What is the difference between range and xrange functions in Python 2 X

Question

Apparently xrange is faster but I have no idea why it s faster  and no proof besides the anecdotal so far that it is faster  or what besides that is different about  for i in range 0  20   for i in xrange 0  20

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range creates a list  so if you do range 1  10000000  it creates a list in memory with 10000000 elements    xrange is a generator  so it evaluates lazily    This brings you two advantages    You can iterate longer lists without getting a MemoryError  As it resolves each number lazily  if you stop iteration early  you won t waste time creating the whole list

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xrange returns an iterator and only keeps one number in memory at a time   range keeps the entire list of numbers in memory

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range  -range will populate everything at once which means every number of the range will occupy the memory   xrange  -xrange is something like generator  it will comes into picture when you want the range of numbers but you dont want them to be stored like when you want to use in for loop so memory efficient

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You will find the advantage of xrange over range in this simple example   import timeit  t1   timeit default timer   a   0 for i in xrange 1  100000000       pass t2   timeit default timer    print  time taken      t2-t1     4 49153590202 seconds  t1   timeit default timer   a   0 for i in range 1  100000000       pass t2   timeit default timer    print  time taken      t2-t1     7 04547905922 seconds   The above example doesn t reflect anything substantially better in case of xrange   Now look at the following case where range is really really slow  compared to xrange   import timeit  t1   timeit default timer   a   0 for i in xrange 1  100000000       if i    10000          break t2   timeit default timer    print  time taken      t2-t1     0 000764846801758 seconds  t1   timeit default timer   a   0 for i in range 1  100000000       if i    10000          break t2   timeit default timer     print  time taken      t2-t1     2 78506207466 seconds   With range  it already creates a list from 0 to 100000000 time consuming   but xrange is a generator and it only generates numbers based on the need  that is  if the iteration continues   In Python-3  the implementation of the range functionality is same as that of xrange in Python-2  while they have done away with xrange in Python-3  Happy Coding

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From the help docs   Python 2 7 12   gt  gt  gt  print range   doc   range stop  - gt  list of integers range start  stop   step   - gt  list of integers  Return a list containing an arithmetic progression of integers  range i  j  returns  i  i 1  i 2       j-1   start     defaults to 0  When step is given  it specifies the increment  or decrement   For example  range 4  returns  0  1  2  3    The end point is omitted  These are exactly the valid indices for a list of 4 elements    gt  gt  gt  print xrange   doc   xrange stop  - gt  xrange object xrange start  stop   step   - gt  xrange object  Like range    but instead of returning a list  returns an object that generates the numbers in the range on demand   For looping  this is  slightly faster than range   and more memory efficient    Python 3 5 2   gt  gt  gt  print range   doc    range stop  - gt  range object range start  stop   step   - gt  range object  Return an object that produces a sequence of integers from start  inclusive  to stop  exclusive  by step   range i  j  produces i  i 1  i 2       j-1  start defaults to 0  and stop is omitted   range 4  produces 0  1  2  3  These are exactly the valid indices for a list of 4 elements  When step is given  it specifies the increment  or decrement     gt  gt  gt  print xrange   doc    Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  NameError  name  xrange  is not defined   Difference is apparent  In Python 2 x  range returns a list  xrange returns an xrange object which is iterable   In Python 3 x  range becomes xrange of Python 2 x  and xrange is removed

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I am shocked nobody read doc      This function is very similar to range    but returns an xrange object instead of a list  This is an opaque sequence type which yields the same values as the corresponding list  without actually storing them all simultaneously  The advantage of xrange   over range   is minimal  since xrange   still has to create the values when asked for them  except when a very large range is used on a memory-starved machine or when all of the range   s elements are never used  such as when the loop is usually terminated with break

