The built-in next() function calls __next__() method of iterator object to retrieve next item in it. So the next question WOULD be – what is an iterator?

An object representing stream of data is an iterator. One element from the stream is retrieved at a time. It follows iterator protocol which requires it to support  __iter__() and __next__() methods. Python’s built-in method iter() implements __iter__() method and next() implements __next__() method. It receives an iterable and returns iterator object.

Python uses iterators  implicitly while working with collection data types such as list, tuple or string. That's why these data types are called iterables. We normally USE ‘for’ loop to iterate through an iterable as follows:

We can use iter() function to obtain iterator object underlying any iterable.

Iterator object has __next__() method. Every time it is called, it returns next element in iterator stream. When the stream gets exhausted, StopIteration error is raised.

Remember that next() function implements __next__() method. Hence next(it) is equivalent to it.__next__()

When we use a for/while loop with any iterable, it actually implements iterator protocol as follows:

A disk file, memory buffer or network stream also acts as an iterator object. Following example shows that each line from a file can be printed using next() function.