Randomly generated Parse Tree using a fix set of vocabulary [closed]

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I am using Python 3.2 and I have tried to build a randomly generated parse tree for a sentence.Though I am sure it generates the sentence, I am not sure how random the parse tree is and also, I do not know if there is a better/more efficient way to improve this code. (I am new to programming and Python as such and I have recently been interested in NLP. Any advice, solution or corrections are welcome.)

 N=['man','dog','cat','telescope','park']  #noun
 P=['in','on','by','with']   #preposition
 det=['a','an','the','my']   #determinant
 V=['saw','ate','walked']    #verb
NP=['John','Mary','Bob']    #noun phrase


from random import choice
 PP=choice(NP)+' '+choice(P)   #preposition phrase
 PP=''.join(PP)
 VP=''.join(choice(V)+' '+choice(NP)) or''.join(choice(V)+' '.choice(NP)+(PP)) #verb phrase         
 VP=''.join(VP) #verb phrase 
 S=choice(NP)+' '+VP  #sentence
 print(S)

Answer 1

try NLTK, http://nltk.org/book/ch08.html

import nltk
from random import choice, shuffle, random

# Sometimes i find reading terminals as values into a dict of POS helps.
vocab={
'Det':['a','an','the','my'],
'N':['man','dog','cat','telescope','park'],
'V':['saw','ate','walked'],
'P':['in','on','by','with'],
'NP':['John','Mary','Bob']
}

vocab2string = [pos + " -> '" + "' | '".join(vocab[pos])+"'" for pos in vocab]

# Rules are simpler to be manually crafted so i left them in strings
rules = '''
S -> NP VP
VP -> V NP
VP -> V NP PP
PP -> NP P
NP -> Det N
'''

mygrammar = rules + "\n".join(vocab2string)
grammar = nltk.parse_cfg(mygrammar) # Loaded your grammar
parser =  nltk.ChartParser(grammar) # Loaded grammar into a parser

# Randomly select one terminal from each POS, based on infinite monkey theorem, i.e. selection of words without grammatical order, see https://en.wikipedia.org/wiki/Infinite_monkey_theorem
words = [choice(vocab[pos]) for pos in vocab if pos != 'P'] # without PP
words = [choice(vocab[pos]) for pos in vocab] + choice(vocab('NP')) # with a PP you need 3 NPs

# To make sure that you always generate a grammatical sentence
trees = []
while trees != []:
  shuffle(words)
  trees = parser.nbest_parse(words)

for t in trees:
  print t