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range x y  returns a list of each number in between x and y if you use a for loop  then range is slower  In fact  range has a bigger Index range  range x y  will print out a list of all the numbers in between x and y  xrange x y  returns xrange x y  but if you used a for loop  then xrange is faster  xrange has a smaller Index range  xrange will not only print out xrange x y  but it will still keep all the numbers that are in it    In  range 1 10   Out   1  2  3  4  5  6  7  8  9   In  xrange 1 10   Out  xrange 1 10    If you use a for loop  then it would work   In  for i in range 1 10           print i  Out  1       2       3       4       5       6       7       8       9  In  for i in xrange 1 10            print i  Out  1       2       3       4       5       6       7       8       9   There isn t much difference when using loops  though there is a difference when just printing it

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I am shocked nobody read doc      This function is very similar to range    but returns an xrange object instead of a list  This is an opaque sequence type which yields the same values as the corresponding list  without actually storing them all simultaneously  The advantage of xrange   over range   is minimal  since xrange   still has to create the values when asked for them  except when a very large range is used on a memory-starved machine or when all of the range   s elements are never used  such as when the loop is usually terminated with break

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The difference decreases for smaller arguments to range       xrange         python -m timeit  for i in xrange 10111      for k in range 100       pass  10 loops  best of 3  59 4 msec per loop    python -m timeit  for i in xrange 10111      for k in xrange 100       pass  10 loops  best of 3  46 9 msec per loop   In this case xrange 100  is only about 20  more efficient

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You will find the advantage of xrange over range in this simple example   import timeit  t1   timeit default timer   a   0 for i in xrange 1  100000000       pass t2   timeit default timer    print  time taken      t2-t1     4 49153590202 seconds  t1   timeit default timer   a   0 for i in range 1  100000000       pass t2   timeit default timer    print  time taken      t2-t1     7 04547905922 seconds   The above example doesn t reflect anything substantially better in case of xrange   Now look at the following case where range is really really slow  compared to xrange   import timeit  t1   timeit default timer   a   0 for i in xrange 1  100000000       if i    10000          break t2   timeit default timer    print  time taken      t2-t1     0 000764846801758 seconds  t1   timeit default timer   a   0 for i in range 1  100000000       if i    10000          break t2   timeit default timer     print  time taken      t2-t1     2 78506207466 seconds   With range  it already creates a list from 0 to 100000000 time consuming   but xrange is a generator and it only generates numbers based on the need  that is  if the iteration continues   In Python-3  the implementation of the range functionality is same as that of xrange in Python-2  while they have done away with xrange in Python-3  Happy Coding

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range creates a list  so if you do range 1  10000000  it creates a list in memory with 9999999 elements  xrange is a generator  so it is a sequence object is a that  evaluates lazily   This is true  but in Python 3  range   will be implemented by the Python 2 xrange    If you need to actually generate the list  you will need to do  list range 1 100

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range x y  returns a list of each number in between x and y if you use a for loop  then range is slower  In fact  range has a bigger Index range  range x y  will print out a list of all the numbers in between x and y  xrange x y  returns xrange x y  but if you used a for loop  then xrange is faster  xrange has a smaller Index range  xrange will not only print out xrange x y  but it will still keep all the numbers that are in it    In  range 1 10   Out   1  2  3  4  5  6  7  8  9   In  xrange 1 10   Out  xrange 1 10    If you use a for loop  then it would work   In  for i in range 1 10           print i  Out  1       2       3       4       5       6       7       8       9  In  for i in xrange 1 10            print i  Out  1       2       3       4       5       6       7       8       9   There isn t much difference when using loops  though there is a difference when just printing it

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Do spend some time with the Library Reference  The more familiar you are with it  the faster you can find answers to questions like this  Especially important are the first few chapters about builtin objects and types      The advantage of the xrange type is that an xrange object will always    take the same amount of memory  no matter the size of the range it represents     There are no consistent performance advantages    Another way to find quick information about a Python construct is the docstring and the help-function   print xrange   doc     def doc x   print x   doc   is super useful help xrange

