Build a basic Python iterator

Question

How would one create an iterative function  or iterator object  in python

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This question is about iterable objects  not about iterators  In Python  sequences are iterable too so one way to make an iterable class is to  make it behave like a sequence  i e  give it   getitem   and   len   methods  I have tested this on Python 2 and 3   class CustomRange       def   init   self  low  high           self low   low         self high   high      def   getitem   self  item           if item  gt   len self               raise IndexError  CustomRange index out of range           return self low   item      def   len   self           return self high - self low   cr   CustomRange 0  10  for i in cr      print i

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If you looking for something short and simple  maybe it will be enough for you   class A object       def   init   self  l           self data   l      def   iter   self           return iter self data    example of usage   In  3   a   A  2 3 4    In  4    i for i in a  Out 4    2  3  4

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Include the following code in your class code   def   iter   self           for x in self iterable              yield x  Make sure that you replace self iterablewith the iterable which you iterate through  Here s an example code class someClass      def   init   self list           self list   list     def   iter   self           for x in self list              yield x   var   someClass  1 2 3 4 5   for num in var       print num    Output 1 2 3 4 5  Note  Since strings are also iterable  they can also be used as an argument for the class foo   someClass  quot Python quot   for x in foo      print x   Output P y t h o n

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This is an iterable function without yield  It make use of the iter function and a closure which keeps it s state in a mutable  list  in the enclosing scope for python 2     def count low  high       counter    0      def tmp            val   low   counter 0          if val  lt  high              counter 0     1             return val         return None     return iter tmp  None    For Python 3  closure state is kept in an immutable in the enclosing scope and nonlocal is used in local scope to update the state variable     def count low  high       counter   0     def tmp            nonlocal counter         val   low   counter         if val  lt  high              counter    1             return val         return None     return iter tmp  None      Test   for i in count 1 10       print i  1 2 3 4 5 6 7 8 9

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Inspired by Matt Gregory s answer here is a bit more complicated iterator that will return a b     z aa ab     zz aaa aab     zzy zzz      class AlphaCounter      def   init   self  low  high           self current   low         self high   high      def   iter   self           return self      def   next   self     Python 3  def   next   self          alpha     abcdefghijklmnopqrstuvwxyz          n current   sum   alpha find self current x    26   len self current -x-1   for x in range len self current             n high   sum   alpha find self high x    26   len self high -x-1   for x in range len self high             if n current  gt  n high              raise StopIteration         else              increment   True             ret                  for x in self current   -1                   if  z     x                      if increment                          ret     a                      else                          ret     z                  else                      if increment                          ret    alpha alpha find x  1                          increment   False                     else                          ret    x             if increment                  ret     a              tmp   self current             self current   ret   -1              return tmp  for c in AlphaCounter  a    zzz        print c

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There are four ways to build an iterative function    create a generator  uses the yield keyword  use a generator expression  genexp  create an iterator  defines   iter   and   next    or next in Python 2 x   create a class that Python can iterate over on its own  defines   getitem      Examples     generator def uc gen text       for char in text upper            yield char    generator expression def uc genexp text       return  char for char in text upper       iterator protocol class uc iter        def   init   self  text           self text   text upper           self index   0     def   iter   self           return self     def   next   self           try              result   self text self index          except IndexError              raise StopIteration         self index    1         return result    getitem method class uc getitem        def   init   self  text           self text   text upper       def   getitem   self  index           return self text index    To see all four methods in action   for iterator in uc gen  uc genexp  uc iter  uc getitem      for ch in iterator  abcde            print ch  end          print     Which results in   A B C D E A B C D E A B C D E A B C D E   Note   The two generator types  uc gen and uc genexp  cannot be reversed    the plain iterator  uc iter  would need the   reversed   magic method  which  according to the docs  must return a new iterator  but returning self works  at least in CPython    and the getitem iteratable  uc getitem  must have the   len   magic method         for uc iter we add   reversed   and update   next       def   reversed   self           self index   -1         return self     def   next   self           try              result   self text self index          except IndexError              raise StopIteration         self index    -1 if self index  lt  0 else  1         return result        for uc getitem     def   len   self          return len self text      To answer Colonel Panic s secondary question about an infinite lazily evaluated iterator  here are those examples  using each of the four methods above     generator def even gen        result   0     while True          yield result         result    2     generator expression def even genexp        return  num for num in even gen       or even iter or even getitem                                           not much value under these circumstances    iterator protocol class even iter        def   init   self           self value   0     def   iter   self           return self     def   next   self           next value   self value         self value    2         return next value    getitem method class even getitem        def   getitem   self  index           return index   2  import random for iterator in even gen  even genexp  even iter  even getitem      limit   random randint 15  30      count   0     for even in iterator            print even          count    1         if count  gt   limit              break     print   Which results in  at least for my sample run    0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32     How to choose which one to use   This is mostly a matter of taste   The two methods I see most often are generators and the iterator protocol  as well as a hybrid    iter   returning a generator    Generator expressions are useful for replacing list comprehensions  they are lazy and so can save on resources    If one needs compatibility with earlier Python 2 x versions use   getitem

