Should you always favor xrange over range

Question

Why or why not

User · Accepted Answer

For performance  especially when you re iterating over a large range  xrange   is usually better   However  there are still a few cases why you might prefer range      In python 3  range   does what xrange   used to do and xrange   does not exist   If you want to write code that will run on both Python 2 and Python 3  you can t use xrange    range   can actually be faster in some cases - eg  if iterating over the same sequence multiple times   xrange   has to reconstruct the integer object every time  but range   will have real integer objects    It will always perform worse in terms of memory however  xrange   isn t usable in all cases where a real list is needed   For instance  it doesn t support slices  or any list methods     Edit  There are a couple of posts mentioning how range   will be upgraded by the 2to3 tool   For the record  here s the output of running the tool on some sample usages of range   and xrange    RefactoringTool  Skipping implicit fixer  buffer RefactoringTool  Skipping implicit fixer  idioms RefactoringTool  Skipping implicit fixer  ws comma --- range test py  original      range test py  refactored     -1 7  1 7      for x in range 20   -    a range 20       a list range 20        b list range 20        c  x for x in range 20        d  x for x in range 20   -    e xrange 20       e range 20    As you can see  when used in a for loop or comprehension  or where already wrapped with list    range is left unchanged

User · Answer

xrange   is more efficient because instead of generating a list of objects  it just generates one object at a time  Instead of 100 integers  and all of their overhead  and the list to put them in  you just have one integer at a time  Faster generation  better memory use  more efficient code   Unless I specifically need a list for something  I always favor xrange

User · Answer

Okay  everyone here as a different opinion as to the tradeoffs and advantages of xrange versus range   They re mostly correct  xrange is an iterator  and range fleshes out and creates an actual list   For the majority of cases  you won t really notice a difference between the two    You can use map with range but not with xrange  but it uses up more memory    What I think you rally want to hear  however  is that the preferred choice is xrange   Since range in Python 3 is an iterator  the code conversion tool 2to3 will correctly convert all uses of xrange to range  and will throw out an error or warning for uses of range   If you want to be sure to easily convert your code in the future  you ll use xrange only  and list xrange  when you re sure that you want a list   I learned this during the CPython sprint at PyCon this year  2008  in Chicago

User · Answer

I would just like to say that it REALLY isn t that difficult to get an xrange object with slice and indexing functionality   I have written some code that works pretty dang well and is just as fast as xrange for when it counts  iterations    from   future   import division  def read xrange xrange object         returns the xrange object s start  stop  and step     start   xrange object 0      if len xrange object   gt  1         step   xrange object 1  - xrange object 0      else          step   1     stop   xrange object -1    step     return start  stop  step  class Xrange object           creates an xrange-like object that supports slicing and indexing      ex  a   Xrange 20      a index 10      will work      Also a  5      will return another Xrange object with the specified attributes      Also allows for the conversion from an existing xrange object             def   init   self   inputs             allow inputs of xrange objects         if len inputs     1              test    inputs             if type test     xrange                  self xrange   test                 self start  self stop  self step   read xrange test                  return            or create one from start  stop  step         self start  self step   0  None         if len inputs     1              self stop    inputs         elif len inputs     2              self start  self stop   inputs         elif len inputs     3              self start  self stop  self step   inputs         else              raise ValueError inputs           self xrange   xrange self start  self stop  self step       def   iter   self           return iter self xrange       def   getitem   self  item           if type item  is int              if item  lt  0                  item    len self               return self xrange item           if type item  is slice                get the indexes  and then convert to the number             start  stop  step   item start  item stop  item step             start   start if start    None else 0   convert start   None to start   0             if start  lt  0                  start    start             start   self start              if start  lt  0  raise IndexError item              step    self step if self step    None else 1     step if step    None else 1              stop   stop if stop is not None else self xrange -1              if stop  lt  0                  stop    stop              stop   self stop              stop   stop              if stop  gt  self stop                  raise IndexError             if start  lt  self start                  raise IndexError             return Xrange start  stop  step       def index self  value           error   ValueError  object index  0     0  not in object  format value           index    value - self start  self step         if index   1    0              raise error         index   int index            try              self xrange index          except  IndexError  TypeError               raise error         return index      def   len   self           return len self xrange    Honestly  I think the whole issue is kind of silly and xrange should do all of this anyway

