python convert list to dictionary

Question

l     a    b    c    d    e     I want to convert this list to a dictionary like   d     a    b    c    d    e         So basically  the evens will be keys whereas the odds will be values  I know that I can do it in a  non-pythonic  way such as a for loop with if statements but I believe that there should be a more  pythonic  way to accomplish this  So  I appreciate any help

User · Accepted Answer

Using the usual grouper recipe  you could do   Python 2   d   dict itertools izip longest   iter l     2  fillvalue        Python 3   d   dict itertools zip longest   iter l     2  fillvalue

User · Answer

Not sure whether it would help you or not but it works to me   l     a    b    c    d    e   outRes   dict  l i   l i 1   if i 1  lt  len l  else  l i       for i in xrange len l

User · Answer

I d go for recursions   l     a    b    c    d    e        d   dict   k  v  for k v in zip  l   2   l 1  2

User · Answer

If you are still thinking what the  You would not be alone  its actually not that complicated really  let me explain   How to turn a list into a dictionary using built-in functions only  We want to turn the following list into a dictionary using the odd entries  counting from 1  as keys mapped to their consecutive even entries   l     a    b    c    d    e     dict    To create a dictionary we can use the built in dict function for Mapping Types as per the manual the following methods are supported   dict one 1  two 2  dict   one   1   two   2   dict zip   one    two     1  2    dict    two   2     one   1      The last option suggests that we supply a list of lists with 2 values or  key  value  tuples  so we want to turn our sequential list into   l      a    b      c    d      e       We are also introduced to the zip function  one of the built-in functions which the manual explains      returns a list of tuples  where the i-th tuple contains the i-th element from each of the arguments   In other words if we can turn our list into two lists a  c  e and b  d then zip will do the rest   slice notation  Slicings which we see used with Strings and also further on in the List section which mainly uses the range or short slice notation but this is what the long slice notation looks like and what we can accomplish with step    gt  gt  gt  l   2    a    c    e     gt  gt  gt  l 1  2    b    d     gt  gt  gt  zip   a    c    e      b    d       a    b      c    d      gt  gt  gt  dict zip l   2   l 1  2      a    b    c    d     Even though this is the simplest way to understand the mechanics involved there is a downside because slices are new list objects each time  as can be seen with this cloning example    gt  gt  gt  a    1  2  3   gt  gt  gt  b   a  gt  gt  gt  b  1  2  3    gt  gt  gt  b is a True   gt  gt  gt  b   a     gt  gt  gt  b  1  2  3    gt  gt  gt  b is a False   Even though b looks like a they are two separate objects now and this is why we prefer to use the grouper recipe instead   grouper recipe  Although the grouper is explained as part of the itertools module it works perfectly fine with the basic functions too   Some serious voodoo right     But actually nothing more than a bit of syntax sugar for spice  the grouper recipe is accomplished by the following expression     iter l   2   Which more or less translates to two arguments of the same iterator wrapped in a list  if that makes any sense  Lets break it down to help shed some light   zip for shortest   gt  gt  gt  l 2   a    b    c    d    e    a    b    c    d    e     gt  gt  gt   l  2    a    b    c    d    e      a    b    c    d    e      gt  gt  gt   iter l   2   lt listiterator object at 0x100486450 gt    lt listiterator object at 0x100486450 gt     gt  gt  gt  zip  iter l   2     lt listiterator object at 0x1004865d0 gt      lt listiterator object at 0x1004865d0 gt       gt  gt  gt  zip   iter l   2     a    b      c    d      gt  gt  gt  dict zip   iter l   2     a    b    c    d     As you can see the addresses for the two iterators remain the same so we are working with the same iterator which zip then first gets a key from and then a value and a key and a value every time stepping the same iterator to accomplish what we did with the slices much more productively   You would accomplish very much the same with the following which carries a smaller What the  factor perhaps    gt  gt  gt  it   iter l        gt  gt  gt  dict zip it  it     a    b    c    d     What about the empty key e if you ve noticed it has been missing from all the examples which is because zip picks the shortest of the two arguments  so what are we to do   Well one solution might be adding an empty value to odd length lists  you may choose to use append and an if statement which would do the trick  albeit slightly boring  right    gt  gt  gt  if len l    2          l append       gt  gt  gt  l   a    b    c    d    e         gt  gt  gt  dict zip   iter l   2     a    b    c    d    e         Now before you shrug away to go type from itertools import izip longest you may be surprised to know it is not required  we can accomplish the same  even better IMHO  with the built in functions alone   map for longest  I prefer to use the map   function instead of izip longest   which not only uses shorter syntax doesn t require an import but it can assign an actual None empty value when required  automagically    gt  gt  gt  l     a    b    c    d    e    gt  gt  gt  l   a    b    c    d    e     gt  gt  gt  dict map None    iter l   2     a    b    c    d    e   None     Comparing performance of the two methods  as pointed out by KursedMetal  it is clear that the itertools module far outperforms the map function on large volumes  as a benchmark against 10 million records show      time python -c  dict map None    iter range 10000000    2    real    0m3 755s user    0m2 815s sys     0m0 869s   time python -c  from itertools import izip longest  dict izip longest   iter range 10000000    2  fillvalue None    real    0m2 102s user    0m1 451s sys     0m0 539s   However the cost of importing the module has its toll on smaller datasets with map returning much quicker up to around 100 thousand records when they start arriving head to head      time python -c  dict map None    iter range 100    2    real    0m0 046s user    0m0 029s sys     0m0 015s   time python -c  from itertools import izip longest  dict izip longest   iter range 100    2  fillvalue None    real    0m0 067s user    0m0 042s sys     0m0 021s    time python -c  dict map None    iter range 100000    2    real    0m0 074s user    0m0 050s sys     0m0 022s   time python -c  from itertools import izip longest  dict izip longest   iter range 100000    2  fillvalue None    real    0m0 075s user    0m0 047s sys     0m0 024s   See nothing to it      nJoy

[python] python convert list to dictionary

Examples related to python

Examples related to list

Examples related to dictionary