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90 lines
2.4 KiB
Python
90 lines
2.4 KiB
Python
# -*- coding: utf-8 -*-
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# Natural Language Toolkit: Generating from a CFG
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#
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# Copyright (C) 2001-2020 NLTK Project
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# Author: Steven Bird <stevenbird1@gmail.com>
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# Peter Ljunglöf <peter.ljunglof@heatherleaf.se>
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# URL: <http://nltk.org/>
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# For license information, see LICENSE.TXT
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#
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import itertools
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import sys
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from nltk.grammar import Nonterminal
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def generate(grammar, start=None, depth=None, n=None):
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"""
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Generates an iterator of all sentences from a CFG.
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:param grammar: The Grammar used to generate sentences.
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:param start: The Nonterminal from which to start generate sentences.
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:param depth: The maximal depth of the generated tree.
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:param n: The maximum number of sentences to return.
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:return: An iterator of lists of terminal tokens.
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"""
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if not start:
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start = grammar.start()
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if depth is None:
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depth = sys.maxsize
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iter = _generate_all(grammar, [start], depth)
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if n:
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iter = itertools.islice(iter, n)
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return iter
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def _generate_all(grammar, items, depth):
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if items:
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try:
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for frag1 in _generate_one(grammar, items[0], depth):
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for frag2 in _generate_all(grammar, items[1:], depth):
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yield frag1 + frag2
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except RuntimeError as _error:
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if _error.message == "maximum recursion depth exceeded":
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# Helpful error message while still showing the recursion stack.
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raise RuntimeError(
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"The grammar has rule(s) that yield infinite recursion!!"
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)
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else:
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raise
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else:
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yield []
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def _generate_one(grammar, item, depth):
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if depth > 0:
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if isinstance(item, Nonterminal):
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for prod in grammar.productions(lhs=item):
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for frag in _generate_all(grammar, prod.rhs(), depth - 1):
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yield frag
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else:
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yield [item]
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demo_grammar = """
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S -> NP VP
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NP -> Det N
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PP -> P NP
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VP -> 'slept' | 'saw' NP | 'walked' PP
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Det -> 'the' | 'a'
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N -> 'man' | 'park' | 'dog'
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P -> 'in' | 'with'
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"""
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def demo(N=23):
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from nltk.grammar import CFG
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print("Generating the first %d sentences for demo grammar:" % (N,))
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print(demo_grammar)
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grammar = CFG.fromstring(demo_grammar)
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for n, sent in enumerate(generate(grammar, n=N), 1):
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print("%3d. %s" % (n, " ".join(sent)))
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if __name__ == "__main__":
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demo()
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