away with needing if indentation via some other safe means. – Mitchell Currie Nov 28 '17 at 05:43
This will use one possible key, but won't necessarily use the first key that exists (sets are not ordered). – Andrew Guy Nov 28 '17 at 05:46
I have edited my code, now it will get the value for first key in expected keys – pushpendra chauhan Nov 28 '17 at 05:50
1

This method can be inefficient if the data is large (it must find all of the intersecting keys first, even though only one is needed). – wim Nov 28 '17 at 06:03

Get value from dictionary for first key that exists

TL;DR

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3 Answers3

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