Python using enumerate inside list comprehension

Question

Lets suppose I have a list like this   mylist     a   b   c   d     To get the values printed along with their index I can use Python s enumerate function like this   gt  gt  gt  for i j in enumerate mylist           print i j     0 a 1 b 2 c 3 d  gt  gt  gt    Now  when I try to use it inside a list comprehension it gives me this error   gt  gt  gt   i j for i j in enumerate mylist     File   lt stdin gt    line 1      i j for i j in enumerate mylist                SyntaxError  invalid syntax   So  my question is  what is the correct way of using enumerate inside list comprehension

User · Answer

All great answer guys  I know the question here is specific to enumeration but how about something like this  just another perspective  from itertools import izip  count a     5    6    1    2   tupleList   list  izip  count    a     print tupleList    It becomes more powerful  if one has to iterate multiple lists in parallel in terms of performance  Just a thought  a     5    6    1    2   b     a    b    c    d   tupleList   list  izip  count    a  b     print tupleList

User · Answer

Try this     i  j  for i  j in enumerate mylist     You need to put i j inside a tuple for the list comprehension to work  Alternatively  given that enumerate   already returns a tuple  you can return it directly without unpacking it first    pair for pair in enumerate mylist     Either way  the result that gets returned is as expected    gt    0   a     1   b     2   c     3   d

User · Answer

Be explicit about the tuples     i  j  for  i  j  in enumerate mylist

User · Answer

Or  if you don t insist on using a list comprehension    gt  gt  gt  mylist     a   b   c   d    gt  gt  gt  list enumerate mylist     0   a     1   b     2   c     3   d

User · Answer

Just to be really clear  this has nothing to do with enumerate and everything to do with list comprehension syntax   This list comprehension returns a list of tuples     i j  for i in range 3  for j in  abc     this a list of dicts     i j  for i in range 3  for j in  abc     a list of lists     i j  for i in range 3  for j in  abc     a syntax error    i j for i in range 3  for j in  abc     Which is inconsistent  IMHO  and confusing with dictionary comprehensions syntax    gt  gt  gt   i j for i j in enumerate  abcdef     0   a   1   b   2   c   3   d   4   e   5   f     And a set of tuples    gt  gt  gt    i j  for i j in enumerate  abcdef    set   0   a     4   e     1   b     2   c     5   f     3   d       As   scar L  pez stated  you can just pass the enumerate tuple directly    gt  gt  gt   t for t in enumerate  abcdef        0   a     1   b     2   c     3   d     4   e     5   f

User · Answer

If you re using long lists  it appears the list comprehension s faster  not to mention more readable      python -mtimeit -s mylist     a   b   c   d     list enumerate mylist    1000000 loops  best of 3  1 61 usec per loop    python -mtimeit -s mylist     a   b   c   d       i  j  for i  j in enumerate mylist    1000000 loops  best of 3  0 978 usec per loop    python -mtimeit -s mylist     a   b   c   d      t for t in enumerate mylist    1000000 loops  best of 3  0 767 usec per loop

User · Answer

Here s a way to do it    gt  gt  gt  mylist     a    b    c    d    gt  gt  gt   item for item in enumerate mylist     0   a     1   b     2   c     3   d      Alternatively  you can do    gt  gt  gt    i  j  for i  j in enumerate mylist     0   a     1   b     2   c     3   d      The reason you got an error was that you were missing the    around i and j to make it a tuple

[python] Python using enumerate inside list comprehension

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