Flatten Nested List into 1-Deep List

Flatten an irregular (arbitrarily nested) list of lists

Using generator functions can make your example easier to read and improve performance.

Python 2

Using the Iterable ABC added in 2.6:

from collections import Iterable

def flatten(xs):
    for x in xs:
        if isinstance(x, Iterable) and not isinstance(x, basestring):
            for item in flatten(x):
                yield item
        else:
            yield x

Python 3

In Python 3, basestring is no more, but the tuple (str, bytes) gives the same effect. Also, the yield from operator returns an item from a generator one at a time.

from collections.abc import Iterable

def flatten(xs):
    for x in xs:
        if isinstance(x, Iterable) and not isinstance(x, (str, bytes)):
            yield from flatten(x)
        else:
            yield x

Flatten nested list into 1-deep list

Replace the rapply part by your own recursion so NULLs are not getting any special treatment:

renquote <- function(l) if (is.list(l)) lapply(l, renquote) else enquote(l)

lapply(unlist(renquote(ml)), eval)

How to convert a nested list into a one-dimensional list in Python?

You need to recursively loop over the list and check if an item is iterable(strings are iterable too, but skip them) or not.

itertools.chain will not work for [1,[2,2,2],4] because it requires all of it's items to be iterable, but 1 and 4 (integers) are not iterable. That's why it worked for the second one because it's a list of lists.

>>> from collections import Iterable
def flatten(lis):
     for item in lis:
         if isinstance(item, Iterable) and not isinstance(item, str):
             for x in flatten(item):
                 yield x
         else:        
             yield item

>>> lis = [1,[2,2,2],4]
>>> list(flatten(lis))
[1, 2, 2, 2, 4]
>>> list(flatten([[1, 2, 3], [4, 5, 6], [7, 8, 9]]))
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Works for any level of nesting:

>>> a = [1,[2,2,[2]],4]
>>> list(flatten(a))
[1, 2, 2, 2, 4]

Unlike other solutions, this will work for strings as well:

>>> lis = [1,[2,2,2],"456"]
>>> list(flatten(lis))
[1, 2, 2, 2, '456']

Flatten nested lists in a list

Loop through the list, unlist recursively, then return as a list:

lapply(LIST2, function(i) list(unlist(i, recursive = TRUE)))

How do I make a flat list out of a list of lists?

Given a list of lists l,

flat_list = [item for sublist in l for item in sublist]

which means:

flat_list = []
for sublist in l:
    for item in sublist:
        flat_list.append(item)

is faster than the shortcuts posted so far. (l is the list to flatten.)

Here is the corresponding function:

def flatten(l):
    return [item for sublist in l for item in sublist]

As evidence, you can use the timeit module in the standard library:

$ python -mtimeit -s'l=[[1,2,3],[4,5,6], [7], [8,9]]*99' '[item for sublist in l for item in sublist]'
10000 loops, best of 3: 143 usec per loop
$ python -mtimeit -s'l=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'sum(l, [])'
1000 loops, best of 3: 969 usec per loop
$ python -mtimeit -s'l=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'reduce(lambda x,y: x+y,l)'
1000 loops, best of 3: 1.1 msec per loop

Explanation: the shortcuts based on + (including the implied use in sum) are, of necessity, O(L**2) when there are L sublists -- as the intermediate result list keeps getting longer, at each step a new intermediate result list object gets allocated, and all the items in the previous intermediate result must be copied over (as well as a few new ones added at the end). So, for simplicity and without actual loss of generality, say you have L sublists of I items each: the first I items are copied back and forth L-1 times, the second I items L-2 times, and so on; total number of copies is I times the sum of x for x from 1 to L excluded, i.e., I * (L**2)/2.

The list comprehension just generates one list, once, and copies each item over (from its original place of residence to the result list) also exactly once.

How to convert a nested list of strings to a one list?

If you're using the more_itertools module try the collapse function:

print(list(more_itertools.collapse(fruits)))

Output:

['apple', 'orange', 'pineapple', 'grapes']

How to flatten a hetrogenous list of list into a single list in python?

Here is a relatively simple recursive version which will flatten any depth of list

l = [35,53,[525,6743],64,63,[743,754,757]]

def flatten(xs):
    result = []
    if isinstance(xs, (list, tuple)):
        for x in xs:
            result.extend(flatten(x))
    else:
        result.append(xs)
    return result

print flatten(l)

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