hasNext in Python iterators?

Haven't Python iterators got a hasNext method?

Solution 1:

There's an alternative to the StopIteration by using next(iterator, default_value).

For exapmle:

>>> a = iter('hi')
>>> print next(a, None)
h
>>> print next(a, None)
i
>>> print next(a, None)
None

So you can detect for None or other pre-specified value for end of the iterator if you don't want the exception way.

Solution 2:

No, there is no such method. The end of iteration is indicated by an exception. See the documentation.

Solution 3:

If you really need a has-next functionality, it's easy to obtain it with a little wrapper class. For example:

class hn_wrapper(object):
  def __init__(self, it):
    self.it = iter(it)
    self._hasnext = None
  def __iter__(self): return self
  def next(self):
    if self._hasnext:
      result = self._thenext
    else:
      result = next(self.it)
    self._hasnext = None
    return result
  def hasnext(self):
    if self._hasnext is None:
      try: self._thenext = next(self.it)
      except StopIteration: self._hasnext = False
      else: self._hasnext = True
    return self._hasnext

now something like

x = hn_wrapper('ciao')
while x.hasnext(): print next(x)

emits

c
i
a
o

as required.

Note that the use of next(sel.it) as a built-in requires Python 2.6 or better; if you're using an older version of Python, use self.it.next() instead (and similarly for next(x) in the example usage). [[You might reasonably think this note is redundant, since Python 2.6 has been around for over a year now -- but more often than not when I use Python 2.6 features in a response, some commenter or other feels duty-bound to point out that they are 2.6 features, thus I'm trying to forestall such comments for once;-)]]

===

For Python3, you would make the following changes:

from collections.abc import Iterator  # since python 3.3 Iterator is here

class hn_wrapper(Iterator):  # need to subclass Iterator rather than object
  def __init__(self, it):
    self.it = iter(it)
    self._hasnext = None
    
  def __iter__(self): 
    return self
  
  def __next__(self):        # __next__ vs next in python 2
    if self._hasnext:
      result = self._thenext
    else:
      result = next(self.it)
    self._hasnext = None
    return result
  
  def hasnext(self):
    if self._hasnext is None:
      try: 
        self._thenext = next(self.it)
      except StopIteration: 
        self._hasnext = False
      else: self._hasnext = True
    return self._hasnext