Hasnext() for Python iterators?

Strategies for hasnext() in Python

Using sentinel values

When the iterable can include None, a unique sentinel object provides a safer way:

# "Nobody expects the Spanish Inquisition!" or this sentinel object
sentinel = object()  
iterator = iter([1, 2, None, 4])
while (current := next(iterator, sentinel)) is not sentinel:  
    print(current if current is not None else 'None value')


Crafting custom wrappers

Custom iterators can encapsulate this logic, providing a cleaner interface for the users:

class IteratorWithHasNext:
    def __init__(self, iterable):
        self._iter = iter(iterable)
        self._cache = next(self._iter, None)

    def __iter__(self):
        return self

    def __next__(self):
        if self._cache is None:
            raise StopIteration
        current = self._cache
        self._cache = next(self._iter, None)
        return current

    def hasnext(self):
        return self._cache is not None

my_iterator = IteratorWithHasNext([1, 2, 3])
while my_iterator.hasnext():
    print(next(my_iterator))


Leveraging Python's in-built features

For loops

In Python, for loops naturally handle the stopping condition of an iteration, efficiently managing the StopIteration:

for element in iter([1, 2, 3]):
    print(element)


Estimating remaining elements

The __length_hint__() function of an iterator can guess how many elements are left, offering a glimpse into the future:

it = iter([1, 2, 3])
while True:
    try:
        value = next(it)
        print(value)
    except StopIteration:
        break
    finally:
        print(f"Approximately {it.__length_hint__()} items remaining")


Detailed examination of Python iteration

Handling potential pitfalls

Python's robust iteration construct can handle various edge cases with grace. Be mindful of naturally occurring None or False elements that could tamper with simplistic end-of-iteration checks.

Mastering advanced iteration

Python's itertools offers potent tools that can supercharge your iteration workflow:

import itertools as it

data = [1, 2, 3, None, 4]
# chaining multiple iterators is as easy as supergluing sticks
for value in it.chain(data, [None], data):  
    print(value)


Creating custom iterators

With a custom iterator, functionality isn't limited to hasnext(). Look forward to lazy evaluation or even infinite sequences, all within Python's protocol!