{"id":1190,"date":"2020-11-17T07:27:23","date_gmt":"2020-11-17T07:27:23","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=1190"},"modified":"2025-03-27T15:01:21","modified_gmt":"2025-03-27T15:01:21","slug":"python-iter","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/advanced-python\/python-iter\/","title":{"rendered":"Python iter"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use the Python iter()<\/code> built-in function effectively.<\/p>\n\n\n\n

Introduction to the Python iter function #<\/a><\/h2>\n\n\n\n

The iter()<\/code> function returns an iterator of a given object:<\/p>\n\n\n

iter(object)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
When you call the iter()<\/code> function on an object, the function first looks for an __iter__()<\/code> method of that object.<\/p>\n\n\n\n
If the __iter__()<\/code> method exists, the iter()<\/code> function calls it to get an iterator. Otherwise, the iter()<\/code> function will look for a __getitem__()<\/code> method. <\/p>\n\n\n\n
If the __getitem__()<\/code> is available, the iter()<\/code> function creates an iterator object and returns that object. Otherwise, it raises a TypeError<\/code> exception.<\/p>\n\n\n\n
The following flowchart illustrates how the iter()<\/code> function works:<\/p>\n\n\n\n
\"How<\/figure>\n\n\n\n
Python iter() function examples #<\/a><\/h2>\n\n\n\n
The following example defines a simple Counter<\/code> class and uses the iter()<\/code> function to get an iterator of the counter object:<\/p>\n\n\n
class<\/span> Counter<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self)<\/span>:<\/span>\n        self.__current = 0<\/span>\n\n\ncounter = Counter()\niterator = iter(counter)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
It’ll raise a TypeError<\/code> because the counter<\/code> object is not an iterable:<\/p>\n\n\n
TypeError: 'Counter'<\/span> object is<\/span> not<\/span> iterable<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The following adds the __getitem__()<\/code> method to the Counter<\/code> class:<\/p>\n\n\n
class<\/span> Counter<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self)<\/span>:<\/span>\n        self.current = 0<\/span>\n\n    def<\/span> __getitem__<\/span>(self, index)<\/span>:<\/span>\n        if<\/span> isinstance(index, int):\n            self.current += 1<\/span>\n            return<\/span> self.current<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Because the Counter<\/code> implements the __getitem__()<\/code> method that returns an element based on an index, it’s a sequence.<\/p>\n\n\n\n
Now, you can use the iter()<\/code> function to get the iterator of the counter:<\/p>\n\n\n
counter = Counter()\n\niterator = iter(counter)\nprint(type(iterator))<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
<class<\/span> 'iterator<\/span>'><\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this case, Python creates an iterator object and returns it. Hence, you can use the iterator object to iterate the counter<\/code>:<\/p>\n\n\n
for<\/span> _ in<\/span> range(1<\/span>, 4<\/span>):\n    print(next(iterator))<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The following adds the CounterIterator<\/code> class to the Counter<\/code> class and implement the iterable protocol:<\/p>\n\n\n
class<\/span> Counter<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self)<\/span>:<\/span>\n        self.current = 0<\/span>\n\n    def<\/span> __getitem__<\/span>(self, index)<\/span>:<\/span>\n        if<\/span> isinstance(index, int):\n            self.current += 1<\/span>\n            return<\/span> self.current\n\n    def<\/span> __iter__<\/span>(self)<\/span>:<\/span>\n        return<\/span> self.CounterIterator(self)\n\n    class<\/span> CounterIterator<\/span>:<\/span>\n        def<\/span> __init__<\/span>(self, counter)<\/span>:<\/span>\n            self.__counter = counter\n\n        def<\/span> __iter__<\/span>(self)<\/span>:<\/span>\n            return<\/span> self\n\n        def<\/span> __next__<\/span>(self)<\/span>:<\/span>\n            self.__counter.current += 1<\/span>\n            return<\/span> self.__counter.current<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
The iter()<\/code> function requires an argument that can be an iterable<\/a> or a sequence<\/a>. In general, the object argument can be any object that supports either iteration or sequence protocol.<\/p>\n\n\n\n