How to implement iter self for a container object Python

Question

I have written a custom container object   According to this page  I need to implement this method on my object     iter   self    However  upon following up the link to Iterator Types in the Python reference manual  there are no examples given of how to implement your own   Can someone post a snippet  or link to a resource   that shows how to do this   The container I am writing  is a map  i e  stores values by unique keys   dicts can be iterated like this   for k  v in mydict items     In this case I need to be able to return two elements  a tuple   in the iterator   It is still not clear how to implement such an iterator  despite the several answers that have been kindly provided   Could someone please shed some more light on how to implement an iterator for a map-like container object   i e  a custom class that acts like a dict

User · Answer

In case you don t want to inherit from dict as others have suggested  here is direct answer to the question on how to implement   iter   for a crude example of a custom dict   class Attribute      def   init   self  key  value           self key   key         self value   value  class Node collections Mapping       def   init   self           self type               self attrs        List of Attributes      def   iter   self           for attr in self attrs              yield attr key   That uses a generator  which is well described here   Since we re inheriting from Mapping  you need to also implement   getitem   and   len         def   getitem   self  key           for attr in self attrs              if key    attr key                  return attr value         raise KeyError      def   len   self           return len self attrs

User · Answer

One option that might work for some cases is to make your custom class inherit from dict  This seems like a logical choice if it acts like a dict  maybe it should be a dict  This way  you get dict-like iteration for free   class MyDict dict       def   init   self  custom attribute           self bar   custom attribute  mydict   MyDict  Some name   mydict  a     1 mydict  b     2  print mydict bar for k  v in mydict items        print k     gt    v   Output   Some name a   gt  1 b   gt  2

User · Answer

Another option is to inherit from the appropriate abstract base class from the  collections module as documented here   In case the container is its own iterator  you can inherit from  collections Iterator  You only need to implement the next method then   An example is    gt  gt  gt  from collections import Iterator  gt  gt  gt  class MyContainer Iterator           def   init   self   data               self data   list data          def next self               if not self data                  raise StopIteration             return self data pop                               gt  gt  gt  c   MyContainer 1   two   3  4 0   gt  gt  gt  for i in c          print i               4 0 3 two 1   While you are looking at the collections module  consider inheriting from Sequence  Mapping or another abstract base class if that is more appropriate  Here is an example for a Sequence subclass    gt  gt  gt  from collections import Sequence  gt  gt  gt  class MyContainer Sequence           def   init   self   data               self data   list data          def   getitem   self  index               return self data index          def   len   self               return len self data                              gt  gt  gt  c   MyContainer 1   two   3  4 0   gt  gt  gt  for i in c          print i               1 two 3 4 0   NB  Thanks to Glenn Maynard for drawing my attention to the need to clarify the difference between iterators on the one hand and containers that are iterables rather than iterators on the other

User · Answer

To answer the question about mappings  your provided   iter   should iterate over the keys of the mapping  The following is a simple example that creates a mapping x - gt  x   x and works on Python3 extending the ABC mapping   import collections abc  class MyMap collections abc Mapping       def   init   self  n           self n   n      def   getitem   self  key     given a key  return it s value         if 0  lt   key  lt  self n              return key   key         else              raise KeyError  Invalid key        def   iter   self     iterate over all keys         for x in range self n               yield x      def   len   self           return self n  m   MyMap 5  for k  v in m items        print k   - gt    v    0 - gt  0   1 - gt  1   2 - gt  4   3 - gt  9   4 - gt  16

User · Answer

The  iterable interface  in python consists of two methods   next     and   iter      The   next   function is the most important  as it defines the iterator behavior - that is  the function determines what value should be returned next  The   iter     method is used to reset the starting point of the iteration  Often  you will find that    iter     can just return self when   init     is used to set the starting point   See the following code for defining a Class Reverse which implements the  iterable interface  and defines an iterator over any instance from any sequence class  The   next     method starts at the end of the sequence and returns values in reverse order of the sequence  Note that instances from a class implementing the  sequence interface  must define a   len     and a   getitem     method    class Reverse         Iterator for looping over a sequence backwards         def   init   self  seq           self data   seq         self index   len seq       def   iter   self           return self      def   next   self           if self index    0              raise StopIteration         self index   self index - 1         return self data self index    gt  gt  gt  rev   Reverse  spam    gt  gt  gt  next rev      note no need to call iter    m   gt  gt  gt  nums   Reverse range 1 10    gt  gt  gt  next nums  9

User · Answer

example for inhert from dict  modify its iter  for example  skip key 2 when in for loop    method 1 class Dict dict       def   iter   self           keys   self keys           for i in keys              if i    2                  continue             yield i    method 2 class Dict dict       def   iter   self           for i in super Dict  self    iter                  if i    2                  continue             yield i

User · Answer

If your object contains a set of data you want to bind your object s iter to  you can cheat and do this    gt  gt  gt  class foo      def   init   self   params              self data   params     def   iter   self           if hasattr self data 0      iter                  return self data 0    iter             return self data   iter      gt  gt  gt  d foo 6 7 3 8   ads   6   gt  gt  gt  for i in d      print i 6 7 3 8 ads 6

User · Answer

I normally would use a generator function  Each time you use a yield statement  it will add an item to the sequence   The following will create an iterator that yields five  and then every item in some list   def   iter   self      yield 5    yield from some list   Pre-3 3  yield from didn t exist  so you would have to do   def   iter   self      yield 5    for x in some list        yield x

User · Answer

usually   iter     just return self if you have already define the next   method  generator object    here is a Dummy example of a generator    class Test object        def   init   self  data          self data   data      def next self           if not self data             raise StopIteration         return self data pop        def   iter   self           return self   but   iter     can also be used like this      http   mail python org pipermail tutor 2006-January 044455 html

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