Explicitly select items from a list or tuple

Question

I have the following Python list  can also be a tuple    myList     foo    bar    baz    quux     I can say   gt  gt  gt  myList 0 3    foo    bar    baz    gt  gt  gt  myList   2    foo    baz    gt  gt  gt  myList 1  2    bar    quux     How do I explicitly pick out items whose indices have no specific patterns  For example  I want to select  0 2 3   Or from a very big list of 1000 items  I want to select  87  342  217  998  500   Is there some Python syntax that does that  Something that looks like    gt  gt  gt  myBigList 87  342  217  998  500

User · Answer

like often when you have a boolean numpy array like mask

[mylist[i] for i in np.arange(len(mask), dtype=int)[mask]]

A lambda that works for any sequence or np.array:

subseq = lambda myseq, mask : [myseq[i] for i in np.arange(len(mask), dtype=int)[mask]]

newseq = subseq(myseq, mask)

User · Answer

gt  gt  gt  map myList   getitem     2 2 1 3     baz    baz    bar    quux     You can also create your own List class which supports tuples as arguments to   getitem   if you want to be able to do myList  2 2 1 3

User · Answer

I just want to point out  even syntax of itemgetter looks really neat  but it s kinda slow when perform on large list   import timeit from operator import itemgetter start timeit default timer   for i in range 1000000       itemgetter 0 2 3  myList  print   Itemgetter took     timeit default timer  -start     Itemgetter took  1 065209062149279  start timeit default timer   for i in range 1000000       myList 0  myList 2  myList 3  print   Multiple slice took     timeit default timer  -start     Multiple slice took  0 6225321444745759

User · Answer

It isn t built-in  but you can make a subclass of list that takes tuples as  indexes  if you d like   class MyList list        def   getitem   self  index           if isinstance index  tuple               return  self i  for i in index          return super MyList  self    getitem   index    seq   MyList  foo bar baaz quux mumble  split    print seq 0  print seq 2 4  print seq 1  2    printing  foo   baaz    mumble     bar    quux

User · Answer

Another possible solution   sek    L  1 2 3 4 5 6 7 8 9 0  for i in  2  4  7  0  3      a  L i      sek sek a print  sek

User · Answer

list  myBigList i  for i in  87  342  217  998  500        I compared the answers with python 2 5 2    19 7 usec    myBigList i  for i in  87  342  217  998  500    20 6 usec  map myBigList   getitem     87  342  217  998  500   22 7 usec  itemgetter 87  342  217  998  500  myBigList  24 6 usec  list  myBigList i  for i in  87  342  217  998  500      Note that in Python 3  the 1st was changed to be the same as the 4th     Another option would be to start out with a numpy array which allows indexing via a list or a numpy array    gt  gt  gt  import numpy  gt  gt  gt  myBigList   numpy array range 1000    gt  gt  gt  myBigList  87  342  217  998  500   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  IndexError  invalid index  gt  gt  gt  myBigList  87  342  217  998  500   array   87  342  217  998  500    gt  gt  gt  myBigList numpy array  87  342  217  998  500    array   87  342  217  998  500     The tuple doesn t work the same way as those are slices

User · Answer

What about this   from operator import itemgetter itemgetter 0 2 3  myList    foo    baz    quux

User · Answer

Maybe a list comprehension is in order   L     a    b    c    d    e    f   print   L index  for index in  1 3 5      Produces     b    d    f     Is that what you are looking for

[python] Explicitly select items from a list or tuple

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