481 lines
16 KiB
Python
481 lines
16 KiB
Python
# Natural Language Toolkit: Shift-Reduce Parser
|
|
#
|
|
# Copyright (C) 2001-2019 NLTK Project
|
|
# Author: Edward Loper <edloper@gmail.com>
|
|
# Steven Bird <stevenbird1@gmail.com>
|
|
# URL: <http://nltk.org/>
|
|
# For license information, see LICENSE.TXT
|
|
|
|
from nltk.grammar import Nonterminal
|
|
from nltk.tree import Tree
|
|
from nltk.compat import unicode_repr
|
|
|
|
from nltk.parse.api import ParserI
|
|
|
|
##//////////////////////////////////////////////////////
|
|
## Shift/Reduce Parser
|
|
##//////////////////////////////////////////////////////
|
|
class ShiftReduceParser(ParserI):
|
|
"""
|
|
A simple bottom-up CFG parser that uses two operations, "shift"
|
|
and "reduce", to find a single parse for a text.
|
|
|
|
``ShiftReduceParser`` maintains a stack, which records the
|
|
structure of a portion of the text. This stack is a list of
|
|
strings and Trees that collectively cover a portion of
|
|
the text. For example, while parsing the sentence "the dog saw
|
|
the man" with a typical grammar, ``ShiftReduceParser`` will produce
|
|
the following stack, which covers "the dog saw"::
|
|
|
|
[(NP: (Det: 'the') (N: 'dog')), (V: 'saw')]
|
|
|
|
``ShiftReduceParser`` attempts to extend the stack to cover the
|
|
entire text, and to combine the stack elements into a single tree,
|
|
producing a complete parse for the sentence.
|
|
|
|
Initially, the stack is empty. It is extended to cover the text,
|
|
from left to right, by repeatedly applying two operations:
|
|
|
|
- "shift" moves a token from the beginning of the text to the
|
|
end of the stack.
|
|
- "reduce" uses a CFG production to combine the rightmost stack
|
|
elements into a single Tree.
|
|
|
|
Often, more than one operation can be performed on a given stack.
|
|
In this case, ``ShiftReduceParser`` uses the following heuristics
|
|
to decide which operation to perform:
|
|
|
|
- Only shift if no reductions are available.
|
|
- If multiple reductions are available, then apply the reduction
|
|
whose CFG production is listed earliest in the grammar.
|
|
|
|
Note that these heuristics are not guaranteed to choose an
|
|
operation that leads to a parse of the text. Also, if multiple
|
|
parses exists, ``ShiftReduceParser`` will return at most one of
|
|
them.
|
|
|
|
:see: ``nltk.grammar``
|
|
"""
|
|
|
|
def __init__(self, grammar, trace=0):
|
|
"""
|
|
Create a new ``ShiftReduceParser``, that uses ``grammar`` to
|
|
parse texts.
|
|
|
|
:type grammar: Grammar
|
|
:param grammar: The grammar used to parse texts.
|
|
:type trace: int
|
|
:param trace: The level of tracing that should be used when
|
|
parsing a text. ``0`` will generate no tracing output;
|
|
and higher numbers will produce more verbose tracing
|
|
output.
|
|
"""
|
|
self._grammar = grammar
|
|
self._trace = trace
|
|
self._check_grammar()
|
|
|
|
def grammar(self):
|
|
return self._grammar
|
|
|
|
def parse(self, tokens):
|
|
tokens = list(tokens)
|
|
self._grammar.check_coverage(tokens)
|
|
|
|
# initialize the stack.
|
|
stack = []
|
|
remaining_text = tokens
|
|
|
|
# Trace output.
|
|
if self._trace:
|
|
print("Parsing %r" % " ".join(tokens))
|
|
self._trace_stack(stack, remaining_text)
|
|
|
|
# iterate through the text, pushing the token onto
|
|
# the stack, then reducing the stack.
|
|
while len(remaining_text) > 0:
|
|
self._shift(stack, remaining_text)
|
|
while self._reduce(stack, remaining_text):
|
|
pass
|
|
|
|
# Did we reduce everything?
|
|
if len(stack) == 1:
|
|
# Did we end up with the right category?
|
|
if stack[0].label() == self._grammar.start().symbol():
|
|
yield stack[0]
|
|
|
|
def _shift(self, stack, remaining_text):
|
|
"""
|
|
Move a token from the beginning of ``remaining_text`` to the
|
|
end of ``stack``.
|
|
|
|
:type stack: list(str and Tree)
|
|
:param stack: A list of strings and Trees, encoding
|
|
the structure of the text that has been parsed so far.
|
|
:type remaining_text: list(str)
|
|
:param remaining_text: The portion of the text that is not yet
|
|
covered by ``stack``.
|
|
:rtype: None
|
|
"""
|
|
stack.append(remaining_text[0])
|
|
remaining_text.remove(remaining_text[0])
|
|
if self._trace:
|
|
self._trace_shift(stack, remaining_text)
|
|
|
|
def _match_rhs(self, rhs, rightmost_stack):
|
|
"""
|
|
:rtype: bool
|
|
:return: true if the right hand side of a CFG production
|
|
matches the rightmost elements of the stack. ``rhs``
|
|
matches ``rightmost_stack`` if they are the same length,
|
|
and each element of ``rhs`` matches the corresponding
|
|
element of ``rightmost_stack``. A nonterminal element of
|
|
``rhs`` matches any Tree whose node value is equal
|
|
to the nonterminal's symbol. A terminal element of ``rhs``
|
|
matches any string whose type is equal to the terminal.
