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

In Python 2.6 or better, the designed-in idiom for such behavioral checks is a "membership check" with the abstract base class in the collections module of the standard library:

>>> import collections
>>> isinstance('ciao', collections.Iterable)
True
>>> isinstance(23, collections.Iterable)
False
>>> isinstance(xrange(23), collections.Iterable)
True

Indeed, this kind of checks is the prime design reason for the new abstract base classes (a second important one is to provide "mixin functionality" in some cases, which is why they're ABCs rather than just interfaces -- but that doesn't apply to collections.Iterable, it exists strictly to allow such checks with isinstance or issubclass). ABCs allow classes that don't actually inherit from them to be "registered" as subclasses anyway, so that such classes can be "subclasses" of the ABC for such checks; and, they can internally perform all needed checks for special methods (__iter__ in this case), so you don't have to.

If you're stuck with older releases of Python, "it's better to ask forgiveness than permission":

def isiterable(x):
  try: iter(x)
  except TypeError: return False
  else: return True

but that's not as fast and concise as the new approach.

Note that for this special case you'll often want to special-case strings (which are iterable but most application contexts want to treat as "scalars" anyway). Whatever approach you're using to check iterableness, if you need such special casing just prepend a check for isinstance(x, basestring) -- for example:

def reallyiterable(x):
  return not isinstance(x, basestring) and isinstance(x, collections.Iterable)

Edit: as pointed out in a comment, the question focuses on whether an object is an iter***ator*** rather than whether it's iter***able*** (all iterators are iterable, but not vice versa -- not all iterables are iterators). isinstance(x, collections.Iterator) is the perfectly analogous way to check for that condition specifically.

Answer 2

This example comes from the book Effective Python and is illustrated in this post.

An iterable produces an iterator. Any iterator is also an iterable, but produces itself as the iterator:

>>> list_iter = iter([])
>>> iter(list_iter) is list_iter
True