How can I check if an object is an iterator in Python?

Question

I can check for a next() method, but is that enough? Is there an ideomatic way?

Answer 1

There is a better method than other answers have suggested.

In Python we have two kinds of things: Iterable and Iterator. An object is Iterable if it can give you Iterator. It does so when you use iter() on it. An object is Iterator if you can use next() to sequentially browse through its elements. For example, map() returns Iterator and list is Iterable.

Here are more details.

Below code illustrates how to check for these types:

from collections.abc import Iterable, Iterator

r = [1, 2, 3]
e = map(lambda x:x, r)

print(isinstance(r, Iterator)) # False, because can't apply next
print(isinstance(e, Iterator)) # True
print(isinstance(r, Iterable)) # True, because can apply iter()
print(isinstance(e, Iterable)) # True, note iter() returns self

Answer 2

To be an iterator an object must pass three tests:

• obj has an __iter__ method
• obj has a next method (or __next__ in Python 3)
• obj.__iter__() returns obj

So, a roll-your-own test would look like:

def is_iterator(obj):
    if (
            hasattr(obj, '__iter__') and
            hasattr(obj, 'next') and      # or __next__ in Python 3
            callable(obj.__iter__) and
            obj.__iter__() is obj
        ):
        return True
    else:
        return False

Answer 3

answer from python sourcecode doc comments:

{python install path}/Versions/3.5/lib/python3.5/types.py

# Iterators in Python aren't a matter of type but of protocol.  A large
# and changing number of builtin types implement *some* flavor of
# iterator.  Don't check the type!  Use hasattr to check for both
# "__iter__" and "__next__" attributes instead.

Answer 4

An object is iterable if it implements the iterator protocol.
You could check the presence of __iter__() method with:

hasattr(object,'__iter__')

in Python 2.x this approach misses str objects and other built-in sequence types like unicode, xrange, buffer. It works in Python 3.

Another way is to test it with iter method :

try:
   iter(object)
except TypeError:
   #not iterable

Answer 5

from collections.abc import Iterator

isinstance(object, Iterator)

Answer 6

As the question is about Iterator not Iterable and considering usage of iterator, a simplest and pythonic way of doing this

iterable = [1,2]
iterator = iter(iterable)


def isIterator(obj):
    try:
        next(obj, None)
        return True
    except TypeError:
        return False

>>> isIterator(iterable)
False
>>> isIterator(iterator)
True

Yes. Checking on next() should be enough