build a dictionary for find key by value [duplicate]

Answer 1

Just create an inverted mapping:

from collections import defaultdict
inverted = defaultdict(list)
for k, v in dictionary.iteritems():
    inverted[v].append(k)

Note that the above code handles duplicate values; inverted[v] returns a list of keys that hold that value.

If your values are also unique, a simple dict can be used instead of defaultdict:

inverted = { v: k for k, v in dictionary.iteritems() }

or, in python 3, where items() is a dictionary view:

inverted = { v: k for k, v in dictionary.items() }