Zip lists in Python

Question

I am trying to learn how to  zip  lists  To this end  I have a program  where at a particular point  I do the following   x1  x2  x3   stuff calculations withdataa    This gives me three lists  x1  x2  and x3  each of  say  size 20   Now  I do   zipall   zip x1  x2  x3    However  when I do   print  len of zipall  s    len zipall    I get 20  which is not what I expected  I expected three  I think I am doing something fundamentally wrong

User · Answer

zip creates a new list  filled with tuples containing elements from the iterable arguments    gt  gt  gt  zip   1 2   3 4     1 3    2 4     I expect what you try to so is create a tuple where each element is a list

User · Answer

In Python 2 7 this might have worked fine    gt  gt  gt  a   b   c   range 20   gt  gt  gt  zip a  b  c    But in Python 3 4 it should be  otherwise  the result will be something like  lt zip object at 0x00000256124E7DC8 gt      gt  gt  gt  a   b   c   range 20   gt  gt  gt  list zip a  b  c

User · Answer

In Python 3 zip returns an iterator instead and needs to be passed to a list function to get the zipped tuples   x    1  2  3   y     a   b   c   z   zip x  y  z   list z  print z   gt  gt  gt    1   a     2   b     3   c      Then to unzip them back just conjugate the zipped iterator   x back  y back   zip  z  print x back   print y back   gt  gt  gt   1  2  3   gt  gt  gt    a    b    c     If the original form of list is needed instead of tuples   x back  y back   zip  z  print list x back    print list y back    gt  gt  gt   1 2 3   gt  gt  gt    a   b   c

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For the completeness s sake   When zipped lists  lengths are not equal  The result list s length will become the shortest one without any error occurred   gt  gt  gt  a    1   gt  gt  gt  b     2   3   gt  gt  gt  zip a b    1   2

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It s worth adding here as it is such a highly ranking question on zip  zip is great  idiomatic Python - but it doesn t scale very well at all for large lists   Instead of   books     AAAAAAA    BAAAAAAA          ZZZZZZZ   words    345  567        672   for book  word in zip books  words      print          format book  word     Use izip  For modern processing  it stores it in L1 Cache memory and is far more performant for larger lists  Use it as simply as adding an i   for book  word in izip books  words      print          format book  word

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Source  My Blog Post  better formatting   Example  numbers    1 2 3  letters    abcd   zip numbers  letters      1   a     2   b     3   c      Input  Zero or more iterables  1   ex  list  string  tuple  dictionary   Output  list      1st tuple    element 1 of numbers  element 1 of letters        2nd tuple    e 2 numbers  e 2 letters                   n-th tuple    e n numbers  e n letters     List of n tuples  n is the length of the shortest argument  input    len numbers     3  lt  len letters     4   short  3   return 3 tuples  Length each tuple     of args  tuple takes an element from each arg    args    numbers letters   len args     2   tuple with 2 elements  ith tuple    element i arg1  element i arg2     element i argn    Edge Cases  1  Empty String  len str   0   no tuples  2  Single String  len str     2 tuples with len args     1 element s   zip        zip          zip  hi        h       i         Zip in Action   1  Build a dictionary  2  out of two lists  keys     drink   band   food   values     La Croix    Daft Punk    Sushi    my favorite   dict  zip keys  values     my favorite  drink      La Croix   my faves   dict   for i in range len keys        my faves keys i     values i     zip is an elegant  clear   amp  concise solution   2  Print columns in a table       3  is called  unpacking   f   arg1 arg2 arg3      f arg1  arg2  arg3   student grades          Morty      1       B           Rick       4       A           Jerry      3       M          Kramer      0       F         row 1   student grades 0  print row 1     Morty   1   B    columns   zip  student grades  names   columns 0  print names     Morty    Rick    Jerry    Kramer       Extra Credit  Unzipping  zip  args  is called    unzipping    because it has the inverse effect of zip  numbers    1 2 3  letters     a   b   c    zipped   zip numbers  letters  print zipped     1   a     2   b     3   c     unzipped   zip  zipped  print unzipped     1  2  3     a    b    c       unzipped  tuple 1   e1 of each zipped tuple  tuple 2   e2 of each zipped     Footnotes   An object capable of returning its members one at a time  ex  list  1 2 3   string  I like codin   tuple  1 2 3   dictionary   a  1   b  2    key1 value1  key2 value2        Unpacking            Code     foo - function  returns sum of two arguments def foo x y       return x   y print foo 3 4    7  numbers    1 2  print foo numbers    TypeError  foo   takes exactly 2 arguments  1 given   print foo  numbers    3     took numbers  1 arg  and    unpacked    its    2 elements into 2 args

User · Answer

I don t think zip returns a list  zip returns a generator  You have got to do list zip a  b   to get a list of tuples   x    1  2  3  y    4  5  6  zipped   zip x  y  list zipped

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zip takes a bunch of lists likes  a  a1 a2 a3 a4 a5 a6 a7    b  b1 b2 b3 b4 b5 b6 b7    c  c1 c2 c3 c4 c5 c6 c7      and  zips  them into one list whose entries are 3-tuples  ai  bi  ci   Imagine drawing a zipper horizontally from left to right

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When you zip   together three lists containing 20 elements each  the result has twenty elements  Each element is a three-tuple   See for yourself   In  1   a   b   c   range 20   In  2   zip a  b  c  Out 2      0  0  0     1  1  1          17  17  17     18  18  18     19  19  19     To find out how many elements each tuple contains  you could examine the length of the first element   In  3   result   zip a  b  c   In  4   len result 0   Out 4   3   Of course  this won t work if the lists were empty to start with

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Basically the zip function works on lists  tuples and dictionaries in Python  If you are using IPython then just type zip  And check what zip   is about   If you are not using IPython then just install it   pip install ipython   For lists  a     a    b    c   b     p    q    r   zip a  b    The output is    a    p      b    q      c    r    For dictionary   c     gaurav   waghs    nilesh   kashid    ramesh   sawant    anu   raje   d     amit   wagh    swapnil   dalavi    anish   mane    raghu   rokda   zip c  d    The output is      gaurav    amit       nilesh    swapnil       ramesh    anish       anu    raghu

[python] Zip lists in Python

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