User · Answer

While xrange is faster than range in most circumstances  the difference in performance is pretty minimal  The little program below compares iterating over a range and an xrange   import timeit   Try various list sizes  for list len in  1  10  100  1000  10000  100000  1000000       Time doing a range and an xrange    rtime   timeit timeit  a 0  nfor n in range  d   a    n  list len  number 1000    xrtime   timeit timeit  a 0  nfor n in xrange  d   a    n  list len  number 1000      Print the result   print  Loop list of len  d  range   4f  xrange   4f   list len  rtime  xrtime    The results below shows that xrange is indeed faster  but not enough to sweat over   Loop list of len 1  range 0 0003  xrange 0 0003 Loop list of len 10  range 0 0013  xrange 0 0011 Loop list of len 100  range 0 0068  xrange 0 0034 Loop list of len 1000  range 0 0609  xrange 0 0438 Loop list of len 10000  range 0 5527  xrange 0 5266 Loop list of len 100000  range 10 1666  xrange 7 8481 Loop list of len 1000000  range 168 3425  xrange 155 8719   So by all means use xrange  but unless you re on a constrained hardware  don t worry too much about it

User · Answer

One other difference is that xrange   can t support numbers bigger than C ints  so if you want to have a range using python s built in large number support  you have to use range       Python 2 7 3  default  Jul 13 2012  22 29 01    GCC 4 7 1  on linux2 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  range 123456787676676767676676 123456787676676767676679   123456787676676767676676L  123456787676676767676677L  123456787676676767676678L   gt  gt  gt  xrange 123456787676676767676676 123456787676676767676679  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  OverflowError  Python int too large to convert to C long   Python 3 does not have this problem   Python 3 2 3  default  Jul 14 2012  01 01 48    GCC 4 7 1  on linux2 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  range 123456787676676767676676 123456787676676767676679  range 123456787676676767676676  123456787676676767676679

User · Answer

You should favour range   over xrange   only when you need an actual list  For instance  when you want to modify the list returned by range    or when you wish to slice it  For iteration or even just normal indexing  xrange   will work fine  and usually much more efficiently   There is a point where range   is a bit faster than xrange   for very small lists  but depending on your hardware and various other details  the break-even can be at a result of length 1 or 2  not something to worry about  Prefer xrange

User · Answer

A good example given in book  Practical Python By Magnus Lie Hetland    gt  gt  gt  zip range 5   xrange 100000000     0  0    1  1    2  2    3  3    4  4     I wouldn   t recommend using range instead of xrange in the preceding example   although only the first five numbers are needed  range calculates all the numbers  and that may take a lot of time  With xrange  this isn   t a problem because it calculates only those numbers needed   Yes I read  Brian s answer  In python 3  range   is a generator anyway and xrange   does not exist

User · Answer

range   returns a list  xrange   returns an xrange object   xrange   is a bit faster  and a bit more memory efficient  But the gain is not very large   The extra memory used by a list is of course not just wasted  lists have more functionality  slice  repeat  insert        Exact differences can be found in the documentation  There is no bonehard rule  use what is needed   Python 3 0 is still in development  but IIRC range   will very similar to xrange   of 2 X and list range    can be used to generate lists

User · Answer

range    range 1  10  returns a list from 1 to 10 numbers  amp  hold whole list in memory  xrange    Like range    but instead of returning a list  returns an object that generates the numbers in the range on demand  For looping  this is lightly faster than range   and more memory efficient  xrange   object like an iterator and generates the numbers on demand  Lazy Evaluation       In  1   range 1 10  Out 1    1  2  3  4  5  6  7  8  9   In  2   xrange 10  Out 2   xrange 10   In  3   print xrange   doc   Out 3   xrange  start   stop   step   - gt  xrange object   range   does the same thing as xrange   used to do in Python 3 and there is not term xrange   exist in Python 3  range   can actually be faster in some scenario if you iterating over the same sequence multiple times  xrange   has to reconstruct the integer object every time  but range   will have real integer objects

User · Answer

No  they both have their uses   Use xrange   when iterating  as it saves memory   Say   for x in xrange 1  one zillion     rather than   for x in range 1  one zillion     On the other hand  use range   if you actually want a list of numbers   multiples of seven   range 7 100 7  print  Multiples of seven  lt  100     multiples of seven

User · Answer

Go with range for these reasons   1  xrange will be going away in newer Python versions   This gives you easy future compatibility   2  range will take on the efficiencies associated with xrange

[python] Should you always favor xrange() over range()?

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