join list of lists in python

Question

Is the a short syntax for joining a list of lists into a single list  or iterator  in python   For example I have a list as follows and I want to iterate over a b and c     x      a   b      c      The best I can come up with is as follows   result        result extend el  for el in x    for el in result    print el

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For one-level flatten  if you care about speed  this is faster than any of the previous answers under all conditions I tried    That is  if you need the result as a list   If you only need to iterate through it on the fly then the chain example is probably better    It works by pre-allocating a list of the final size and copying the parts in by slice  which is a lower-level block copy than any of the iterator methods    def join a          Joins a sequence of sequences into a single sequence    One-level flattening       E g   join   1 2 3    4  5    6   7  8  9   10       1 2 3 4 5 6  7 8 9  10      This is very efficient  especially when the subsequences are long              n   sum  len b  for b in a       l    None  n     i   0     for b in a          j   i len b          l i j    b         i   j     return l   Sorted times list with comments     0 5391559600830078   flatten4b      join   above    0 5400412082672119   flatten4c      Same  with sum len b  for b in a    0 5419249534606934   flatten4a      Similar  using zip     0 7351131439208984   flatten1b      list itertools chain from iterable a     0 7472689151763916   flatten1      list itertools chain  a     1 5468521118164062   flatten3       i for j in a for i in j    26 696547985076904   flatten2      sum a

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import itertools a      a   b      c    print list itertools chain from iterable a

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Late to the party but       I m new to python and come from a lisp background  This is what I came up with  check out the var names for lulz    def flatten lst       if lst          car  cdr lst         if isinstance car  list tuple                if cdr  return flatten car    flatten cdr              return flatten car          if cdr  return  car    flatten cdr          return  car    Seems to work  Test   flatten  1 2 3  4 5 6  7 8    1 2          returns    1  2  3  4  5  6  7  8  1  2

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What you re describing is known as flattening a list  and with this new knowledge you ll be able to find many solutions to this on Google  there is no built-in flatten method   Here is one of them  from http   www daniel-lemire com blog archives 2006 05 10 flattening-lists-in-python    def flatten x       flat   True     ans          for i in x          if   i   class   is list               ans   flatten i          else              ans append i      return ans

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This is known as flattening  and there are a LOT of implementations out there    More on python flatten   Python tricks   Flattening lists in Python   How about this  although it will only work for 1 level deep nesting    gt  gt  gt  x      a   b      c     gt  gt  gt  for el in sum x               print el     a b c   From those links  apparently the most complete-fast-elegant-etc implementation is the following   def flatten l  ltypes  list  tuple        ltype   type l      l   list l      i   0     while i  lt  len l           while isinstance l i   ltypes               if not l i                   l pop i                  i -  1                 break             else                  l i i   1    l i          i    1     return ltype l

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I had a similar problem when I had to create a dictionary that contained the elements of an array and their count  The answer is relevant because  I flatten a list of lists  get the elements I need and then do a group and count  I used Python s map function to produce a tuple of element and it s count and groupby over the array  Note that the groupby takes the array element itself as the keyfunc  As a relatively new Python coder  I find it to me more easier to comprehend  while being Pythonic as well    Before I discuss the code  here is a sample of data I had to flatten first       id    ObjectId  4fe3a90783157d765d000011     status       opencalais        content length    688   open calais extract       entities          type    Person   name     Iman Samdura   rel score    0 223         type     Company     name     Associated Press       rel score    0 321                  type     Country     name     Indonesia      rel score    0 321               title     Indonesia Police Arrest Bali Bomb Planner    time     06 42  ET               filename     021121bn 01    month     November    utctime    1037836800     date     November 21  2002    news type     bn    day     21      It is a query result from Mongo  The code below flattens a collection of such lists   def flatten list items     return sorted  entity  name   for entity in  entities for sublist in       item  open calais extract    entities   for item in items      for entities in sublist     First  I would extract all the  entities  collection  and then for each entities collection  iterate over the dictionary and extract the name attribute

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l      map l extend  list of lists    shortest

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Sadly  Python doesn t have a simple way to flatten lists   Try this   def flatten some list       for element in some list          if type element  in  tuple  list               for item in flatten element                   yield item         else              yield element   Which will recursively flatten a list  you can then do  result        result extend el  for el in x    for el in flatten result         print el

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x      a   b      c     result   sum x

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A performance comparison   import itertools import timeit big list     0  1000 for i in range 1000   timeit repeat lambda  list itertools chain from iterable big list    number 100  timeit repeat lambda  list itertools chain  big list    number 100  timeit repeat lambda   lambda b  map b extend  big list        number 100  timeit repeat lambda   el for list  in big list for el in list    number 100   100 x for x in timeit repeat lambda  sum big list       number 1     Producing    gt  gt  gt  import itertools  gt  gt  gt  import timeit  gt  gt  gt  big list     0  1000 for i in range 1000    gt  gt  gt  timeit repeat lambda  list itertools chain from iterable big list    number 100   3 016212113769325  3 0148865239060227  3 0126415732791028   gt  gt  gt  timeit repeat lambda  list itertools chain  big list    number 100   3 019953987082083  3 528754223385439  3 02181439266457   gt  gt  gt  timeit repeat lambda   lambda b  map b extend  big list        number 100   1 812084445152557  1 7702404451095965  1 7722977998725362   gt  gt  gt  timeit repeat lambda   el for list  in big list for el in list    number 100   5 409658160700605  5 477502077679354  5 444318360412744   gt  gt  gt   100 x for x in timeit repeat lambda  sum big list       number 1    399 27587954973444  400 9240571138051  403 7521153804846    This is with Python 2 7 1 on Windows XP 32-bit  but  temoto in the comments above got from iterable to be faster than map extend  so it s quite platform and input dependent   Stay away from sum big list

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Or a recursive operation   def flatten input       ret          if not isinstance input   list  tuple            return  input      for i in input          if isinstance i   list  tuple                ret extend flatten i           else              ret append i      return ret

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If you need a list  not a generator  use list     from itertools import chain x      a   b      c    y   list chain  x

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This works recursively for infinitely nested elements   def iterFlatten root       if isinstance root   list  tuple            for element in root              for e in iterFlatten element                   yield e     else          yield root   Result        b      a     b    c      d       list iterFlatten b     a    b    c    d

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If you re only going one level deep  a nested comprehension will also work    gt  gt  gt  x      a   b      c     gt  gt  gt   inner         for outer in x             for inner in outer    a    b    c     On one line  that becomes    gt  gt  gt   j for i in x for j in i    a    b    c

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There s always reduce  being deprecated to functools     gt  gt  gt  x        a    b      c      gt  gt  gt  for el in reduce lambda a b  a b  x            print el       main   1  DeprecationWarning  reduce   not supported in 3 x  use functools reduce   a b c  gt  gt  gt  import functools  gt  gt  gt  for el in functools reduce lambda a b  a b  x             print el     a b c  gt  gt  gt    Unfortunately the plus operator for list concatenation can t be used as a function -- or fortunate  if you prefer lambdas to be ugly for improved visibility

[python] join list of lists in python

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