Getting number of elements in an iterator in Python

Question

Is there an efficient way to know how many elements are in an iterator in Python  in general  without iterating through each and counting

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I like the cardinality package for this  it is very lightweight and tries to use the fastest possible implementation available depending on the iterable   Usage    gt  gt  gt  import cardinality  gt  gt  gt  cardinality count  1  2  3   3  gt  gt  gt  cardinality count i for i in range 500   500  gt  gt  gt  def gen            yield  hello          yield  world   gt  gt  gt  cardinality count gen    2   The actual count   implementation is as follows   def count iterable       if hasattr iterable     len              return len iterable       d   collections deque enumerate iterable  1   maxlen 1      return d 0  0  if d else 0

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def count iter iter       sum   0     for   in iter  sum    1     return sum

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No  any method will require you to resolve every result  You can do   iter length   len list iterable     but running that on an infinite iterator will of course never return  It also will consume the iterator and it will need to be reset if you want to use the contents   Telling us what real problem you re trying to solve might help us find you a better way to accomplish your actual goal   Edit  Using list   will read the whole iterable into memory at once  which may be undesirable  Another way is to do  sum 1 for   in iterable    as another person posted  That will avoid keeping it in memory

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It s common practice to put this type of information in the file header  and for pysam to give you access to this   I don t know the format  but have you checked the API   As others have said  you can t know the length from the iterator

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There are two ways to get the length of  something  on a computer   The first way is to store a count - this requires anything that touches the file data to modify it  or a class that only exposes interfaces -- but it boils down to the same thing    The other way is to iterate over it and count how big it is

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This code should work    gt  gt  gt  iter    i for i in range 50    gt  gt  gt  sum 1 for   in iter  50   Although it does iterate through each item and count them  it is the fastest way to do so   It also works for when the iterator has no item    gt  gt  gt  sum 1 for   in range 0   0   Of course  it runs forever for an infinite input  so remember that iterators can be infinite    gt  gt  gt  sum 1 for   in itertools count     nothing happens  forever    Also  be aware that the iterator will be exhausted by doing this  and further attempts to use it will see no elements  That s an unavoidable consequence of the Python iterator design  If you want to keep the elements  you ll have to store them in a list or something

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I thought it could be worthwhile to have a micro-benchmark comparing the run-times of the different approaches mentioned here  Disclaimer  I m using simple benchmark  a library written by me  for the benchmarks and also include iteration utilities count items  a function in a third-party-library written by me   To provide a more differentiated result I ve done two benchmarks  one only including the approaches that don t build an intermediate container just to throw it away and one including these  from simple benchmark import BenchmarkBuilder import more itertools as mi import iteration utilities as iu  b1   BenchmarkBuilder   b2   BenchmarkBuilder     b1 add function    b2 add function   def summation it       return sum 1 for   in it    b1 add function   def len list it       return len list it     b1 add function   def len listcomp it       return len    for   in it     b1 add function    b2 add function   def more itertools ilen it       return mi ilen it    b1 add function    b2 add function   def iteration utilities count items it       return iu count items it    b1 add arguments  length    b2 add arguments  length   def argument provider        for exp in range 2  18           size   2  exp         yield size   0  size  r1   b1 run   r2   b2 run    import matplotlib pyplot as plt  f   ax1  ax2    plt subplots 2  1  sharex True  figsize  15  18   r1 plot ax ax2  r2 plot ax ax1  plt savefig  result png    The results were   It uses log-log-axis so that all ranges  small values  large values  can be inspected  Since the plots are intended for qualitative comparison the actual values aren t too interesting  In general the y-axis  vertical  represents the time and the x-axis  horizontal  represents the number of elements in the input  quot iterable quot   Lower on the vertical axis means faster  The upper plot shows the approaches where no intermediate list was used  Which shows that the iteration utilities approach was fastest  followed by more itertools and the slowest was using sum 1 for   in iterator   The lower plot also included the approaches that used len   on an intermediate list  once with list and once with a list comprehension  The approach with len list  was fastest here  but the difference to the iteration utilities approach is almost negligible  The approach using the comprehension was significantly slower than using list directly  Summary Any approach mentioned here did show a dependency on the length of the input and iterated over ever element in the iterable  There is no way to get the length without the iteration  even if the iteration is hidden   If you don t want third-party extensions then using len list iterable   is definitely the fastest approach of the tested approaches  it however generates an intermediate list which could use significant more memory  If you don t mind additional packages then iteration utilities count items would be almost as fast as the len list       function but doesn t require additional memory  However it s important to note that the micro-benchmark used a list as input  The result of the benchmark could be different depending on the iterable you want to get the length of  I also tested with range and a simple genertor-expression and the trends were very similar  however I cannot exclude that the timing won t change depending on the type of input

User · Answer

Presumably  you want count the number of items without iterating through  so that the iterator is not exhausted  and you use it again later  This is possible with copy or deepcopy  import copy  def get iter len iterator       return sum 1 for   in copy copy iterator                                                     iterator   range 0  10  print get iter len iterator    if len tuple iterator    gt  1      print  Finding the length did not exhaust the iterator    else      print  oh no  it s all gone     The output is  Finding the length did not exhaust the iterator    Optionally  and unadvisedly   you can shadow the built-in len function as follows   import copy  def len obj     len len       try          if hasattr obj     len                  r   len obj          elif hasattr obj     next                  r   sum 1 for   in copy copy obj           else              r   len obj      finally          pass     return r

