Python: Using iter() - The Iteration Protocol

Using iter() in Python - The Iteration Protocol in Python

Using iter() in Python - The Iteration Protocol in Python

Sequences and file handlers in Python have an __iter__() magic method. When the sequence is used is in conjunction with a for loop, the __iter__() method of the sequence calls the builtin iter() method and returns an iterator object. This is called the 'The Iterator Protocol'. The iterator object thus received, has a __next__() method, which gives the element in the sequence. When there is no element left and the __next__() method is called, the iterator object raises a StopIteration exception.

>>> numbersListIterator = iter([1, 2, 3])
>>> numbersListIterator
<listiterator object at 0x03040830>
>>> numbersListIterator.__next__()
1
>>> numbersListIterator.__next__()
2
>>> numbersListIterator.__next__()
3
>>> numbersListIterator.__next__()
Traceback (most recent call last):
    numbersListIterator.__next__()
StopIteration

Once you have an iterator object, you can iterate over the values in the following 3 ways:

1. Using the __next__() magic function of the iterator object, as done above. The __next__() is calling the builtin next() method and passing itself (i.e. numbersListIterator) to it.
2. Using the builtin next() function explicitly, such as next(numbersListIterator).
3. Using a for loop, such as for number in numbersListIterator: print(number).

See also:

• Python 101
• Object Oriented Python
• Design Patterns in Python
• 50+ Handy Standard Library Modules
• 60+ Handy Third Library Modules
• 50+ Tips & Tricks for Python Developers
• 50+ Know-How(s) Every Pythonista Must Know