_sentinel=object()

# like the tp_iter slot on new-style classes
# http://hg.python.org/cpython/file/c6e529c1f87c/Objects/typeobject.c#l5907
def _tp_iter(object):
    typ = type(object)
    __iter__ = getattr(typ, '__iter__', _sentinel)
    if __iter__ is not _sentinel:
        return __iter__()
    __next__ = getattr(typ, '__next__', _sentinel)
    if __next__ is not _sentinel:
        return object
    __getitem__ = getattr(type, '__getitem__', _sentinel)
    if __getitem__ is _sentinel:
        raise TypeError("'%.200s' object is not iterable' % typ.__name__)
    return SequenceIterator(object)

# like the PyObject_GetIter function
# http://hg.python.org/cpython/file/c6e529c1f87c/Objects/abstract.c#l2644
def _iter(object):
    it = _tp_iter(object)
    if not isinstance(it, Iterable):
        raise TypeError("iter() returned non-iterator of type '%.100s'" % type(it).__name__)
    return it

# like the actual tier function
# http://hg.python.org/cpython/file/c6e529c1f87c/Python/bltinmodule.c#l1290
def iter(object, sentinel=_sentinel):
    if sentinel is _sentinel:
        return _iter(object)
    if not callable(object):
        raise TypeError("iter(v, w): v must be callable")
    return CallableIterator(object, sentinel)