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What  range returns a static list at runtime  xrange returns an object  which acts like a generator  although it s certainly not one  from which values are generated as and when required   When to use which      Use xrange if you want to generate a list for a gigantic range  say 1 billion  especially when you have a  memory sensitive system  like a cell phone  Use range if you want to iterate over the list several times    PS  Python 3 x s range function    Python 2 x s xrange function

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See this post to find difference between range and xrange   To quote      range returns exactly what you think  a list of consecutive   integers  of a defined length beginning with 0  xrange  however    returns an  xrange object   which acts a great deal like an iterator

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The difference decreases for smaller arguments to range       xrange         python -m timeit  for i in xrange 10111      for k in range 100       pass  10 loops  best of 3  59 4 msec per loop    python -m timeit  for i in xrange 10111      for k in xrange 100       pass  10 loops  best of 3  46 9 msec per loop   In this case xrange 100  is only about 20  more efficient

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It is for optimization reasons   range   will create a list of values from start to end  0    20 in your example   This will become an expensive operation on very large ranges   xrange   on the other hand is much more optimised  it will only compute the next value when needed  via an xrange sequence object  and does not create a list of all values like range   does

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Some of the other answers mention that Python 3 eliminated 2 x s range and renamed 2 x s xrange to range  However  unless you re using 3 0 or 3 1  which nobody should be   it s actually a somewhat different type   As the 3 1 docs say      Range objects have very little behavior  they only support indexing  iteration  and the len function    However  in 3 2   range is a full sequence   it supports extended slices  and all of the methods of collections abc Sequence with the same semantics as a list    And  at least in CPython and PyPy  the only two 3 2  implementations that currently exist   it also has constant-time implementations of the index and count methods and the in operator  as long as you only pass it integers   This means writing 123456 in r is reasonable in 3 2   while in 2 7 or 3 1 it would be a horrible idea       The fact that issubclass xrange  collections Sequence  returns True in 2 6-2 7 and 3 0-3 1 is a bug that was fixed in 3 2 and not backported

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range creates a list  so if you do range 1  10000000  it creates a list in memory with 9999999 elements  xrange is a generator  so it is a sequence object is a that  evaluates lazily   This is true  but in Python 3  range   will be implemented by the Python 2 xrange    If you need to actually generate the list  you will need to do  list range 1 100

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Everyone has explained it greatly  But I wanted it to see it for myself  I use python3  So  I opened the resource monitor  in Windows    and first  executed the following command first   a 0 for i in range 1 100000       a a i   and then checked the change in  In Use  memory  It was insignificant  Then  I ran the following code   for i in list range 1 100000        a a i   And it took a big chunk of the memory for use  instantly  And  I was convinced  You can try it for yourself   If you are using Python 2X  then replace  range    with  xrange    in the first code and  list range     with  range

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xrange   and range   in python works similarly as for the user   but the difference comes when we are talking about how the memory is allocated in using both the function   When we are using range   we allocate memory for all the variables it is generating  so it is not recommended to use with larger no  of variables to be generated   xrange   on the other hand generate only a particular value at a time and can only be used with the for loop to print all the values required

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Range returns a list while xrange returns an xrange object which takes the same memory irrespective of the range size as in this case only one element is generated and available per iteration whereas in case of using range  all the elements are generated at once and are available in the memory

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xrange only stores the range params and generates the numbers on demand  However the C implementation of Python currently restricts its args to C longs   xrange 2  32-1  2  32 1     When long is 32 bits  OverflowError  Python int too large to convert to C long range 2  32-1  2  32 1      OK -- gt   4294967295L  4294967296L    Note that in Python 3 0 there is only range and it behaves like the 2 x xrange but without the limitations on minimum and maximum end points

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Do spend some time with the Library Reference  The more familiar you are with it  the faster you can find answers to questions like this  Especially important are the first few chapters about builtin objects and types      The advantage of the xrange type is that an xrange object will always    take the same amount of memory  no matter the size of the range it represents     There are no consistent performance advantages    Another way to find quick information about a Python construct is the docstring and the help-function   print xrange   doc     def doc x   print x   doc   is super useful help xrange

