How to make a custom object iterable?

by

__iter__ is what gets called when you try to iterate over a class instance:

>>> class Foo(object):
...     def __iter__(self):
...         return (x for x in range(4))
...
>>> list(Foo())
[0, 1, 2, 3]

__next__ is what gets called on the object which is returned from __iter__ (on python2.x, it’s next, not __next__ — I generally alias them both so that the code will work with either…):

class Bar(object):
    def __init__(self):
        self.idx = 0
        self.data = range(4)
    def __iter__(self):
        return self
    def __next__(self):
        self.idx += 1
        try:
            return self.data[self.idx-1]
        except IndexError:
            self.idx = 0
            raise StopIteration  # Done iterating.
    next = __next__  # python2.x compatibility.

In the comments, it was asked how you would construct and object that could be iterated multiple times. In this case, I’d recommend taking the same approach that Python takes and split the iterator from the data container:

class BarIterator(object):
    def __init__(self, data_sequence):
        self.idx = 0
        self.data = data_sequence
    def __iter__(self):
        return self
    def __next__(self):
        self.idx += 1
        try:
            return self.data[self.idx-1]
        except IndexError:
            self.idx = 0
            raise StopIteration  # Done iterating.


class Bar(object):
    def __init__(self, data_sequence):
        self.data_sequence = data_sequence
    def __iter__(self):
        return BarIterator(self.data_sequence)

More Related Contents:

  1. Python – TypeError: ‘int’ object is not iterable
  2. Is there a way to ordinate numbers in a list?
  3. How to create the following lists with list comprehensions? [closed]
  4. Unable to use convert strings into integers for a for loop
  5. Printing the number from a list and its square on the same line in python3 [closed]
  6. Python3 – create global variable(list) from function
  7. how to unpack a tuple in this form (‘abc’, [20, 20])? [closed]
  8. What do ellipsis […] mean in a list?
  9. Access multiple elements of list knowing their index
  10. Are lists thread-safe?
  11. Count frequency of words in a list and sort by frequency
  12. Calling filter returns [duplicate]
  13. Python 3 turn range to a list
  14. Getting only element from a single-element list in Python?
  15. Unsupported operand type(s) for +: ‘int’ and ‘str’ [duplicate]
  16. Check if all elements of a list are of the same type
  17. One liner: creating a dictionary from list with indices as keys
  18. Python range() and zip() object type
  19. Best way to check if an item is present in a list of lists? [duplicate]
  20. How do I return dictionary keys as a list in Python?
  21. “TypeError: ‘type’ object is not subscriptable” in a function signature
  22. Changing one dict changes all dicts in a list of dicts [duplicate]
  23. Appending to list in Python dictionary [duplicate]
  24. Deleting consonants from a string in Python
  25. Python – How to convert only numbers in a mixed list into float?
  26. Nested lambda statements when sorting lists
  27. Removing an item from a list of lists based on each of the lists first element
  28. Size of a Python list in memory
  29. How to convert only numbers in a mixed list into float
  30. Star * operator on left vs right side of an assignment statement

Leave a Comment