python：dict值的平面列表

Question

I have a list of dicts like so:

a = [ {'list':[1,2,3]}, {'list':[1,4,5]} ]

Am trying to get a flat set of the values in the list key like {1,2,3,4,5} . What's the quickest way?

Answer 1

You can write a loop like:

result = set()
for row in a:
    result.update(row['list'])

which I think will work reasonably fast.

Or you can simply use set comprehension and that will result in the following one-liner :

result = {x for row in a for x in row['list']}

In case not all elements contain a 'list' key, you can use .get(..) with an empty tuple (this will reduce construction time):

result = {x for row in a for x in row.get('list',())}

Answer 2

It is not clear what your definition of "quickest" is, but whether it is speed or number of lines I would use a combination of itertools and a generator.

>>> import itertools
>>> a = [ {'list':[1,2,3]}, {'list':[1,4,5]} ]
>>> b = set(itertools.chain.from_iterable(x['list'] for x in a if 'list' in x))

Note that I have added a guard against any elements that may not contain a 'list' key; you can omit that if you know this will always be true.

Answer 3

flat list can be made through reduce easily.

All you need to use initializer - third argument in the reduce function.

reduce(
    lambda  _set, _dict, key='list': _set.update(
          _dict.get(key) or set()) or _set,
    a, 
    set())

Above code works for both python2 and python3, but you need to import reduce module as from functools import reduce . Refer below link for details.

• for python2

• for python3