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On a requirement for scanning printing of 0-N items   range and xrange works as follows   range   - creates a new list in the memory and takes the whole 0 to N items totally N 1  and prints them  xrange   - creates a iterator instance that scans through the items and keeps only the current encountered item into the memory   hence utilising same amount of memory all the time   In case the required element is somewhat at the beginning of the list only then it saves a good amount of time and memory

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See this post to find difference between range and xrange   To quote      range returns exactly what you think  a list of consecutive   integers  of a defined length beginning with 0  xrange  however    returns an  xrange object   which acts a great deal like an iterator

User · Answer

Do spend some time with the Library Reference  The more familiar you are with it  the faster you can find answers to questions like this  Especially important are the first few chapters about builtin objects and types      The advantage of the xrange type is that an xrange object will always    take the same amount of memory  no matter the size of the range it represents     There are no consistent performance advantages    Another way to find quick information about a Python construct is the docstring and the help-function   print xrange   doc     def doc x   print x   doc   is super useful help xrange

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Remember  use the timeit module to test which of small snippets of code is faster    python -m timeit  for i in range 1000000      pass  10 loops  best of 3  90 5 msec per loop   python -m timeit  for i in xrange 1000000      pass  10 loops  best of 3  51 1 msec per loop  Personally  I always use range    unless I were dealing with really huge lists -- as you can see  time-wise  for a list of a million entries  the extra overhead is only 0 04 seconds   And as Corey points out  in Python 3 0 xrange   will go away and range   will give you nice iterator behavior anyway

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xrange returns an iterator and only keeps one number in memory at a time   range keeps the entire list of numbers in memory

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When testing range against xrange in a loop  I know I should use timeit  but this was swiftly hacked up from memory using a simple list comprehension example  I found the following   import time  for x in range 1  10        t   time time        v 10 for v in range 1  10000       print  range     4f      time time  -t  100       t   time time        v 10 for v in xrange 1  10000       print  xrange    4f      time time  -t  100    which gives    python range tests py range   0 4273 xrange  0 3733 range   0 3881 xrange  0 3507 range   0 3712 xrange  0 3565 range   0 4031 xrange  0 3558 range   0 3714 xrange  0 3520 range   0 3834 xrange  0 3546 range   0 3717 xrange  0 3511 range   0 3745 xrange  0 3523 range   0 3858 xrange  0 3997  lt - garbage collection    Or  using xrange in the for loop   range   0 4172 xrange  0 3701 range   0 3840 xrange  0 3547 range   0 3830 xrange  0 3862  lt - garbage collection  range   0 4019 xrange  0 3532 range   0 3738 xrange  0 3726 range   0 3762 xrange  0 3533 range   0 3710 xrange  0 3509 range   0 3738 xrange  0 3512 range   0 3703 xrange  0 3509   Is my snippet testing properly  Any comments on the slower instance of xrange  Or a better example  -

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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    xrange  start   stop   step   - gt  xrange object

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xrange uses an iterator  generates values on the fly   range returns a list

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In Python 2 x   range creates a list  so if you do range 1  10000000  it creates a list in memory with 9999999 elements   xrange is a sequence object that  evaluates lazily    In Python 3   range does the equivalent of Python 2 s xrange  To get the list  you have to explicitly use list range        xrange no longer exists

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range creates a list  so if you do range 1  10000000  it creates a list in memory with 9999999 elements  xrange is a generator  so it is a sequence object is a that  evaluates lazily   This is true  but in Python 3  range   will be implemented by the Python 2 xrange    If you need to actually generate the list  you will need to do  list range 1 100

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It is for optimization reasons   range   will create a list of values from start to end  0    20 in your example   This will become an expensive operation on very large ranges   xrange   on the other hand is much more optimised  it will only compute the next value when needed  via an xrange sequence object  and does not create a list of all values like range   does

