在Python中反转字典

Question

I want to know which would be an efficient method to invert dictionaries in python. I also want to get rid of duplicate values by comparing the keys and choosing the larger over the smaller assuming they can be compared. Here is inverting a dictionary:

inverted = dict([[v,k] for k,v in d.items()])

Answer 1

To remove duplicates by using the largest key, sort your dictionary iterator by value. The call to dict will use the last key inserted:

import operator
inverted = dict((v,k) for k,v in sorted(d.iteritems(), key=operator.itemgetter(1)))

Answer 2

Here is a simple and direct implementation of inverting a dictionary and keeping the larger of any duplicate values:

inverted = {}
for k, v in d.iteritems():
    if v in inverted:
        inverted[v] = max(inverted[v], k)
    else:
        inverted[v] = k  

This can be tightened-up a bit with dict.get() :

inverted = {}
for k, v in d.iteritems():
    inverted[v] = max(inverted.get(v, k), k)

This code makes fewer comparisons and uses less memory than an approach using sorted() .