How do I concatenate two lists in Python

Question

How do I concatenate two lists in Python   Example   listone    1  2  3  listtwo    4  5  6    Expected outcome    gt  gt  gt  joinedlist  1  2  3  4  5  6

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If you are using NumPy  you can concatenate two arrays of compatible dimensions with this command  numpy concatenate  a b

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This question directly asks about joining two lists  However it s pretty high in search even when you are looking for a way of joining many lists  including the case when you joining zero lists    I think the best option is to use list comprehensions    gt  gt  gt  a     1 2 3    4 5 6    7 8 9    gt  gt  gt   x for xs in a for x in xs   1  2  3  4  5  6  7  8  9    You can create generators as well    gt  gt  gt  map str   x for xs in a for x in xs     1    2    3    4    5    6    7    8    9     Old Answer  Consider this more generic approach   a     1 2 3    4 5 6    7 8 9   reduce lambda c  x  c   x  a        Will output    1  2  3  4  5  6  7  8  9    Note  this also works correctly when a is    or   1 2 3     However  this can be done more efficiently with itertools   a     1 2 3    4 5 6    7 8 9   list itertools chain  a     If you don t need a list  but just an iterable  omit list     Update  Alternative suggested by Patrick Collins in the comments could also work for you   sum a

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It s worth noting that the itertools chain function accepts variable number of arguments    gt  gt  gt  l1     a    l2     b    c    l3     d    e    f    gt  gt  gt   i for i in itertools chain l1  l2     a    b    c    gt  gt  gt   i for i in itertools chain l1  l2  l3     a    b    c    d    e    f     If an iterable  tuple  list  generator  etc   is the input  the from iterable class method may be used    gt  gt  gt  il      a      b    c      d    e    f     gt  gt  gt   i for i in itertools chain from iterable il     a    b    c    d    e    f

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You can use the union   function in python  joinedlist   union listone  listtwo  print joinedlist   Essentially what this is doing is its removing one of every duplicate in the two lists  Since your lists don t have any duplicates it  it just returns the concatenated version of the two lists

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A really concise way to combine a list of lists is  list of lists     1 2 3    4 5 6    7 8 9   reduce list   add    list of lists    which gives us   1  2  3  4  5  6  7  8  9

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It s also possible to create a generator that simply iterates over the items in both lists using itertools chain    This allows you to chain lists  or any iterable  together for processing without copying the items to a new list   import itertools for item in itertools chain listone  listtwo         Do something with each list item

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I assume you are wanting one of the two methods  Keep Duplicate elements It is very easy  just concatenate like string  def concat list l1 l2       l3   l1 l2     return l3  Next if you want to eliminate duplicate elements def concat list l1 l2      l3         for i in  l1 l2        for j in i            if j not in l3               Check if element exists in final list  if no then add element to list          l3 append j     return l3

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As already pointed out by many  itertools chain   is the way to go if one needs to apply exactly the same treatment to both lists  In my case  I had a label and a flag which were different from one list to the other  so I needed something slightly more complex  As it turns out  behind the scenes itertools chain   simply does the following   for it in iterables      for element in it          yield element    see https   docs python org 2 library itertools html   so I took inspiration from here and wrote something along these lines   for iterable  header  flag in    newList   New         modList   Modified    -f         print header           for path in iterable                        command    cp -r  if os path isdir srcPath  else  cp          print  gt  gt  SCRIPT   command  flag  srcPath  mergedDirPath                 The main points to understand here are that lists are just a special case of iterable  which are objects like any other  and that for     in loops in python can work with tuple variables  so it is simple to loop on multiple variables at the same time

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You can use the   operator to combine them   listone    1 2 3  listtwo    4 5 6   joinedlist   listone   listtwo   Output    gt  gt  gt  joinedlist  1 2 3 4 5 6

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If you want to merge the two lists in sorted form  you can use the merge function from the heapq library   from heapq import merge  a    1  2  4  b    2  4  6  7   print list merge a  b