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xrange only stores the range params and generates the numbers on demand  However the C implementation of Python currently restricts its args to C longs   xrange 2  32-1  2  32 1     When long is 32 bits  OverflowError  Python int too large to convert to C long range 2  32-1  2  32 1      OK -- gt   4294967295L  4294967296L    Note that in Python 3 0 there is only range and it behaves like the 2 x xrange but without the limitations on minimum and maximum end points

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Additionally  if do list xrange       will be equivalent to range        So list is slow   Also xrange really doesn t fully finish the sequence  So that s why its not a list  it s a xrange object

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range creates a list  so if you do range 1  10000000  it creates a list in memory with 9999999 elements  xrange is a generator  so it is a sequence object is a that  evaluates lazily   This is true  but in Python 3  range   will be implemented by the Python 2 xrange    If you need to actually generate the list  you will need to do  list range 1 100

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range creates a list  so if you do range 1  10000000  it creates a list in memory with 10000000 elements    xrange is a generator  so it evaluates lazily    This brings you two advantages    You can iterate longer lists without getting a MemoryError  As it resolves each number lazily  if you stop iteration early  you won t waste time creating the whole list

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range   in Python 2 x  This function is essentially the old range   function that was available in Python 2 x and returns an instance of a list object that contains the elements in the specified range     However  this implementation is too inefficient when it comes to initialise a list with a range of numbers  For example  for i in range 1000000  would be a very expensive command to execute  both in terms of memory and time usage as it requires the storage of this list into the memory      range   in Python 3 x and xrange   in Python 2 x  Python 3 x introduced a newer implementation of range    while the newer implementation was already available in Python 2 x through the xrange   function     The range   exploits a strategy known as lazy evaluation  Instead of creating a huge list of elements in range  the newer implementation introduces the class range  a lightweight object that represents the required elements in the given range  without storing them explicitly in memory  this might sound like generators but the concept of lazy evaluation is different       As an example  consider the following      Python 2 x  gt  gt  gt  a   range 10   gt  gt  gt  type a   lt type  list  gt   gt  gt  gt  b   xrange 10   gt  gt  gt  type b   lt type  xrange  gt    and     Python 3 x  gt  gt  gt  a   range 10   gt  gt  gt  type a   lt class  range  gt

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In python 2 x  range x  returns a list  that is created in memory with x elements    gt  gt  gt  a   range 5   gt  gt  gt  a  0  1  2  3  4    xrange x  returns an xrange object which is a generator obj which generates the numbers on demand  they are computed during for-loop Lazy Evaluation    For looping  this is slightly faster than range   and more memory efficient    gt  gt  gt  b   xrange 5   gt  gt  gt  b xrange 5

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xrange   and range   in python works similarly as for the user   but the difference comes when we are talking about how the memory is allocated in using both the function   When we are using range   we allocate memory for all the variables it is generating  so it is not recommended to use with larger no  of variables to be generated   xrange   on the other hand generate only a particular value at a time and can only be used with the for loop to print all the values required

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range   in Python 2 x  This function is essentially the old range   function that was available in Python 2 x and returns an instance of a list object that contains the elements in the specified range     However  this implementation is too inefficient when it comes to initialise a list with a range of numbers  For example  for i in range 1000000  would be a very expensive command to execute  both in terms of memory and time usage as it requires the storage of this list into the memory      range   in Python 3 x and xrange   in Python 2 x  Python 3 x introduced a newer implementation of range    while the newer implementation was already available in Python 2 x through the xrange   function     The range   exploits a strategy known as lazy evaluation  Instead of creating a huge list of elements in range  the newer implementation introduces the class range  a lightweight object that represents the required elements in the given range  without storing them explicitly in memory  this might sound like generators but the concept of lazy evaluation is different       As an example  consider the following      Python 2 x  gt  gt  gt  a   range 10   gt  gt  gt  type a   lt type  list  gt   gt  gt  gt  b   xrange 10   gt  gt  gt  type b   lt type  xrange  gt    and     Python 3 x  gt  gt  gt  a   range 10   gt  gt  gt  type a   lt class  range  gt

