Hacker News Reader

Comment by lqdc13 | Hacker News Reader

zip to unzip a dict is a very slow approach to do it

Instead of

    mi = dict(zip(m.values(), m.keys()))

    mi = {v: k for (k, v) in m.iteritems()}

View on HN

When did support for dictionary comprehensions make it into 2.x? I could've sworn it didn't used to work, but I just tried it in the shell and sure enough, it does in 2.7.3.

2.7:

http://legacy.python.org/dev/peps/pep-0274/

Wow I didn't know that! I was always doing something like this

    mi = dict((v,k) for k,v in m.iteritems())

more importantly it's wrong - the iteration order of .values() and .keys() is not guaranteed to be consistent

The order of items in the dictionary isn't stable, but the order of .keys() is guaranteed to be the same as the order of .values() (as long as you don't modify the dict in between calling one then the other).

http://docs.python.org/2/library/stdtypes.html#dict.items

Reply on news.ycombinator.com