Python list comprehension for joining lists (flattened) [duplicate]

Question

This question already has answers here:

How do I make a flat list out of a list of lists? (35 answers)

Closed 8 years ago.

So I'm currently working with mongoengine, although anything with properties will have the same behavior.

I have a list called collections. Technically it's a mongoengine QuerySet, which is an iterable, but it has virtually all the same behaviors as a list.

Each element collections is an object, with a property called addons which is also a list.

I want to create a flat list of all the addons. Currently doing this produces the desired result

addons = []
for col in collections:
    addons+=col.addons

But when I try this, which I assume is the equivalent list comprehension, I get a list of lists (it essentially appends each list together instead of adding them)

addons = [col.addons for col in collections]

I've been reading about nested list comprehensions and even using itertools but haven't been able to figure out how to get either to work properly

Answer 1

You can use itertools.chain.from_iterable like so:

from itertools import chain
addons = list(chain.from_iterable(col.addons for col in collections))

Available on Python >= 2.6. In the past, it's been quite fast. Faster even than the double list comprehension.

Answer 2

you need a double list comprehension, like:

addons = [item for item in col.addons for col in collections]