Set to dict Python

Question

is there any pythonic way to convert a set into a dict?

I got the following set

s = {1,2,4,5,6}

and want the following dict

c = {1:0, 2:0, 3:0, 4:0, 5:0, 6:0}

with a list you would do

a = [1,2,3,4,5,6]
b = []

while len(b) < len(a):
   b.append(0)

c = dict(itertools.izip(a,b))

Solution 1

Use dict.fromkeys():

c = dict.fromkeys(s, 0)

Demo:

>>> s = {1,2,4,5,6}
>>> dict.fromkeys(s, 0)
{1: 0, 2: 0, 4: 0, 5: 0, 6: 0}

This works for lists as well; it is the most efficient method to create a dictionary from a sequence. Note all values are references to that one default you passed into dict.fromkeys(), so be careful when that default value is a mutable object.

Solution 2

Besides the method given by @Martijn Pieters, you can also use a dictionary comprehension like this:

s = {1,2,4,5,6}
d = {e:0 for e in s}

This method is slower than dict.fromkeys(), but it allows you to set the values in the dict to whatever you need, in case you don't always want it to be zero.

You can also use it to create lists, lists comprehensions are faster and more pythonic that the loop that you have in your question. You can learn more about comprehensions here: http://docs.python.org/2/tutorial/datastructures.html#list-comprehensions

Solution 3

This is also another way to do

s = {1,2,3,4,5}
dict([ (elem, 0) for elem in s ])

Comments

[Martijn Pieters]

Closely related: Most Pythonic Way to Build Dictionary From Single List.

[Martijn Pieters]

And instead of a while loop appending 0, why not make b = [0] * len(a)? Or use itertools.izip(a, itertools.repeat(0)).

[KRoy]

I wish there is a way to use map and do some lambda !

[Martijn Pieters]

This is the method to use if the default value is mutable and you need a new object for each key. dict.fromkeys() is far faster otherwise.

[Martijn Pieters]

See Most Pythonic Way to Build Dictionary From Single List for a comparison.

[papirrin]

True, comprehensions are slower.

[Danica]

@MartijnPieters I just timed it and got a factor of 1.5x for set(xrange(6)) and a factor of 3x for set(xrange(10000)). Definitely faster and there's no reason not to use fromkeys, but the difference isn't that huge and this is pretty unlikely to be a bottleneck.