How do I tell whether a Python value is iterable or an iterator?

They describe related but different roles. An iterable is a value from which Python can get an iterator using iter(value). An iterator delivers values one at a time when you call next(); when it has no more values, it signals exhaustion with StopIteration. [S1]

A list makes the distinction clear: the list is iterable, and iter(the_list) produces an iterator. In ordinary code, a for loop handles the iteration process for you. These terms describe how a value can be used, not just what kind of data it contains.

Here’s a diagnostic example. Each successful next() call advances the iterator:

values = [1, 2, 3]
it = iter(values)

print(next(it))  # 1
print(next(it))  # 2
print(next(it))  # 3

try:
    print(next(it))
except StopIteration:
    print("iterator is exhausted")

The first three calls deliver the list’s values in order. Once they have all been delivered, the next call signals exhaustion with StopIteration. [S1] The try/except lets the example handle that signal and print a message. It also shows why it matters which object you use: calling next(it) consumes a value from the iterator.

First decide what you need to check. To see whether Python can obtain an iterator from a value, try iter(value) and handle TypeError if one is raised. This does not retrieve a value with next().

To check whether an iterator has a next value, you must try to retrieve one. If successful, that value has been consumed from the iterator. Keep it and process it as part of your program’s normal work if you still need it. If the iterator has no more values, it signals exhaustion with StopIteration. [S1] The key distinction is that obtaining an iterator and retrieving a value are different operations.

This pattern first tries to obtain an iterator, then—if successful—tries to retrieve one value:

value = [1, 2, 3]  # replace with your value

try:
    it = iter(value)
except TypeError:
    print("Could not get an iterator from this value")
else:
    print("Could get an iterator")
    try:
        first = next(it)
    except StopIteration:
        print("The iterator has no next value")
    else:
        print("Next value:", first)

For the list shown, the first value is 1. The first test asks whether Python can obtain an iterator; the second retrieves a value and advances that iterator. If you need to continue through the values, keep using it: an iterator advances forward and does not reset just because you call next() again. [S1]