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venv/lib/python3.7/site-packages/nltk/parse/generate.py

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+# -*- coding: utf-8 -*-
+# Natural Language Toolkit: Generating from a CFG
+#
+# Copyright (C) 2001-2019 NLTK Project
+# Author: Steven Bird <stevenbird1@gmail.com>
+#         Peter Ljunglöf <peter.ljunglof@heatherleaf.se>
+# URL: <http://nltk.org/>
+# For license information, see LICENSE.TXT
+#
+from __future__ import print_function
+
+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 RuntimeError as _error:
+            if _error.message == "maximum recursion depth exceeded":
+                # Helpful error message while still showing the recursion stack.
+                raise RuntimeError(
+                    "The grammar has rule(s) that yield infinite recursion!!"
+                )
+            else:
+                raise
+    else:
+        yield []
+
+
+def _generate_one(grammar, item, depth):
+    if depth > 0:
+        if isinstance(item, Nonterminal):
+            for prod in grammar.productions(lhs=item):
+                for frag in _generate_all(grammar, prod.rhs(), depth - 1):
+                    yield frag
+        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()