Python return key from value, but its a list in the dictionary

Question

So Im quiet new to python and maybe I´ve searced the wrong words on google...

My current problem:

In python you can return the key to a value when its mentioned in a dictionary.

One thing I wonder, is it possible to return the key if the used value is part of a list of values to the key? So my testing skript is the following

MainDict={'FAQ':['FAQ','faq','Faq']}


def key_return(X):
    for Y, value in MainDict.items():
        if X == value:
            return Y
    return "Key doesnt exist"

print(key_return(['FAQ', 'faq', 'Faq'])) 

print(key_return('faq'))

So I can just return the Key if I ask for the whole list, How can I return the key if I just ask for one value of that list as written for the second print? On current code I get the "Key doesnt exist" as an answer.

Does this answer your question? [Is there a short contains function for lists?](https://stackoverflow.com/questions/12934190/is-there-a-short-contains-function-for-lists) — depperm, Sep 10 '21 at 15:19
`MainDict` contains 1 key ('FAQ') which points to a value which is a list. The only way to get the value in this dict is to use `MainDict['FAQ']` — balderman, Sep 10 '21 at 15:19

Bill the Lizard · Accepted Answer · 2021-09-10T15:27:49.013

You can check to see if a value in the dict is a list, and if it is check to see if the value you're searching for is in the list.

MainDict = {'FAQ':['FAQ','faq','Faq']}

def key_return(X):
    for key, value in MainDict.items():
        if X == value:
            return key
        if isinstance(value, list) and X in value:
            return key
    return "Key doesnt exist"


print(key_return(['FAQ', 'faq', 'Faq']))
print(key_return('faq'))

Note: You should also consider making MainDict a parameter that you pass to key_return instead of a global variable.

score 1 · Answer 2 · answered Sep 10 '21 at 15:34

You can do this using next and a simple comprehension:

next(k for k, v in MainDict.items() if x == v or x in v)

So your code would look like:

MainDict = {'FAQ':['FAQ','faq','Faq']}

def key_return(x):
    return next(k for k, v in MainDict.items() if x == v or x in v)

print(key_return(['FAQ', 'faq', 'Faq'])) 
#FAQ
print(key_return('faq'))
#FAQ

score 0 · Answer 3 · answered Sep 10 '21 at 15:27

You can create a dict that maps from values in the lists to keys in MainDict:

MainDict={'FAQ':['FAQ','faq','Faq']}
back_dict = {value: k for k,values in MainDict.items() for value in values}

Then rewrite key_return to use this dict:

def key_return(X):
    return back_dict[X]

print(key_return('faq'))

The line back_dict = {value: k for k,values in MainDict.items() for value in values} is a dictionary comprehension expression, which is equivalent to:

back_dict = {}
for k,values in MainDict.items():
    for value in values:
        back_dict[value] = k

This approach is more time-efficient that looping over every item of MainDict every time you search, since it only requires a single loopkup rather than a loop.

Python return key from value, but its a list in the dictionary

3 Answers3

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