Tell us about how iterators work in Python. How to implement your own iterator? What is the difference between iterators and iterable objects?

Answer.

In Python, iterators are objects that implement the iterator protocol: the methods __iter__() and __next__(). Iterators manage their internal state and return the next element of the sequence with each call to next() or raise a StopIteration exception when finished.

Iterable objects are those over which you can iterate (for example, lists, strings, dictionaries). All of them have a method __iter__(), which returns an iterator.

To create your own iterator, you need to create a class with methods __iter__() (returns self) and __next__() (implements the logic of getting the next element):

class Counter:
    def __init__(self, low, high):
        self.current = low
        self.high = high
    def __iter__(self):
        return self
    def __next__(self):
        if self.current > self.high:
            raise StopIteration
        else:
            self.current += 1
            return self.current - 1

for num in Counter(1, 3):
    print(num)
# 1 2 3

Trick question.

What is the difference between iterable objects and iterators? Can you iterate over an iterator more than once?

Answer: An iterable object implements only the method __iter__() that returns an iterator. An iterator is an object that has the method __next__(). An iterator is usually iterated only once: after reaching the end of the sequence, re-iteration is not possible without creating a new iterator.

Examples of real errors due to ignorance of the subtleties of the topic.

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Story

In a log analysis project, a developer wrote a function that accepted an iterator and tried to iterate over it twice (for log in logs:), expecting both iterations to yield the same results. The second iteration returned nothing because the iterator was already "exhausted".

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Story

In one module, a developer returned not a list from a function but a generator, allowing a single iteration. When trying to pass it to another function that expected an iterable object for multiple passes, an unexpected error occurred, resulting in an empty outcome.

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Story

In a Web API project, a developer tried to serialize an iterator directly to JSON. Iterators are not lists, and conversion is impossible without first converting them to a list (otherwise serialization will end in an exception).