better one-liner to flip keys and values of a dictionary

Question

I've written a one-liner to accomplish this:

vocab_tag = dict(zip(*reversed(zip(*tag_vocab.items()))))

Can anybody write one that's more comprehensible/direct?

Answer 1

A readable and short dict comprehension is probably the best one can do:

vocab_tage = {value: key for key, value in tag_vocab.items()}

Pre 2.7, dictionary comprehensions don't exist, but we can replace them trivially with dict() and a generator expression:

vocab_tage = dict((value, key) for key, value in tag_vocab.items())

It's worth noting that this presumes there are no keys with the same value (as does the example in the question).

Answer 2

Try this one-liner: dict(map(reversed, table.items()))

Answer 3

Performance + Readability

Considering time performance, dictionary comprehension is the best solution. It is both readable and performant.

Given a dictionary, a, defined as

a = {'1' : 'a', '2' : 'b', '3' : 'c', '4' : 'd'}

the implementations perform in the following manner:

Original

%%timeit
b = dict(zip(*reversed(zip(*a.items()))))

---

100000 loops, best of 3: 3.09 µs per loop

Gareth Latty's answer

%%timeit 
c = {v: k for k, v in a.items()}

---

1000000 loops, best of 3: 776 ns per loop

Noctis Skytower's answer

%%timeit 
d = dict(map(reversed, a.items()))

---

100000 loops, best of 3: 4.38 µs per loop

Modification to Gareth Latty's answer using .iteritems() method

If using < Python 3, .iteritems() will perform slightly faster and is more memory efficient.

%%timeit
e = {v: k for k, v in a.iteritems()}

---

1000000 loops, best of 3: 598 ns per loop