How do I flatten a list of lists/nested lists? [duplicate]

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.

1. So you can use list comprehension like this

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

2. Or you can use chain from itertools like this

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

3. 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))
   

4. Using sum function

   newlist = sum(newlist, [])
   

5. Using reduce function

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

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

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

Edit: For the sake of completeness, included the answers found in Making a flat list out of list of lists in Python 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.