Merge dictionary values into a set

Question

How do I do the following in the form of a oneliner:

replacements = set()
for i in replacement_per_file.values():
    replacements.update(i)

Answer 1

Something like this:

replacements = set(x for i in replacement_per_file.values() for x in i)

Answer 2

If i is a list (or iterable), then you're looking for this:

replacements = set([
    x
    for i in replacement_per_file.values()
    for x in i
])

Answer 3

An itertools version inspired from this post:

>>> from itertools import chain
>>> a = [[1, 2], [2, 3]]
>>> set(chain(*a))
set([1, 2, 3])

Answer 4

You can do this:

set().union(replacement_per_file.values())

This is slightly less efficient than your multi-line way, as it requires creating a copy of value references for the arguments of union.