{"id":3231,"date":"2020-02-09T18:14:50","date_gmt":"2020-02-09T18:14:50","guid":{"rendered":"https:\/\/www.askpython.com\/?p=3231"},"modified":"2022-08-06T13:10:54","modified_gmt":"2022-08-06T13:10:54","slug":"python-iterator","status":"publish","type":"post","link":"https:\/\/www.askpython.com\/python\/built-in-methods\/python-iterator","title":{"rendered":"Python Iterator"},"content":{"rendered":"\n

A Python Iterator is an object that can be iterated upon. Any kind of object that supports this kind of iteration is called as an iterator<\/strong>.<\/p>\n\n\n\n

Now, you may be confused. There exists another object called iterable<\/strong>. What is that? Let’s take a look.<\/p>\n\n\n\n

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Iterators and Iterables<\/h2>\n\n\n\n

Any iterable is an object that can be iterated upon. That is, we can use iterators to move through this object.<\/p>\n\n\n\n

Examples of Iterable objects are tuples, lists, strings, arrays, etc.<\/p>\n\n\n\n

To construct an iterator<\/strong> from an iterable<\/em>, we can use the iter()<\/code> method.<\/p>\n\n\n

\niterable_list = [1, 2, 3]\niterable_string = "Hello"\n\niterator_1 = iter(iterable_list)\niterator_2 = iter(iterable_string)\n\nprint(iterator_1, type(iterator_1))\nprint(iterator_2, type(iterator_2))\n<\/pre><\/div>\n\n\n
Output<\/p>\n\n\n
\n<list_iterator object at 0x7f887b01aed0> <class 'list_iterator'>\n<str_iterator object at 0x7f887b01af50> <class 'str_iterator'>\n<\/pre><\/div>\n\n\n
The output shows that we have created two iterators; one for the list and one for the string.<\/p>\n\n\n\n
Now let’s look at the methods which the iterator object supports.<\/p>\n\n\n\n
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Python Iterator Methods<\/h2>\n\n\n\n
An iterator object has two special methods that can be used with it, called iter()<\/strong> and next()<\/strong>.<\/p>\n\n\n\n
The iter()<\/code> method was used earlier, to get a Python iterator<\/strong> object from an iterable. <\/p>\n\n\n\n
Now, to traverse through the iterator, we can use the next()<\/code> method to get the next element in the iterable.<\/p>\n\n\n\n
Format:<\/p>\n\n\n
\nnext_iterable_object = next(iterator)\n<\/pre><\/div>\n\n\n
When there are no more elements left to go to, this will terminate and raise a StopIteration<\/code> Exception<\/a>.<\/p>\n\n\n\n
To illustrate all this, let’s print all the elements of our list iterator.<\/p>\n\n\n
\n>>> iterable_list = [1, 2, 3]\n>>> iterator_1 = iter(iterable_list)\n>>> next(iterator_1)\n1\n>>> next(iterator_1)\n2\n>>> next(iterator_1)\n3\n>>> next(iterator_1)\nTraceback (most recent call last):\n  File "<stdin>", line 1, in <module>\nStopIteration\n<\/pre><\/div>\n\n\n
As you can see, when you go beyond the length of the iterable, this indeed raises the StopIteration<\/code> Exception.<\/p>\n\n\n\n
Now, let’s go to the next step: Making our own Iterator!<\/p>\n\n\n\n
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Constructing our own Iterator in Python<\/h2>\n\n\n\n
Any iterator object has a countable number of elements, that can be traversed. But how can we make our own iterator? We need to create our own Class.<\/p>\n\n\n\n
In Python, constructing any iterator involves a protocol called the Iterator Protocol<\/strong>.<\/p>\n\n\n\n
This protocol contains two specific methods, called __iter__()<\/code> and __next__()<\/code>, similar to the general iterator methods, but since they are inside a class, it is prefixed and suffixed with this symbol, to show the distinction.<\/p>\n\n\n\n
The iter()<\/code> and next()<\/code> methods internally call these methods, and hence, to make the iterator, we need to define our own __iter__()<\/code> and __next__()<\/code> methods inside our class.<\/p>\n\n\n\n
Let’s create a simple iterator that traverses across a list<\/strong> only and raises a StopIteration<\/strong> Exception if the count of elements is greater than 5.<\/p>\n\n\n\n
We will also print the number of elements iterated so far, in our next()<\/code> method.<\/p>\n\n\n
\nclass MyClass():\n    def __init__(self):\n        self.counter = 0\n        # Set the limit\n        self.limit = 5\n\n    def __iter__(self):\n        # The iter() method internally calls this\n        print('Call to __iter__()')\n        return self\n\n    def __next__(self):\n        print('Call to __next__()')\n        if self.counter > self.limit:\n            raise StopIteration\n        else:\n            # Increment counter and return it\n            self.counter += 1\n            return self.counter\n\n\n# Create the iterable\nmy_obj = MyClass()\n\n# Create the iterator object from the iterable\nmy_it = iter(my_obj)\n\nfor i in range(7):\n    print(next(my_it))\n<\/pre><\/div>\n\n\n
Output<\/strong><\/p>\n\n\n
\nCall to __iter__()\nCall to __next__()\n1\nCall to __next__()\n2\nCall to __next__()\n3\nCall to __next__()\n4\nCall to __next__()\n5\nCall to __next__()\n6\nCall to __next__()\nTraceback (most recent call last):\n  File "iter.py", line 29, in <module>\n    print(next(my_it))\n  File "iter.py", line 15, in __next__\n    raise StopIteration\nStopIteration\n<\/pre><\/div>\n\n\n
Here, this prints numbers from 1 to 6, and the next call will trigger the StopIteration<\/code> exception, since we’ve crossed the limit.<\/p>\n\n\n\n
We’ve made our own iterator!<\/p>\n\n\n\n
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Conclusion<\/h2>\n\n\n\n
I hope you now have a good understanding of iterators and any doubts regarding this concept are cleared after reading this article. If not, do ask them in the comment section below!<\/p>\n\n\n\n
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References<\/h2>\n\n\n\n
  • JournalDev article on Iterators<\/li><\/ul>\n\n\n\n
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