Can't iterate over a list class in Python

Question

I'm trying to write a simple GUI front end for Plurk using pyplurk.

I have successfully got it to create the API connection, log in, and retrieve and display a list of friends. Now I'm trying to retrieve and display a list of Plurks.

pyplurk provides a GetNewPlurks function as follows:

  def GetNewPlurks(self, since):
    '''Get new plurks since the specified time.
    Args:
      since: [datetime.datetime] the timestamp criterion.
    Returns:
      A PlurkPostList object or None.
    '''
    offset = jsonizer.conv_datetime(since)
    status_code, result = self._CallAPI('/Polling/getPlurks', offset=offset)
    return None if status_code != 200 else \
           PlurkPostList(result['plurks'], result['plurk_users'].values())

As you can see this returns a PlurkPostList, which in turn is defined as follows:

class PlurkPostList:
  '''A list of plurks and the set of users that posted them.'''
  def __init__(self, plurk_json_list, user_json_list=[]):
    self._plurks = [PlurkPost(p) for p in plurk_json_list]
    self._users = [PlurkUser(u) for u in user_json_list]
  def __iter__(self):
    return self._plurks
  def GetUsers(self):
    return self._users
  def __eq__(self, other):
    if other.__class__ != PlurkPostList: return False
    if self._plurks != other._plurks: return False
    if self._users != other._users: return False
    return True

Now I expected to be able to do something like this:

api = plurk_api_urllib2.PlurkAPI(open('api.key').read().strip(), debug_level=1)
plurkproxy = PlurkProxy(api, json.loads)
user = plurkproxy.Login('my_user', 'my_pass')
ps = plurkproxy.GetNewPlurks(datetime.datetime(2009, 12, 12, 0, 0, 0))
print ps
for p in ps:
  print str(p)

When I run this, what I actually get is:

<plurk.PlurkPostList instance at 0x01E8D738>

from the "print ps", then:

    for p in ps:
TypeError: __iter__ returned non-iterator of type 'list'

I don't understand - surely a list is iterable? Where am I going wrong - how do I access the Plurks in the PlurkPostList?

Answer 1

When you define your own __iter__ method, you should realize that that __iter__ method should return an iterator, not an iterable. You are returning a list, not an iterator to a list, so it fails. You can fix it by doing return iter(self._plurks), for example.

If you wanted to do something a little more complex, like process each item in self._plurks as it's being iterated over, the usual trick is to make your __iter__ method be a generator. That way, the returnvalue of the call to __iter__ is the generator, which is an iterator:

def __iter__(self):
    for item in self._plurks:
        yield process(item)

Answer 2

The __iter__ method should return an object which implements the next() method.

A list does not have a next() method, but it has an __iter__ method, which returns a listiterator object. The listiterator object has a next() method.

You should write:

def __iter__(self):
    return iter(self._plurks)

Answer 3

As an alternative, you can also define the next() function and have __iter__() return self. See http://stackoverflow.com/questions/19151/build-a-basic-python-iterator for a nice example.