How can I get a flat result from a list comprehension instead of a nested list?

Question

I have a list A, and a function f which takes an item of A and returns a list. I can use a list comprehension to convert everything in A like [f(a) for a in A], but this returns a list of lists. Suppose my input is [a1,a2,a3], resulting in [[b11,b12],[b21,b22],[b31,b32]].

How can I get the flattened list [b11,b12,b21,b22,b31,b32] instead? In other words, in Python, how can I get what is traditionally called flatmap in functional programming languages, or SelectMany in .NET?

(In the actual code, A is a list of directories, and f is os.listdir. I want to build a flat list of subdirectories.)

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_{See also: How do I make a flat list out of a list of lists? for the more general problem of flattening a list of lists after it's been created.}

Answer 1

You can have nested iterations in a single list comprehension:

[filename for path in dirs for filename in os.listdir(path)]

which is equivalent (at least functionally) to:

filenames = []
for path in dirs:
    for filename in os.listdir(path):
        filenames.append(filename)

Answer 2

>>> from functools import reduce  # not needed on Python 2
>>> list_of_lists = [[1, 2],[3, 4, 5], [6]]
>>> reduce(list.__add__, list_of_lists)
[1, 2, 3, 4, 5, 6]

The itertools solution is more efficient, but this feels very pythonic.

Answer 3

You can find a good answer in the itertools recipes:

import itertools

def flatten(list_of_lists):
    return list(itertools.chain.from_iterable(list_of_lists))

Answer 4

The question proposed flatmap. Some implementations are proposed but they may unnecessary creating intermediate lists. Here is one implementation that's based on iterators.

def flatmap(func, *iterable):
    return itertools.chain.from_iterable(map(func, *iterable))

In [148]: list(flatmap(os.listdir, ['c:/mfg','c:/Intel']))
Out[148]: ['SPEC.pdf', 'W7ADD64EN006.cdr', 'W7ADD64EN006.pdf', 'ExtremeGraphics', 'Logs']

In Python 2.x, use itertools.map in place of map.

Answer 5

You could just do the straightforward:

subs = []
for d in dirs:
    subs.extend(os.listdir(d))

Answer 6

You can concatenate lists using the normal addition operator:

>>> [1, 2] + [3, 4]
[1, 2, 3, 4]

The built-in function sum will add the numbers in a sequence and can optionally start from a specific value:

>>> sum(xrange(10), 100)
145

Combine the above to flatten a list of lists:

>>> sum([[1, 2], [3, 4]], [])
[1, 2, 3, 4]

You can now define your flatmap:

>>> def flatmap(f, seq):
...   return sum([f(s) for s in seq], [])
... 
>>> flatmap(range, [1,2,3])
[0, 0, 1, 0, 1, 2]

Edit: I just saw the critique in the comments for another answer and I guess it is correct that Python will needlessly build and garbage collect lots of smaller lists with this solution. So the best thing that can be said about it is that it is very simple and concise if you're used to functional programming :-)

Answer 7

subs = []
map(subs.extend, (os.listdir(d) for d in dirs))

(but Ants's answer is better; +1 for him)

Answer 8

import itertools
x=[['b11','b12'],['b21','b22'],['b31']]
y=list(itertools.chain(*x))
print y

itertools will work from python2.3 and greater

Answer 9

You could try itertools.chain(), like this:

import itertools
import os
dirs = ["c:\\usr", "c:\\temp"]
subs = list(itertools.chain(*[os.listdir(d) for d in dirs]))
print subs

itertools.chain() returns an iterator, hence the passing to list().

Answer 10

This is the most simple way to do it:

def flatMap(array):
  return reduce(lambda a,b: a+b, array) 

The 'a+b' refers to concatenation of two lists

Answer 11

You can use pyxtension:

from pyxtension.streams import stream
stream([ [1,2,3], [4,5], [], [6] ]).flatMap() == range(7)

Answer 12

Google brought me next solution:

def flatten(l):
   if isinstance(l,list):
      return sum(map(flatten,l))
   else:
      return l

Answer 13

I was looking for flatmap and found this question first. flatmap is basically a generalization of what the original question asks for. If you are looking for a concise way of defining flatmap for summable collections such as lists you can use

sum(map(f,xs),[])

It's only a little longer than just writing

flatmap(f,xs)

but also potentially less clear at first.

The sanest solution would be to have flatmap as a basic function inside the programming language but as long as it is not, you can still define it using a better or more concrete name:

# `function` must turn the element type of `xs` into a summable type.
# `function` must be defined for arguments constructed without parameters.
def aggregate(function, xs):
    return sum( map(function, xs), type(function( type(xs)() ))() )

# or only for lists
aggregate_list = lambda f,xs: sum(map(f,xs),[])

Strings are not summable unfortunately, it won't work for them.
You can do

assert( aggregate_list( lambda x: x * [x], [2,3,4] ) == [2,2,3,3,3,4,4,4,4] )

but you can't do

def get_index_in_alphabet(character):
    return (ord(character) & ~0x20) - ord('A')

assert(aggregate( lambda x: get_index_in_alphabet(x) * x, "abcd") == "bccddd")

For strings, you need to use

aggregate_string = lambda f,s: "".join(map(f,s))  # looks almost like sum(map(f,s),"")

assert( aggregate_string( lambda x: get_index_in_alphabet(x) * x, "abcd" ) == "bccddd" )

It's obviously a mess, requiring different function names and even syntax for different types. Hopefully, Python's type system will be improved in the future.

Answer 14

You can also use the flatten function using numpy:

import numpy as np

matrix = [[i+k for i in range(10)] for k in range(10)]
matrix_flat = np.array(arr).flatten()

numpy documentation flatten

Answer 15

If listA=[list1,list2,list3]
flattened_list=reduce(lambda x,y:x+y,listA)

This will do.