Python Iterator vs Iterable

Summary: in this tutorial, you’ll learn about Python iterator and iterable and their differences.

Iterators

An iterator is an object that implements the iterator protocol. In other words, an iterator is an object that implements the following methods:

  • __iter__ returns the iterator object itself.
  • __next__ returns the next element.

Once you complete iterating a collection using an iterator, the iterator becomes exhausted.

It means that you cannot use the iterator object again.

Iterables

An iterable is an object that you can iterate over.

An object is iterable when it implements the __iter__ method. And its __iter__ method returns a new iterator.

Examining the built-in list and list iterator

In Python, a list is an ordered collection of items. It’s also an iterable because a list object has the __iter__ method that returns an iterator. For example:

numbers = [1, 2, 3] number_iterator = numbers.__iter__() print(type(number_iterator))
Code language: Python (python)

Output:

<class 'list_iterator'>
Code language: HTML, XML (xml)

In this example, the __iter__ method returns an iterator with the type list_iterator.

Because the list_iterator implements the __iter__ method, you can use the iter built-in function to get the iterator object:

numbers = [1, 2, 3] number_iterator = iter(numbers)
Code language: Python (python)

Since the list_iterator also implements the __next__ method, you can use the built-in function next to iterate over the list:

numbers = [1, 2, 3] number_iterator = iter(numbers) next(number_iterator) next(number_iterator) next(number_iterator)
Code language: Python (python)

If you call the next function once more, you’ll get a StopIteration exception.

next(number_iterator)

Error:

StopIteration
Code language: JavaScript (javascript)

This is because the list iterator has been exhausted. To iterate the list again, you need to create a new iterator.

This illustrates the separating the list from its iterator. The list is created once while the iterator is created every time you need to iterate over the list.

Python Iterator and Iterable

The following defines the Colors class:

class Colors: def __init__(self): self.rgb = ['red', 'green', 'blue'] self.__index = 0 def __iter__(self): return self def __next__(self): if self.__index &gt;= len(self.rgb): raise StopIteration # return the next color color = self.rgb[self.__index] self.__index += 1 return color
Code language: Python (python)

In this example, the Colors class plays two roles: iterable and iterator.

The Colors class is an iterator because it implements both __iter__ and __next__ method. The __iter__ method returns the object itself. And the __next__ method returns the next item from a list.

The Colors class is also an iterable because it implements the __iter__ method that returns an object itself, which is an iterator.

The following creates a new instance of the Colors class and iterates over its elements using a for loop:

colors = Colors() for color in colors: print(color)
Code language: Python (python)

Once you complete iterating, the colors object becomes useless. If you attempt to iterate it again, you’ll get a StopIteration exception:

next(colors)
Code language: Python (python)

Error:

StopIteration
Code language: JavaScript (javascript)

If you use the for loop, you’ll get nothing back. The iterator is empty:

for color in colors: print(color)
Code language: Python (python)

To iterate again, you need to create a new colors object with the rgb attribute. This is inefficient.

Separating an iterator from an iterable

Let’s separate the color iterator from its iterable like what Python does with the list iterator and list.

The following defines the Colors class:

class Colors: def __init__(self): self.rgb = ['red', 'green', 'blue'] def __len__(self): return len(self.rgb)
Code language: Python (python)

The following defines the ColorIterator class:

class ColorIterator: def __init__(self, colors): self.__colors = colors self.__index = 0 def __iter__(self): return self def __next__(self): if self.__index >= len(self.__colors): raise StopIteration # return the next color color = self.__colors.rgb[self.__index] self.__index += 1 return color
Code language: Python (python)

How it works.

  • The __init__ method accepts an iterable which is an instance of the Colors class.
  • The __iter__ method returns the iterator itself.
  • The __next__ method returns the next element from the Colors object.

The following shows how to use the ColorIterator to iterate over the Colors object:

colors = Colors() color_iterator = ColorIterator(colors) for color in color_iterator: print(color)
Code language: Python (python)

To iterate the Colors object again, you just need to create a new instance of the ColorIterator.

There’s one problem!

When you want to iterate the Colors object, you need to manually create a new ColorIterator object. And you also need to remember the iterator name ColorIterator.

It would be great if you can automate this. To do it, you can make the Colors class iterable by implementing the __iter__ method:

class Colors: def __init__(self): self.rgb = ['red', 'green', 'blue'] def __len__(self): return len(self.rgb) def __iter__(self): return ColorIterator(self)
Code language: Python (python)

The __iter__ method returns a new instance of the ColorIterator class.

Now, you can iterate the Colors object without explicitly creating the ColorIterator object:

colors = Colors() for color in colors: print(color)
Code language: Python (python)

Internally, the for loop calls the __iter__ method of the colors object to get the iterator and uses this iterator to iterate over the elements of the colors object.

The following places the ColorIterator class inside the Colors class to encapsulate them into a single class:

class Colors: def __init__(self): self.rgb = ['red', 'green', 'blue'] def __len__(self): return len(self.rgb) def __iter__(self): return self.ColorIterator(self) class ColorIterator: def __init__(self, colors): self.__colors = colors self.__index = 0 def __iter__(self): return self def __next__(self): if self.__index >= len(self.__colors): raise StopIteration # return the next color color = self.__colors.rgb[self.__index] self.__index += 1 return color
Code language: Python (python)

Summary

  • An iterable is an object that implements the __iter__ method which returns an iterator.
  • An iterator is an object that implements the __iter__ method which returns itself and the __next__ method which returns the next element.
  • Iterators are also iterables. However, they’re iterables that become exhausted while iterables will never exhausted.
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