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range generates the entire list and returns it   xrange does not -- it generates the numbers in the list on demand

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From the help docs   Python 2 7 12   gt  gt  gt  print range   doc   range stop  - gt  list of integers range start  stop   step   - gt  list of integers  Return a list containing an arithmetic progression of integers  range i  j  returns  i  i 1  i 2       j-1   start     defaults to 0  When step is given  it specifies the increment  or decrement   For example  range 4  returns  0  1  2  3    The end point is omitted  These are exactly the valid indices for a list of 4 elements    gt  gt  gt  print xrange   doc   xrange stop  - gt  xrange object xrange start  stop   step   - gt  xrange object  Like range    but instead of returning a list  returns an object that generates the numbers in the range on demand   For looping  this is  slightly faster than range   and more memory efficient    Python 3 5 2   gt  gt  gt  print range   doc    range stop  - gt  range object range start  stop   step   - gt  range object  Return an object that produces a sequence of integers from start  inclusive  to stop  exclusive  by step   range i  j  produces i  i 1  i 2       j-1  start defaults to 0  and stop is omitted   range 4  produces 0  1  2  3  These are exactly the valid indices for a list of 4 elements  When step is given  it specifies the increment  or decrement     gt  gt  gt  print xrange   doc    Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  NameError  name  xrange  is not defined   Difference is apparent  In Python 2 x  range returns a list  xrange returns an xrange object which is iterable   In Python 3 x  range becomes xrange of Python 2 x  and xrange is removed

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xrange only stores the range params and generates the numbers on demand  However the C implementation of Python currently restricts its args to C longs   xrange 2  32-1  2  32 1     When long is 32 bits  OverflowError  Python int too large to convert to C long range 2  32-1  2  32 1      OK -- gt   4294967295L  4294967296L    Note that in Python 3 0 there is only range and it behaves like the 2 x xrange but without the limitations on minimum and maximum end points

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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    xrange  start   stop   step   - gt  xrange object

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What  range returns a static list at runtime  xrange returns an object  which acts like a generator  although it s certainly not one  from which values are generated as and when required   When to use which      Use xrange if you want to generate a list for a gigantic range  say 1 billion  especially when you have a  memory sensitive system  like a cell phone  Use range if you want to iterate over the list several times    PS  Python 3 x s range function    Python 2 x s xrange function

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range  -range will populate everything at once which means every number of the range will occupy the memory   xrange  -xrange is something like generator  it will comes into picture when you want the range of numbers but you dont want them to be stored like when you want to use in for loop so memory efficient

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range creates a list  so if you do range 1  10000000  it creates a list in memory with 10000000 elements    xrange is a generator  so it evaluates lazily    This brings you two advantages    You can iterate longer lists without getting a MemoryError  As it resolves each number lazily  if you stop iteration early  you won t waste time creating the whole list

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range generates the entire list and returns it   xrange does not -- it generates the numbers in the list on demand

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In python 2 x  range x  returns a list  that is created in memory with x elements    gt  gt  gt  a   range 5   gt  gt  gt  a  0  1  2  3  4    xrange x  returns an xrange object which is a generator obj which generates the numbers on demand  they are computed during for-loop Lazy Evaluation    For looping  this is slightly faster than range   and more memory efficient    gt  gt  gt  b   xrange 5   gt  gt  gt  b xrange 5

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Range returns a list while xrange returns an xrange object which takes the same memory irrespective of the range size as in this case only one element is generated and available per iteration whereas in case of using range  all the elements are generated at once and are available in the memory

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See this post to find difference between range and xrange   To quote      range returns exactly what you think  a list of consecutive   integers  of a defined length beginning with 0  xrange  however    returns an  xrange object   which acts a great deal like an iterator

