One liner for extend loop python

Question

I have the following code lines:

records = []

for future in futures:
  records.extends(future.result())

each future returns a list.

How can I write the above code but in one liner?

records = [future.result() for future in futures]

would result in a list inside list.

I have millions of records, i would rather not flat it after creating lists inside list

Answer 1

records = [r for future in futures for r in future.result()]

Answer 2

There are many ways to do this:

1. Use itertools.chain:
   records = list(itertools.chain.from_iterable(future.result() for future in futures))
2. Use the itertools consume recipe:
   records = collections.deque((records.extend(future.result()) for future in futures), maxlen=0)
3. Use a throw-away list:
   [records.extend(future.result()) for future in futures]. records will now have all the required content, and you will have temporarily made a list of Nones

You could also do functools.reduce(operator.add, (future.result() for future in futures)), but that wouldn't scale very well