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How do I concatenate two lists in Python   As of 3 9  these are the most popular stdlib methods for concatenating two  or more  lists in python    Footnotes  This is a slick solution because of its succinctness  But sum performs concatenation in a pairwise fashion  which means this is a quadratic operation as memory has to be allocated for each step  DO NOT USE if your lists are large   See chain and chain from iterable from the docs  You will need to import itertools first  Concatenation is linear in memory  so this is the best in terms of performance and version compatibility  chain from iterable was introduced in 2 6   This method uses Additional Unpacking Generalizations  PEP 448   but cannot generalize to N lists unless you manually unpack each one yourself   a    b and a extend b  are more or less equivalent for all practical purposes     when called on a list will internally call list   iadd    which extends the first list by the second      Performance 2-List Concatenation1  There s not much difference between these methods but that makes sense given they all have the same order of complexity  linear   There s no particular reason to prefer one over the other except as a matter of style  N-List Concatenation  Plots have been generated using the perfplot module  Code  for your reference  1  The iadd      and extend methods operate in-place  so a copy has to be generated each time before testing  To keep things fair  all methods have a pre-copy step for the left-hand list which can be ignored   Comments on Other Solutions  DO NOT USE THE DUNDER METHOD list   add   directly in any way  shape or form  In fact  stay clear of dunder methods  and use the operators and operator functions like they were designed for  Python has careful semantics baked into these which are more complicated than just calling the dunder directly  Here is an example  So  to summarise  a   add   b    gt  BAD  a   b   gt  GOOD   Some answers here offer reduce operator add   a  b   for pairwise concatenation -- this is the same as sum  a  b       only more wordy   Any method that uses set will drop duplicates and lose ordering  Use with caution   for i in b  a append i  is more wordy  and slower than a extend b   which is single function call and more idiomatic  append is slower because of the semantics with which memory is allocated and grown for lists  See here for a similar discussion   heapq merge will work  but its use case is for merging sorted lists in linear time  Using it in any other situation is an anti-pattern   yielding list elements from a function is an acceptable method  but chain does this faster and better  it has a code path in C  so it is fast    operator add a  b  is an acceptable functional equivalent to a   b  It s use cases are mainly for dynamic method dispatch  Otherwise  prefer a   b which is shorter and more readable  in my opinion  YMMV

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With Python 3 3  you can use yield from   listone    1 2 3  listtwo    4 5 6   def merge l1  l2       yield from l1     yield from l2   gt  gt  gt  list merge listone  listtwo    1  2  3  4  5  6    Or  if you want to support an arbitrary number of iterators   def merge  iters       for it in iters          yield from it   gt  gt  gt  list merge listone  listtwo   abcd    20  21  22     1  2  3  4  5  6   a    b    c    d   20  21  22

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Use a simple list comprehension   joined list    item for list  in  list one  list two  for item in list     It has all the advantages of the newest approach of using Additional Unpacking Generalizations - i e  you can concatenate an arbitrary number of different iterables  for example  lists  tuples  ranges  and generators  that way - and it s not limited to Python 3 5 or later

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Python  gt   3 5 alternative    l1   l2   Another alternative has been introduced via the acceptance of PEP 448 which deserves mentioning    The PEP  titled Additional Unpacking Generalizations  generally reduced some syntactic restrictions when using the starred   expression in Python  with it  joining two lists  applies to any iterable  can now also be done with    gt  gt  gt  l1    1  2  3   gt  gt  gt  l2    4  5  6   gt  gt  gt  joined list     l1   l2     unpack both iterables in a list literal  gt  gt  gt  print joined list   1  2  3  4  5  6    This functionality was defined for Python 3 5 it hasn t been backported to previous versions in the 3 x family  In unsupported versions a SyntaxError is going to be raised   As with the other approaches  this too creates as shallow copy of the elements in the corresponding lists     The upside to this approach is that you really don t need lists in order to perform it  anything that is iterable will do  As stated in the PEP      This is also useful as a more readable way of summing iterables into a   list  such as my list   list my tuple    list my range  which is now   equivalent to just   my list   my tuple   my range     So while addition with   would raise a TypeError due to type mismatch   l    1  2  3  r   range 4  7  res   l   r   The following won t   res     l   r    because it will first unpack the contents of the iterables and then simply create a list from the contents

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You can use sets to obtain merged list of unique values  mergedlist   list set listone   listtwo

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list set listone    set listtwo     The above code  does not preserve order  removes duplicate from each list  but not from the concatenated list

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You could use the append   method defined on list objects    mergedlist     for elem in listone      mergedlist append elem  for elem in listtwo      mergedlist append elem