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All answers on this page are really great for a complex object  But for those containing builtin iterable types as attributes  like str  list  set or dict  or any implementation of collections Iterable  you can omit certain things in your class   class Test object       def   init   self  string           self string   string      def   iter   self             since your string is already iterable         return  ch for ch in self string            or simply         return self string   iter               also         return iter self string    It can  be used like   for x in Test  abcde        print x     prints   a   b   c   d   e

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I see some of you doing return self in   iter    I just wanted to note that   iter   itself can be a generator  thus removing the need for   next   and raising StopIteration exceptions   class range    def   init   self a b       self a   a     self b   b   def   iter   self       i   self a     while i  lt  self b        yield i       i  1   Of course here one might as well directly make a generator  but for more complex classes it can be useful

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Iterator objects in python conform to the iterator protocol  which basically means they provide two methods    iter      and    next          The   iter   returns the iterator object and is implicitly called at the start of loops  The   next     method returns the next value and is implicitly called at each loop increment   This method raises a StopIteration exception when there are no more value to return  which is implicitly captured by looping constructs to stop iterating    Here s a simple example of a counter   class Counter      def   init   self  low  high           self current   low - 1         self high   high      def   iter   self           return self      def   next   self     Python 2  def next self          self current    1         if self current  lt  self high              return self current         raise StopIteration   for c in Counter 3  9       print c    This will print   3 4 5 6 7 8   This is easier to write using a generator  as covered in a previous answer   def counter low  high       current   low     while current  lt  high          yield current         current    1  for c in counter 3  9       print c    The printed output will be the same   Under the hood  the generator object supports the iterator protocol and does something roughly similar to the class Counter   David Mertz s article  Iterators and Simple Generators  is a pretty good introduction

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First of all the itertools module is incredibly useful for all sorts of cases in which an iterator would be useful  but here is all you need to create an iterator in python      yield   Isn t that cool   Yield can be used to replace a normal return in a function   It returns the object just the same  but instead of destroying state and exiting  it saves state for when you want to execute the next iteration   Here is an example of it in action pulled directly from the itertools function list   def count n 0       while True          yield n         n    1   As stated in the functions description  it s the count   function from the itertools module       it produces an iterator that returns consecutive integers starting with n   Generator expressions are a whole other can of worms  awesome worms     They may be used in place of a List Comprehension to save memory  list comprehensions create a list in memory that is destroyed after use if not assigned to a variable  but generator expressions can create a Generator Object    which is a fancy way of saying Iterator   Here is an example of a generator expression definition   gen    n for n in xrange 0 11     This is very similar to our iterator definition above except the full range is predetermined to be between 0 and 10   I just found xrange    suprised I hadn t seen it before     and added it to the above example   xrange   is an iterable version of range   which has the advantage of not prebuilding the list   It would be  very useful if you had a giant corpus of data to iterate over and only had so much memory to do it in

[python] Build a basic Python iterator

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