How to retrieve an element from a set without removing it

Question

Suppose the following    gt  gt  gt  s   set  1  2  3     How do I get a value  any value  out of s without doing s pop    I want to leave the item in the set until I am sure I can remove it - something I can only be sure of after an asynchronous call to another host   Quick and dirty    gt  gt  gt  elem   s pop    gt  gt  gt  s add elem    But do you know of a better way  Ideally in constant time

User · Accepted Answer

Two options that don t require copying the whole set   for e in s      break   e is now an element from s   Or     e   next iter s     But in general  sets don t support indexing or slicing

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Least code would be    gt  gt  gt  s   set  1  2  3    gt  gt  gt  list s  0  1   Obviously this would create a new list which contains each member of the set  so not great if your set is very large

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Since you want a random element  this will also work    gt  gt  gt  import random  gt  gt  gt  s   set  1 2 3    gt  gt  gt  random sample s  1   2    The documentation doesn t seem to mention performance of random sample  From a really quick empirical test with a huge list and a huge set  it seems to be constant time for a list but not for the set  Also  iteration over a set isn t random  the order is undefined but predictable    gt  gt  gt  list set range 10       range 10  True    If randomness is important and you need a bunch of elements in constant time  large sets   I d use random sample and convert to a list first    gt  gt  gt  lst   list s    once  O len s         gt  gt  gt  e   random sample lst  1  0    constant time

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How about s copy   pop    I haven t timed it  but it should work and it s simple  It works best for small sets however  as it copies the whole set

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I wondered how the functions will perform for different sets  so I did a benchmark   from random import sample  def ForLoop s       for e in s          break     return e  def IterNext s       return next iter s    def ListIndex s       return list s  0   def PopAdd s       e   s pop       s add e      return e  def RandomSample s       return sample s  1   def SetUnpacking s       e       s     return e  from simple benchmark import benchmark  b   benchmark  ForLoop  IterNext  ListIndex  PopAdd  RandomSample  SetUnpacking                  2  i  set range 2  i   for i in range 1  20                  argument name  set size                 function aliases  first   First     b plot       This plot clearly shows that some approaches  RandomSample  SetUnpacking and ListIndex  depend on the size of the set and should be avoided in the general case  at least if performance might be important   As already shown by the other answers the fastest way is ForLoop   However as long as one of the constant time approaches is used the performance difference will be negligible     iteration utilities  Disclaimer  I m the author  contains a convenience function for this use-case  first    gt  gt  gt  from iteration utilities import first  gt  gt  gt  first  1 2 3 4   1   I also included it in the benchmark above  It can compete with the other two  fast  solutions but the difference isn t much either way

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Following  wr  post  I get similar results  for Python3 5   from timeit import    stats     for i in range 1000   next iter s               for i in range 1000    n tfor x in s   n t tbreak             for i in range 1000   s add s pop       for stat in stats      t   Timer stat  setup  s set range 100000         try          print  Time for  s  t  f   stat  t timeit number 1000        except          t print exc     Output   Time for for i in range 1000   next iter s       0 205888 Time for for i in range 1000        for x in s           break                                    0 083397 Time for for i in range 1000   s add s pop       0 226570   However  when changing the underlying set  e g  call to remove    things go badly for the iterable examples  for  iter    from timeit import    stats     while s  n ta   next iter s   n ts remove a              while s  n tfor x in s  break n ts remove x              while s  n tx s pop   n ts add x  n ts remove x     for stat in stats      t   Timer stat  setup  s set range 100000         try          print  Time for  s  t  f   stat  t timeit number 1000        except          t print exc     Results in   Time for while s      a   next iter s       s remove a               2 938494 Time for while s      for x in s  break     s remove x               2 728367 Time for while s      x s pop       s add x      s remove x               0 030272

