API changes for Python 3

Proposed Changes for Consideration

• methods that return sequences should produce iterators by default; there would be no special iter methods
  • should we remove the ..._iter methods completely, or just deprecate them?
• make Tree(s) synonymous with Tree(s,[]) and use Tree.parse(s) directly
  • this would simplify the code in tree.py a lot! I'm all for, --Peter Ljunglöf
  • but the name parse is unfortunate -- it reminds too much of all the NLTK parsers -- how about fromstring? (used in the libraries array, xml.etree, lxml, numpy, ...)
• The same argument could be made for nltk.align.Alignment, --Peter Ljunglöf
  • __new__ is used to be able to give a Giza string instead of a list of pairs.
  • Suggestion: add classmethod Alignment.fromGiza and let the constructor only allow a list of pairs.
• not use __new__ in Abstract Base Classes, for propagating the constructor to subclasses --Peter Ljunglöf
  • example: FeatStruct(x) returns a FeatDict or a FeatList, depending on x. This means that type(T(x)) != T for some classes T, which is unintuitive (and un-object-oriented).
  • Suggestion: the same as above -- use FeatStruct.parse(s) or something like that
  • This also holds for nltk.sourcedstring.SourcedString, I think
  • I'm not sure if it will work for nltk.util.AbstractLazySequence
• remove/deprecate nltk.misc.babelfish?
  • we should go ahead and remove it, yeah. https://github.com/nltk/nltk/issues/265 --Alex Rudnick
• perhaps this could be used to simplify sem/logic.py?
  • http://sigusr2.net/2008/Sep/30/python-type-constructors-like-ocaml.html
• ConditionalFreqDist.conditions() currently returns a sorted list, which is inefficient:
  • Suggestion: Just let it return .keys() without sorting.
• we may need to wrap word_tokenize() in sent_tokenize(), since some users (and the book?) apply word_tokenize to un-sentence-segmented text
• Tree should not be a subclass of list --Peter Ljunglöf
  • Almost all list-operations are anyway unsupported on trees. E.g., + or * are not supported, but += is.