[Tutor] Question about Dictionaries

[Steven D'Aprano]

On Tue, 17 Aug 2010 03:44:33 am Chorn, Guillaume wrote:
> Hi All,
>
> I know that I can look up the value for a particular key in a
> dictionary, but can I look up the key associated with a particular
> value?  I understand that this could be problematic from the
> standpoint of multiple keys having the same value, but even then I
> feel like Python could just return a list of keys with that value.

There is no built-in way of doing so, but it's easy to write your own.

def reverse_lookup(d, target, first_only=False):
    found = []
    for key, value in d.items():
        if value == target:
            if first_only: return key
            found.append(key)
    return found


Or if you want a one-liner, use a list-comp:

[k for (k,v) in d.items() if v == target]


-- 
Steven D'Aprano