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On a requirement for scanning printing of 0-N items   range and xrange works as follows   range   - creates a new list in the memory and takes the whole 0 to N items totally N 1  and prints them  xrange   - creates a iterator instance that scans through the items and keeps only the current encountered item into the memory   hence utilising same amount of memory all the time   In case the required element is somewhat at the beginning of the list only then it saves a good amount of time and memory

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In Python 2 x   range creates a list  so if you do range 1  10000000  it creates a list in memory with 9999999 elements   xrange is a sequence object that  evaluates lazily    In Python 3   range does the equivalent of Python 2 s xrange  To get the list  you have to explicitly use list range        xrange no longer exists

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xrange only stores the range params and generates the numbers on demand  However the C implementation of Python currently restricts its args to C longs   xrange 2  32-1  2  32 1     When long is 32 bits  OverflowError  Python int too large to convert to C long range 2  32-1  2  32 1      OK -- gt   4294967295L  4294967296L    Note that in Python 3 0 there is only range and it behaves like the 2 x xrange but without the limitations on minimum and maximum end points

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xrange uses an iterator  generates values on the fly   range returns a list

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Everyone has explained it greatly  But I wanted it to see it for myself  I use python3  So  I opened the resource monitor  in Windows    and first  executed the following command first   a 0 for i in range 1 100000       a a i   and then checked the change in  In Use  memory  It was insignificant  Then  I ran the following code   for i in list range 1 100000        a a i   And it took a big chunk of the memory for use  instantly  And  I was convinced  You can try it for yourself   If you are using Python 2X  then replace  range    with  xrange    in the first code and  list range     with  range

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xrange returns an iterator and only keeps one number in memory at a time   range keeps the entire list of numbers in memory

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Remember  use the timeit module to test which of small snippets of code is faster    python -m timeit  for i in range 1000000      pass  10 loops  best of 3  90 5 msec per loop   python -m timeit  for i in xrange 1000000      pass  10 loops  best of 3  51 1 msec per loop  Personally  I always use range    unless I were dealing with really huge lists -- as you can see  time-wise  for a list of a million entries  the extra overhead is only 0 04 seconds   And as Corey points out  in Python 3 0 xrange   will go away and range   will give you nice iterator behavior anyway

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range creates a list  so if you do range 1  10000000  it creates a list in memory with 10000000 elements    xrange is a generator  so it evaluates lazily    This brings you two advantages    You can iterate longer lists without getting a MemoryError  As it resolves each number lazily  if you stop iteration early  you won t waste time creating the whole list

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range generates the entire list and returns it   xrange does not -- it generates the numbers in the list on demand

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xrange returns an iterator and only keeps one number in memory at a time   range keeps the entire list of numbers in memory

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See this post to find difference between range and xrange   To quote      range returns exactly what you think  a list of consecutive   integers  of a defined length beginning with 0  xrange  however    returns an  xrange object   which acts a great deal like an iterator

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In Python 2 x   range creates a list  so if you do range 1  10000000  it creates a list in memory with 9999999 elements   xrange is a sequence object that  evaluates lazily    In Python 3   range does the equivalent of Python 2 s xrange  To get the list  you have to explicitly use list range        xrange no longer exists

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Do spend some time with the Library Reference  The more familiar you are with it  the faster you can find answers to questions like this  Especially important are the first few chapters about builtin objects and types      The advantage of the xrange type is that an xrange object will always    take the same amount of memory  no matter the size of the range it represents     There are no consistent performance advantages    Another way to find quick information about a Python construct is the docstring and the help-function   print xrange   doc     def doc x   print x   doc   is super useful help xrange

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It is for optimization reasons   range   will create a list of values from start to end  0    20 in your example   This will become an expensive operation on very large ranges   xrange   on the other hand is much more optimised  it will only compute the next value when needed  via an xrange sequence object  and does not create a list of all values like range   does

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xrange uses an iterator  generates values on the fly   range returns a list

