Getting a map to return a list in Python 3 x

Question

I m trying to map a list into hex  and then use the list elsewhere  In python 2 6  this was easy   A  Python 2 6    gt  gt  gt  map chr   66  53  0  94     B    5     x00          However  in Python 3 1  the above returns a map object   B  Python 3 1    gt  gt  gt  map chr   66  53  0  94    lt map object at 0x00AF5570 gt    How do I retrieve the mapped list  as in A above  on Python 3 x    Alternatively  is there a better way of doing this  My initial list object has around 45 items and id like to convert them to hex

User · Answer

Using list comprehension in python and basic map function utility  one can do this also   chi    x for x in map chr  66 53 0 94

User · Answer

List-returning map function has the advantage of saving typing  especially during interactive sessions  You can define lmap function  on the analogy of python2 s imap  that returns list   lmap   lambda func   iterable  list map func   iterable     Then calling lmap instead of map will do the job   lmap str  x  is shorter by 5 characters  30  in this case  than list map str  x   and is certainly shorter than  str v  for v in x    You may create similar functions for filter too   There was a comment to the original question      I would suggest a rename to Getting map   to return a list in Python 3   as it applies to all Python3 versions  Is there a way to do this       meawoppl Jan 24 at 17 58   It is possible to do that  but it is a very bad idea  Just for fun  here s how you may  but should not  do it     global map   map  keep reference to the original map lmap   lambda func   iterable  list   global map func   iterable     using  map  here will cause infinite recursion map   lmap x    1  2  3  map str  x   test map     global map  restore the original map and don t do that again map str  x   iterator

User · Answer

Do this   list map chr  66 53 0 94      In Python 3   many processes that iterate over iterables return iterators themselves   In most cases  this ends up saving memory  and should make things go faster      If all you re going to do is iterate over this list eventually  there s no need to even convert it to a list  because you can still iterate over the map object like so     Prints  ABCD  for ch in map chr  65 66 67 68        print ch

User · Answer

You can try getting a list from the map object by just iterating each item in the object and store it in a different variable   a   map chr   66  53  0  94   b    item for item in a  print b   gt  gt  gt   B    5     x00

User · Answer

list map chr   66  53  0  94         map func   iterables  --  map object      Make an iterator that computes the function using arguments from      each of the iterables   Stops when the shortest iterable is exhausted        Make an iterator    means it will return an iterator       that computes the function using arguments from each of the iterables    means that the next   function of the iterator will take one value of each iterables and pass each of them to one positional parameter of the function   So you get an iterator from the map   funtion and jsut pass it to the list   builtin function or use list comprehensions

User · Answer

New and neat in Python 3 5     map chr   66  53  0  94      Thanks to Additional Unpacking Generalizations  UPDATE  Always seeking for shorter ways  I discovered this one also works    map chr   66  53  0  94      Unpacking works in tuples too  Note the comma at the end  This makes it a tuple of 1 element  That is  it s equivalent to   map chr   66  53  0  94      It s shorter by only one char from the version with the list-brackets  but  in my opinion  better to write  because you start right ahead with the asterisk - the expansion syntax  so I feel it s softer on the mind

User · Answer

Why aren t you doing this    chr x  for x in  66 53 0 94     It s called a list comprehension   You can find plenty of information on Google  but here s the link to the Python  2 6  documentation on list comprehensions   You might be more interested in the Python 3 documenation  though

User · Answer

Converting my old comment for better visibility  For a  better way to do this  without map entirely  if your inputs are known to be ASCII ordinals  it s generally much faster to convert to bytes and decode  a la bytes list of ordinals  decode  ascii    That gets you a str of the values  but if you need a list for mutability or the like  you can just convert it  and it s still faster   For example  in ipython microbenchmarks converting 45 inputs    gt  gt  gt    timeit -r5 ordinals   list range 45       list map chr  ordinals       3 91   s    60 2 ns per loop  mean    std  dev  of 5 runs  100000 loops each    gt  gt  gt    timeit -r5 ordinals   list range 45         map chr  ordinals       3 84   s    219 ns per loop  mean    std  dev  of 5 runs  100000 loops each    gt  gt  gt    timeit -r5 ordinals   list range 45         bytes ordinals  decode  ascii        1 43   s    49 7 ns per loop  mean    std  dev  of 5 runs  1000000 loops each    gt  gt  gt    timeit -r5 ordinals   list range 45       bytes ordinals  decode  ascii       781 ns    15 9 ns per loop  mean    std  dev  of 5 runs  1000000 loops each    If you leave it as a str  it takes  20  of the time of the fastest map solutions  even converting back to list it s still less than 40  of the fastest map solution  Bulk convert via bytes and bytes decode then bulk converting back to list saves a lot of work  but as noted  only works if all your inputs are ASCII ordinals  or ordinals in some one byte per character locale specific encoding  e g  latin-1

User · Answer

In addition to above answers in Python 3  we may simply create a list of result values from a map as   li      for x in map chr  66 53 0 94        li append x   print  li   gt  gt  gt   B    5     x00          We may generalize by another example where I was struck  operations on map can also be handled in similar fashion like in regex problem  we can write function to obtain list of items to map and get result set at the same time  Ex    b    Strings  1 072  Another String  474   li      for x in map int map int  re findall   d    b         li append x   print  li   gt  gt  gt  1  72  474

User · Answer

Another option is to create a shortcut  returning a list  from functools import reduce  compose   lambda f  g  lambda  args  f g  args   lmap   reduce  compose   list  map     gt  gt  gt  lmap chr   66  53  0  94     B    5     x00

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