has_next in Python iterators?

has_next in Python iterators?

Theres 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 dont want the exception way.

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

has_next in Python iterators?

If you really need a has-next functionality, its 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 youre 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 Im 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