Python and NLTK: How to analyze sentence grammar?

Question

I have this code which should show the syntactic structure of the sentence according to defined grammar. However it is returning an empty []. What am I missing or doing wron

Answer 1

You don't have a Det defined in your grammar, but each NP (and consequently S) has to have one by grammar definition.

Compare with

>>> grammar = nltk.parse_cfg("""
... S -> NP VP
... NP -> Det N | Det N PP
... VP -> V NP | VP PP
... Det -> 'a' | 'the'
... N -> 'Kim' | 'Dana' | 'everyone'
... V -> 'arrived' | 'left' |'cheered'
... """)
>>>
>>> parser = nltk.ChartParser(grammar)
>>> parser.nbest_parse('the Kim left a Dana'.split())
[Tree('S', [Tree('NP', [Tree('Det', ['the']), Tree('N', ['Kim'])]), Tree('VP', [Tree('V', ['left']), Tree('NP', [Tree('Det', ['a']), Tree('N', ['Dana'])])])])]