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In Python 2 x   range creates a list  so if you do range 1  10000000  it creates a list in memory with 9999999 elements   xrange is a sequence object that  evaluates lazily    In Python 3   range does the equivalent of Python 2 s xrange  To get the list  you have to explicitly use list range        xrange no longer exists

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It is for optimization reasons   range   will create a list of values from start to end  0    20 in your example   This will become an expensive operation on very large ranges   xrange   on the other hand is much more optimised  it will only compute the next value when needed  via an xrange sequence object  and does not create a list of all values like range   does

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Remember  use the timeit module to test which of small snippets of code is faster    python -m timeit  for i in range 1000000      pass  10 loops  best of 3  90 5 msec per loop   python -m timeit  for i in xrange 1000000      pass  10 loops  best of 3  51 1 msec per loop  Personally  I always use range    unless I were dealing with really huge lists -- as you can see  time-wise  for a list of a million entries  the extra overhead is only 0 04 seconds   And as Corey points out  in Python 3 0 xrange   will go away and range   will give you nice iterator behavior anyway

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Some of the other answers mention that Python 3 eliminated 2 x s range and renamed 2 x s xrange to range  However  unless you re using 3 0 or 3 1  which nobody should be   it s actually a somewhat different type   As the 3 1 docs say      Range objects have very little behavior  they only support indexing  iteration  and the len function    However  in 3 2   range is a full sequence   it supports extended slices  and all of the methods of collections abc Sequence with the same semantics as a list    And  at least in CPython and PyPy  the only two 3 2  implementations that currently exist   it also has constant-time implementations of the index and count methods and the in operator  as long as you only pass it integers   This means writing 123456 in r is reasonable in 3 2   while in 2 7 or 3 1 it would be a horrible idea       The fact that issubclass xrange  collections Sequence  returns True in 2 6-2 7 and 3 0-3 1 is a bug that was fixed in 3 2 and not backported

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When testing range against xrange in a loop  I know I should use timeit  but this was swiftly hacked up from memory using a simple list comprehension example  I found the following   import time  for x in range 1  10        t   time time        v 10 for v in range 1  10000       print  range     4f      time time  -t  100       t   time time        v 10 for v in xrange 1  10000       print  xrange    4f      time time  -t  100    which gives    python range tests py range   0 4273 xrange  0 3733 range   0 3881 xrange  0 3507 range   0 3712 xrange  0 3565 range   0 4031 xrange  0 3558 range   0 3714 xrange  0 3520 range   0 3834 xrange  0 3546 range   0 3717 xrange  0 3511 range   0 3745 xrange  0 3523 range   0 3858 xrange  0 3997  lt - garbage collection    Or  using xrange in the for loop   range   0 4172 xrange  0 3701 range   0 3840 xrange  0 3547 range   0 3830 xrange  0 3862  lt - garbage collection  range   0 4019 xrange  0 3532 range   0 3738 xrange  0 3726 range   0 3762 xrange  0 3533 range   0 3710 xrange  0 3509 range   0 3738 xrange  0 3512 range   0 3703 xrange  0 3509   Is my snippet testing properly  Any comments on the slower instance of xrange  Or a better example  -

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Remember  use the timeit module to test which of small snippets of code is faster    python -m timeit  for i in range 1000000      pass  10 loops  best of 3  90 5 msec per loop   python -m timeit  for i in xrange 1000000      pass  10 loops  best of 3  51 1 msec per loop  Personally  I always use range    unless I were dealing with really huge lists -- as you can see  time-wise  for a list of a million entries  the extra overhead is only 0 04 seconds   And as Corey points out  in Python 3 0 xrange   will go away and range   will give you nice iterator behavior anyway

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range generates the entire list and returns it   xrange does not -- it generates the numbers in the list on demand

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xrange uses an iterator  generates values on the fly   range returns a list

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Additionally  if do list xrange       will be equivalent to range        So list is slow   Also xrange really doesn t fully finish the sequence  So that s why its not a list  it s a xrange object

[python] What is the difference between range and xrange functions in Python 2.X?

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