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a  1 2 3   b  4 5 6    c a b  print c   OUTPUT    gt  gt  gt   1  2  3  4  5  6    In the above code  quot   quot  operator is used to concatenate the 2 lists into a single list   ANOTHER SOLUTION   a  1 2 3   b  4 5 6   c     Empty list in which we are going to append the values of list  a  and  b    for i in a       c append i   for j in b       c append j    print c   OUTPUT   gt  gt  gt   1  2  3  4  5  6

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import itertools  A   list zip  1 3 5 7 9   2 4 6 8 10    B    1 3 5 7 9   2 4 6 8 10  C   list set  1 3 5 7 9     2 4 6 8 10     D    1 3 5 7 9  D append  2 4 6 8 10    E    1 3 5 7 9  E extend  2 4 6 8 10    F      for a in itertools chain  1 3 5 7 9    2 4 6 8 10        F append a    print   A      str A   print   B      str B   print   C      str C   print   D      str D   print   E      str E   print   F      str F     Output   A    1  2    3  4    5  6    7  8    9  10   B   1  3  5  7  9  2  4  6  8  10  C   1  2  3  4  5  6  7  8  9  10  D   1  3  5  7  9   2  4  6  8  10   E   1  3  5  7  9  2  4  6  8  10  F   1  3  5  7  9  2  4  6  8  10

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So there are two easy ways    Using    It creates a new list from provided lists   Example   In  1   a    1  2  3   In  2   b    4  5  6   In  3   a   b Out 3    1  2  3  4  5  6   In  4    timeit a   b 10000000 loops  best of 3  126 ns per loop    Using extend  It appends new list to existing list  That means it does not create a separate list    Example   In  1   a    1  2  3   In  2   b    4  5  6   In  3    timeit a extend b  10000000 loops  best of 3  91 1 ns per loop   Thus we see that out of two of most popular methods  extend is efficient

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This is quite simple  and I think it was even shown in the tutorial    gt  gt  gt  listone    1 2 3   gt  gt  gt  listtwo    4 5 6   gt  gt  gt   gt  gt  gt  listone   listtwo  1  2  3  4  5  6

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You could also use the list extend   method in order to add a list to the end of another one   listone    1 2 3  listtwo    4 5 6   listone extend listtwo    If you want to keep the original list intact  you can create a new list object  and extend both lists to it   mergedlist      mergedlist extend listone  mergedlist extend listtwo

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If you can t use the plus operator       you can use the operator import   import operator  listone    1 2 3  listtwo    4 5 6   result   operator add listone  listtwo  print result    gt  gt  gt   1  2  3  4  5  6    Alternatively  you could also use the   add   dunder function   listone    1 2 3  listtwo    4 5 6   result   list   add   listone  listtwo  print result    gt  gt  gt   1  2  3  4  5  6

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For cases with a low number of lists you can simply add the lists together or use in-place unpacking  available in Python-3 5    In  1   listone    1  2  3           listtwo    4  5  6                                                                                                                                                                                    In  2   listone   listtwo                                                                                                                                                                                    Out 2    1  2  3  4  5  6                                                                                                                                                                                        In  3     listone   listtwo                                                                                                                                                                                  Out 3    1  2  3  4  5  6   As a more general way for cases with more number of lists  as a pythonic approach  you can use chain from iterable  1 function from itertoold module  Also  based on this answer this function is the best  or at least a very food way for flatting a nested list as well   gt  gt  gt  l   1  2  3    4  5  6    7  8  9    gt  gt  gt  import itertools  gt  gt  gt  list itertools chain from iterable l    1  2  3  4  5  6  7  8  9     1  Note that  chain from iterable    is available in Python 2 6 and later  In other versions  use  chain  l

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If you need to merge two ordered lists with complicated sorting rules  you might have to roll it yourself like in the following code  using a simple sorting rule for readability  -      list1    1 2 5  list2    2 3 4  newlist       while list1 and list2      if list1 0     list2 0           newlist append list1 pop 0           list2 pop 0      elif list1 0   lt  list2 0           newlist append list1 pop 0       else          newlist append list2 pop 0    if list1      newlist extend list1  if list2      newlist extend list2   assert newlist     1  2  3  4  5

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You could simply use the   or    operator as follows   a    1  2  3  b    4  5  6   c   a   b   Or   c      a    1  2  3  b    4  5  6   c     a   b    Also  if you want the values in the merged list to be unique you can do   c   list set a   b

[python] How do I concatenate two lists in Python?

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