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| # Natural Language Toolkit: Generating from a CFG | |
| # | |
| # Copyright (C) 2001-2023 NLTK Project | |
| # Author: Steven Bird <[email protected]> | |
| # Peter Ljunglöf <[email protected]> | |
| # URL: <https://www.nltk.org/> | |
| # For license information, see LICENSE.TXT | |
| # | |
| import itertools | |
| import sys | |
| from nltk.grammar import Nonterminal | |
| def generate(grammar, start=None, depth=None, n=None): | |
| """ | |
| Generates an iterator of all sentences from a CFG. | |
| :param grammar: The Grammar used to generate sentences. | |
| :param start: The Nonterminal from which to start generate sentences. | |
| :param depth: The maximal depth of the generated tree. | |
| :param n: The maximum number of sentences to return. | |
| :return: An iterator of lists of terminal tokens. | |
| """ | |
| if not start: | |
| start = grammar.start() | |
| if depth is None: | |
| depth = sys.maxsize | |
| iter = _generate_all(grammar, [start], depth) | |
| if n: | |
| iter = itertools.islice(iter, n) | |
| return iter | |
| def _generate_all(grammar, items, depth): | |
| if items: | |
| try: | |
| for frag1 in _generate_one(grammar, items[0], depth): | |
| for frag2 in _generate_all(grammar, items[1:], depth): | |
| yield frag1 + frag2 | |
| except RecursionError as error: | |
| # Helpful error message while still showing the recursion stack. | |
| raise RuntimeError( | |
| "The grammar has rule(s) that yield infinite recursion!" | |
| ) from error | |
| else: | |
| yield [] | |
| def _generate_one(grammar, item, depth): | |
| if depth > 0: | |
| if isinstance(item, Nonterminal): | |
| for prod in grammar.productions(lhs=item): | |
| yield from _generate_all(grammar, prod.rhs(), depth - 1) | |
| else: | |
| yield [item] | |
| demo_grammar = """ | |
| S -> NP VP | |
| NP -> Det N | |
| PP -> P NP | |
| VP -> 'slept' | 'saw' NP | 'walked' PP | |
| Det -> 'the' | 'a' | |
| N -> 'man' | 'park' | 'dog' | |
| P -> 'in' | 'with' | |
| """ | |
| def demo(N=23): | |
| from nltk.grammar import CFG | |
| print("Generating the first %d sentences for demo grammar:" % (N,)) | |
| print(demo_grammar) | |
| grammar = CFG.fromstring(demo_grammar) | |
| for n, sent in enumerate(generate(grammar, n=N), 1): | |
| print("%3d. %s" % (n, " ".join(sent))) | |
| if __name__ == "__main__": | |
| demo() | |