hasNext in Python iterators

Question

Haven t Python iterators got a hasNext method

User · Answer

very interesting question  but this  quot hasnext quot  design had been put into leetcode  https   leetcode com problems iterator-for-combination  here is my implementation  class CombinationIterator   def   init   self  characters  str  combinationLength  int       from itertools import combinations     from collections import deque     self iter   combinations characters  combinationLength      self res   deque     def next self  - gt  str      if len self res     0          return    join next self iter       else          return    join self res pop      def hasNext self  - gt  bool      try          self res insert 0  next self iter           return True     except          return len self res   gt  0

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If you really need a has-next functionality  because you re just faithfully transcribing an algorithm from a reference implementation in Java  say  or because you re writing a prototype that will need to be easily transcribed to Java when it s finished   it s easy to obtain it with a little wrapper class   For example   class hn wrapper object     def   init   self  it       self it   iter it      self  hasnext   None   def   iter   self   return self   def next self       if self  hasnext        result   self  thenext     else        result   next self it      self  hasnext   None     return result   def hasnext self       if self  hasnext is None        try  self  thenext   next self it        except StopIteration  self  hasnext   False       else  self  hasnext   True     return self  hasnext   now something like  x   hn wrapper  ciao   while x hasnext    print next x    emits  c i a o   as required   Note that the use of next sel it  as a built-in requires Python 2 6 or better  if you re using an older version of Python  use self it next   instead  and similarly for next x  in the example usage     You might reasonably think this note is redundant  since Python 2 6 has been around for over a year now -- but more often than not when I use Python 2 6 features in a response  some commenter or other feels duty-bound to point out that they are 2 6 features  thus I m trying to forestall such comments for once -

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Try the   length hint     method from any iterator object   iter        length hint      gt  0

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hasNext somewhat translates to the StopIteration exception  e g     gt  gt  gt  it   iter  hello    gt  gt  gt  it next    h   gt  gt  gt  it next    e   gt  gt  gt  it next    l   gt  gt  gt  it next    l   gt  gt  gt  it next    o   gt  gt  gt  it next   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  StopIteration    StopIteration docs  http   docs python org library exceptions html exceptions StopIteration Some article about iterators and generator in python  http   www ibm com developerworks library l-pycon html

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There s an alternative to the StopIteration by using next iterator  default value    For exapmle    gt  gt  gt  a   iter  hi    gt  gt  gt  print next a  None  h  gt  gt  gt  print next a  None  i  gt  gt  gt  print next a  None  None   So you can detect for None or other pre-specified value for end of the iterator if you don t want the exception way

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You can tee the iterator using  itertools tee  and check for StopIteration on the teed iterator

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The way I solved my problem is to keep the count of the number of objects iterated over  so far  I wanted to iterate over a set using calls to an instance method  Since I knew the length of the set  and the number of items counted so far  I effectively had an hasNext method   A simple version of my code   class Iterator        s is a string  say     def   init   self  s           self s   set list s           self done   False         self iter   iter s          self charCount   0      def next self           if self done              return None         self char   next self iter          self charCount    1         self done    self charCount  lt  len self s           return self char      def hasMore self           return not self done   Of course  the example is a toy one  but you get the idea  This won t work in cases where there is no way to get the length of the iterable  like a generator etc

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The use case that lead me to search for this is the following  def setfrom self f          Set from iterable f        fi   iter f      for i in range self n           try              x   next fi          except StopIteration              fi   iter f              x   next fi          self a i    x    where hasnext   is available  one could do  def setfrom self f          Set from iterable f        fi   iter f      for i in range self n           if not hasnext fi               fi   iter f    restart         self a i    next fi    which to me is cleaner   Obviously you can work around issues by defining utility classes  but what then happens is you have a proliferation of twenty-odd different almost-equivalent workarounds each with their quirks  and if you wish to reuse code that uses different workarounds  you have to either have multiple near-equivalent in your single application  or go around picking through and rewriting code to use the same approach   The  do it once and do it well  maxim fails badly   Furthermore  the iterator itself needs to have an internal  hasnext  check to run to see if it needs to raise an exception   This internal check is then hidden so that it needs to be tested by trying to get an item  catching the exception and running the handler if thrown   This is unnecessary hiding IMO

User · Answer

Suggested way is StopIteration  Please see Fibonacci example from tutorialspoint    usr bin python3  import sys def fibonacci n    generator function    a  b  counter   0  1  0    while True        if  counter  gt  n             return       yield a       a  b   b  a   b       counter    1 f   fibonacci 5   f is iterator object  while True     try        print  next f   end         except StopIteration        sys exit

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No  The most similar concept is most likely a StopIteration exception

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In addition to all the mentions of StopIteration  the Python  for  loop simply does what you want    gt  gt  gt  it   iter  hello    gt  gt  gt  for i in it          print i     h e l l o

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No  there is no such method  The end of iteration is indicated by an exception  See the documentation

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I believe python just has next   and according to the doc  it throws an exception is there are no more elements   http   docs python org library stdtypes html iterator-types

[python] hasNext in Python iterators?

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