NLTK getting dependencies from raw text - python-2.7

Question

I need get dependencies in sentences from raw text using NLTK.
As far as I understood, stanford parser allows us just to create tree, but how to get dependencies in sentences from this tree I didn't find out (maybe it's possible, maybe not)
So I've started using MaltParser. Here is a peace code I'm using:
import os
from nltk.parse.stanford import StanfordParser
from nltk.tokenize import sent_tokenize
from nltk.parse.dependencygraph import DependencyGraph
from nltk.parse.malt import MaltParser
os.environ['JAVAHOME'] = r"C:\Program Files (x86)\Java\jre1.8.0_45\bin\java.exe"
os.environ['MALT_PARSER'] = r"C:\maltparser-1.8.1"
maltParser = MaltParser(r"C:\maltparser-1.8.1\engmalt.poly-1.7.mco")
class Parser(object):
#staticmethod
def Parse (text):
rawSentences = sent_tokenize(text)
treeSentencesStanford = stanfordParser.raw_parse_sents(rawSentences)
a=maltParser.raw_parse(rawSentences[0])
but last line throws exception "'str' object has no attribute 'tag'"
changing the code above like this:
rawSentences = sent_tokenize(text)
treeSentencesStanford = stanfordParser.raw_parse_sents(rawSentences)
splitedSentences = []
for sentence in rawSentences:
splitedSentence = word_tokenize(sentence)
splitedSentences.append(splitedSentence)
a=maltParser.parse_sents(splitedSentences)
throws the same exception.
So, what I'm I doing wrong.
And in general: I'm I going in right way to get dependencies like this: http://www.nltk.org/images/depgraph0.png (but I need access these dependencies from code)
Traceback (most recent call last):
File "E:\Google drive\Python multi tries\Python multi tries\Parser.py", line 51, in <module>
Parser.Parse("Some random sentence. Hopefully it will be parsed.")
File "E:\Google drive\Python multi tries\Python multi tries\Parser.py", line 32, in Parse
a=maltParser.parse_sents(splitedSentences)
File "C:\Python27\lib\site-packages\nltk-3.0.1-py2.7.egg\nltk\parse\malt.py", line 113, in parse_sents
tagged_sentences = [self.tagger.tag(sentence) for sentence in sentences]
AttributeError: 'str' object has no attribute 'tag'

Answer 1

You are instantiating MaltParser with an unsuitable argument.
Running help(MaltParser) gives the following information:
Help on class MaltParser in module nltk.parse.malt:
class MaltParser(nltk.parse.api.ParserI)
| Method resolution order:
| MaltParser
| nltk.parse.api.ParserI
| __builtin__.object
|
| Methods defined here:
|
| __init__(self, tagger=None, mco=None, working_dir=None, additional_java_args=None)
| An interface for parsing with the Malt Parser.
|
| :param mco: The name of the pre-trained model. If provided, training
| will not be required, and MaltParser will use the model file in
| ${working_dir}/${mco}.mco.
| :type mco: str
...
So when you call maltParser = MaltParser(r"C:\maltparser-1.8.1\engmalt.poly-1.7.mco") then the keyword argument tagger is set to the path to the pretrained model.
Unfortunately this argument is not documented, but apparently it is a PoS tagger, as can be seen from inspecting the source.
(You don't have to specify a PoS tagger; there's a default RegEx-based tagger for English hard-coded in that class.)
So change your code to maltParser = MaltParser(mco=r"C:\maltparser-1.8.1\engmalt.poly-1.7.mco"), and you should be fine (at least until you find the next bug).
Your other questions: I think you're on the right track. If you're interested in dependencies, it's probably best to actually use dependency parsing, just as you are doing now. It is indeed possible to transform constituent parses into depencies (this has been proven), but it's probably more work.