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To provide some timing figures behind the different approaches  consider the following code  The get   is my custom addition to Python s setobject c  being just a pop   without removing the element   from timeit import    stats     for i in xrange 1000   iter s  next                  for i in xrange 1000    n tfor x in s   n t tbreak             for i in xrange 1000   s add s pop                   for i in xrange 1000   s get                for stat in stats      t   Timer stat  setup  s set range 100         try          print  Time for  s  t  f   stat  t timeit number 1000       except          t print exc     The output is       test get py Time for for i in xrange 1000   iter s  next             0 433080 Time for for i in xrange 1000           for x in s                  break    0 148695 Time for for i in xrange 1000   s add s pop              0 317418 Time for for i in xrange 1000   s get                    0 146673   This means that the for break solution is the fastest  sometimes faster than the custom get   solution

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Another option is to use a dictionary with values you don t care about   E g     poor man set      poor man set 1    None poor man set 2    None poor man set 3    None       You can treat the keys as a set except that they re just an array    keys   poor man set keys   print  Some key    s    keys 0    A side effect of this choice is that your code will be backwards compatible with older  pre-set versions of Python   It s maybe not the best answer but it s another option   Edit  You can even do something like this to hide the fact that you used a dict instead of an array or set    poor man set      poor man set 1    None poor man set 2    None poor man set 3    None poor man set   poor man set keys

User · Answer

I f you want just the first element try this  b    a-set    pop

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Yet another way in Python 3  s   iter       next

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tl dr  for first item in muh set  break remains the optimal approach in Python 3 x  Curse you  Guido   y u do this  Welcome to yet another set of Python 3 x timings  extrapolated from wr  s excellent Python 2 x-specific response  Unlike AChampion s equally helpful Python 3 x-specific response  the timings below also time outlier solutions suggested above     including    list s  0   John s novel sequence-based solution  random sample s  1   dF  s eclectic RNG-based solution    Code Snippets for Great Joy  Turn on  tune in  time it   from timeit import Timer  stats          for i in range 1000    n tfor x in s   n t tbreak        for i in range 1000   next iter s          for i in range 1000   s add s pop           for i in range 1000   list s  0         for i in range 1000   random sample s  1       for stat in stats      t   Timer stat  setup  import random ns set range 100         try          print  Time for  s  t  f   stat  t timeit number 1000        except          t print exc     Quickly Obsoleted Timeless Timings  Behold  Ordered by fastest to slowest snippets       test get py Time for for i in range 1000        for x in s           break    0 249871 Time for for i in range 1000   next iter s       0 526266 Time for for i in range 1000   s add s pop       0 658832 Time for for i in range 1000   list s  0     4 117106 Time for for i in range 1000   random sample s  1    21 851104   Faceplants for the Whole Family  Unsurprisingly  manual iteration remains at least twice as fast as the next fastest solution  Although the gap has decreased from the Bad Old Python 2 x days  in which manual iteration was at least four times as fast   it disappoints the PEP 20 zealot in me that the most verbose solution is the best  At least converting a set into a list just to extract the first element of the set is as horrible as expected  Thank Guido  may his light continue to guide us   Surprisingly  the RNG-based solution is absolutely horrible  List conversion is bad  but random really takes the awful-sauce cake  So much for the Random Number God   I just wish the amorphous They would PEP up a set get first   method for us already  If you re reading this  They   Please  Do something

User · Answer

Seemingly the most compact  6 symbols  though very slow way to get a set element  made possible by PEP 3132    e    s   With Python 3 5  you can also use this 7-symbol expression  thanks to PEP 448      s  0    Both options are roughly 1000 times slower on my machine than the for-loop method

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You can unpack the values to access the elements  s   set  1  2  3    v1  v2  v3   s  print v1 v2 v3   1 2 3

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What I usually do for small collections is to create kind of parser converter method like this  def convertSetToList setName   return list setName    Then I can use the new list and access by index number  userFields   convertSetToList user  name   request json userFields 0     As a list you will have all the other methods that you may need to work with

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I use a utility function I wrote   Its name is somewhat misleading because it kind of implies it might be a random item or something like that   def anyitem iterable       try          return iter iterable  next       except StopIteration          return None

[python] How to retrieve an element from a set without removing it?

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