How do I flatten a list of lists/nested lists?

python, python-2.7

Solution

What you are trying to do is called flattening the list. And according to the Zen of Python, you are trying to do the right thing. Quoting from that

Flat is better than nested.

So you can use list comprehension like this

 newlist = [item for items in newlist for item in items]

Or you can use `chain` from `itertools` like this

 from itertools import chain
 newlist = list(chain(*newlist))

Or you can use `chain.from_iterable`, where unpacking of the list is not necessary

 from itertools import chain
 newlist = list(chain.from_iterable(newlist))

Using `sum` function

 newlist = sum(newlist, [])

Using `reduce` function

 newlist = reduce(lambda x,y: x+y, newlist)

Using `operator.add`. This will be faster than the `reduce` with `lambda` version.

 import operator
 from functools import reduce
 newlist = reduce(operator.add, newlist)

Edit: For the sake of completeness, included the answers found in How do I make a flat list out of a list of lists? as well.

I tried to time all of them in Python 2.7, like this

from timeit import timeit
print(timeit("[item for items in newlist for item in items]", "from __main__ import newlist"))
print(timeit("sum(newlist, [])", "from __main__ import newlist"))
print(timeit("reduce(lambda x,y: x+y, newlist)", "from __main__ import newlist"))
print(timeit("reduce(add, newlist)", "from __main__ import newlist; from operator import add"))
print(timeit("list(chain(*newlist))", "from __main__ import newlist; from itertools import chain"))
print(timeit("list(chain.from_iterable(newlist))", "from __main__ import newlist; from itertools import chain"))

Output on my machine

2.26074504852
2.45047688484
3.50180387497
2.56596302986
1.78825688362
1.61612296104

So, the most efficient way to do this is to use `list(chain.from_iterable(newlist))`, in Python 2.7. Ran the same test in Python 3.3

from timeit import timeit
print(timeit("[item for items in newlist for item in items]", "from __main__ import newlist"))
print(timeit("sum(newlist, [])", "from __main__ import newlist"))
print(timeit("reduce(lambda x,y: x+y, newlist)", "from __main__ import newlist; from functools import reduce"))
print(timeit("reduce(add, newlist)", "from __main__ import newlist; from operator import add; from functools import reduce"))
print(timeit("list(chain(*newlist))", "from __main__ import newlist; from itertools import chain"))
print(timeit("list(chain.from_iterable(newlist))", "from __main__ import newlist; from itertools import chain"))

Output on my machine

2.26074504852
2.45047688484
3.50180387497
2.56596302986
1.78825688362
1.61612296104

So, be it Python 2.7 or 3.3, use `list(chain.from_iterable(newlist))` to flatten the nested lists.

Problem

Original source

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