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One simple way is using set   built-in function  iter   zip  1 2 3    a   b   c    print len set iter     set iter      1   a     2   b     3   c    Out 45   3  or iter   range 1 10  print len set iter     set iter     1  2  3  4  5  6  7  8  9  Out 47   9

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An iterator is just an object which has a pointer to the next object to be read by some kind of buffer or stream  it s like a LinkedList where you don t know how many things you have until you iterate through them  Iterators are meant to be efficient because all they do is tell you what is next by references instead of using indexing  but as you saw you lose the ability to see how many entries are next

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So  for those who would like to know the summary of that discussion  The final top scores for counting a 50 million-lengthed generator expression using     len list gen     len    for   in gen     sum 1 for   in gen    ilen gen   from more itertool    reduce lambda c  i  c   1  gen  0      sorted by performance of execution  including memory consumption   will make you surprised        1  test list py 8  0 492 KiB  gen    i for i in data 1000   t0   monotonic    len list gen       list  sec   1 9684218849870376   2  test list compr py 8  0 867 KiB  gen    i for i in data 1000   t0   monotonic    len  i for i in gen       list compr  sec   2 5885991149989422   3  test sum py 8  0 859 KiB  gen    i for i in data 1000   t0   monotonic    sum 1 for i in gen   t1   monotonic       sum  sec   3 441088170016883   4  more itertools more py 413  1 266 KiB  d   deque enumerate iterable  1   maxlen 1   test ilen py 10  0 875 KiB gen    i for i in data 1000   t0   monotonic    ilen gen      ilen  sec   9 812256851990242   5  test reduce py 8  0 859 KiB  gen    i for i in data 1000   t0   monotonic    reduce lambda counter  i  counter   1  gen  0      reduce  sec   13 436614598002052       So  len list gen   is the most frequent and less memory consumable

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Regarding your original question  the answer is still that there is no way in general to know the length of an iterator in Python   Given that you question is motivated by an application of the pysam library  I can give a more specific answer  I m a contributer to PySAM and the definitive answer is that SAM BAM files do not provide an exact count of aligned reads   Nor is this information easily available from a BAM index file   The best one can do is to estimate the approximate number of alignments by using the location of the file pointer after reading a number of alignments and extrapolating based on the total size of the file   This is enough to implement a progress bar  but not a method of counting alignments in constant time

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A quick benchmark   import collections import itertools  def count iter items iterable       counter   itertools count       collections deque itertools izip iterable  counter   maxlen 0      return next counter   def count lencheck iterable       if hasattr iterable     len              return len iterable       d   collections deque enumerate iterable  1   maxlen 1      return d 0  0  if d else 0  def count sum iterable                  return sum 1 for   in iterable   iter   lambda y   x for x in xrange y     timeit count iter items iter 1000    timeit count lencheck iter 1000    timeit count sum iter 1000     The results   10000 loops  best of 3  37 2   s per loop 10000 loops  best of 3  47 6   s per loop 10000 loops  best of 3  61   s per loop   I e  the simple count iter items is the way to go   Adjusting this for python3   61 9   s    275 ns per loop  mean    std  dev  of 7 runs  10000 loops each  74 4   s    190 ns per loop  mean    std  dev  of 7 runs  10000 loops each  82 6   s    164 ns per loop  mean    std  dev  of 7 runs  10000 loops each

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This is against the very definition of an iterator  which is a pointer to an object  plus information about how to get to the next object   An iterator does not know how many more times it will be able to iterate until terminating   This could be infinite  so infinity might be your answer

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No  It s not possible   Example   import random  def gen n       for i in xrange n           if random randint 0  1     0              yield i  iterator   gen 10    Length of iterator is unknown until you iterate through it

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Although it s not possible in general to do what s been asked  it s still often useful to have a count of how many items were iterated over after having iterated over them  For that  you can use jaraco itertools Counter or similar  Here s an example using Python 3 and rwt to load the package     rwt -q jaraco itertools -- -q  gt  gt  gt  import jaraco itertools  gt  gt  gt  items   jaraco itertools Counter range 100    gt  gt  gt      list counted   gt  gt  gt  items count 100  gt  gt  gt  import random  gt  gt  gt  def gen n           for i in range n               if random randint 0  1     0                  yield i       gt  gt  gt  items   jaraco itertools Counter gen 100    gt  gt  gt      list counted   gt  gt  gt  items count 48

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You cannot  except the type of a particular iterator implements some specific methods that make it possible    Generally  you may count iterator items only by consuming the iterator  One of probably the most efficient ways   import itertools from collections import deque  def count iter items iterable               Consume an iterable not reading it into memory  return the number of items              counter   itertools count       deque itertools izip iterable  counter   maxlen 0      consume at C speed      return next counter     For Python 3 x replace itertools izip with zip

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Kinda  You could check the   length hint   method  but be warned that  at least up to Python 3 4  as gsnedders helpfully points out  it s a undocumented implementation detail  following message in thread   that could very well vanish or summon nasal demons instead   Otherwise  no  Iterators are just an object that only expose the next   method  You can call it as many times as required and they may or may not eventually raise StopIteration  Luckily  this behaviour is most of the time transparent to the coder

[python] Getting number of elements in an iterator in Python

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