|
|
:type rhs: list(terminal and Nonterminal)
|
|
:param rhs: The right hand side of a CFG production.
|
|
:type rightmost_stack: list(string and Tree)
|
|
:param rightmost_stack: The rightmost elements of the parser's
|
|
stack.
|
|
"""
|
|
|
|
if len(rightmost_stack) != len(rhs):
|
|
return False
|
|
for i in range(len(rightmost_stack)):
|
|
if isinstance(rightmost_stack[i], Tree):
|
|
if not isinstance(rhs[i], Nonterminal):
|
|
return False
|
|
if rightmost_stack[i].label() != rhs[i].symbol():
|
|
return False
|
|
else:
|
|
if isinstance(rhs[i], Nonterminal):
|
|
return False
|
|
if rightmost_stack[i] != rhs[i]:
|
|
return False
|
|
return True
|
|
|
|
def _reduce(self, stack, remaining_text, production=None):
|
|
"""
|
|
Find a CFG production whose right hand side matches the
|
|
rightmost stack elements; and combine those stack elements
|
|
into a single Tree, with the node specified by the
|
|
production's left-hand side. If more than one CFG production
|
|
matches the stack, then use the production that is listed
|
|
earliest in the grammar. The new Tree replaces the
|
|
elements in the stack.
|
|
|
|
:rtype: Production or None
|
|
:return: If a reduction is performed, then return the CFG
|
|
production that the reduction is based on; otherwise,
|
|
return false.
|
|
:type stack: list(string and Tree)
|
|
:param stack: A list of strings and Trees, encoding
|
|
the structure of the text that has been parsed so far.
|
|
:type remaining_text: list(str)
|
|
:param remaining_text: The portion of the text that is not yet
|
|
covered by ``stack``.
|
|
"""
|
|
if production is None:
|
|
productions = self._grammar.productions()
|
|
else:
|
|
productions = [production]
|
|
|
|
# Try each production, in order.
|
|
for production in productions:
|
|
rhslen = len(production.rhs())
|
|
|
|
# check if the RHS of a production matches the top of the stack
|
|
if self._match_rhs(production.rhs(), stack[-rhslen:]):
|
|
|
|
# combine the tree to reflect the reduction
|
|
tree = Tree(production.lhs().symbol(), stack[-rhslen:])
|
|
stack[-rhslen:] = [tree]
|
|
|
|
# We reduced something
|
|
if self._trace:
|
|
self._trace_reduce(stack, production, remaining_text)
|
|
return production
|
|
|
|
# We didn't reduce anything
|
|
return None
|
|
|
|
def trace(self, trace=2):
|
|
"""
|
|
Set the level of tracing output that should be generated when
|
|
parsing a text.
|
|
|
|
:type trace: int
|
|
:param trace: The trace level. A trace level of ``0`` will
|
|
generate no tracing output; and higher trace levels will
|
|
produce more verbose tracing output.
|
|
:rtype: None
|
|
"""
|
|
# 1: just show shifts.
|
|
# 2: show shifts & reduces
|
|
# 3: display which tokens & productions are shifed/reduced
|
|
self._trace = trace
|
|
|
|
def _trace_stack(self, stack, remaining_text, marker=" "):
|
|
"""
|
|
Print trace output displaying the given stack and text.
|
|
|
|
:rtype: None
|
|
:param marker: A character that is printed to the left of the
|
|
stack. This is used with trace level 2 to print 'S'
|
|
before shifted stacks and 'R' before reduced stacks.
|
|
"""
|
|
s = " " + marker + " [ "
|
|
for elt in stack:
|
|
if isinstance(elt, Tree):
|
|
s += unicode_repr(Nonterminal(elt.label())) + " "
|
|
else:
|
|
s += unicode_repr(elt) + " "
|
|
s += "* " + " ".join(remaining_text) + "]"
|
|
print(s)
|
|
|
|
def _trace_shift(self, stack, remaining_text):
|
|
"""
|
|
Print trace output displaying that a token has been shifted.
|
|
|
|
:rtype: None
|
|
"""
|
|
if self._trace > 2:
|
|
print("Shift %r:" % stack[-1])
|
|
if self._trace == 2:
|
|
self._trace_stack(stack, remaining_text, "S")
|
|
elif self._trace > 0:
|
|
self._trace_stack(stack, remaining_text)
|
|
|
|
def _trace_reduce(self, stack, production, remaining_text):
|
|
"""
|
|
Print trace output displaying that ``production`` was used to
|
|
reduce ``stack``.
|
|
|
|
:rtype: None
|
|
"""
|
|
if self._trace > 2:
|
|
rhs = " ".join(production.rhs())
|
|
print("Reduce %r <- %s" % (production.lhs(), rhs))
|
|
if self._trace == 2:
|
|
self._trace_stack(stack, remaining_text, "R")
|
|
elif self._trace > 1:
|
|
self._trace_stack(stack, remaining_text)
|
|
|
|
def _check_grammar(self):
|
|
"""
|
|
Check to make sure that all of the CFG productions are
|
|
potentially useful. If any productions can never be used,
|
|
then print a warning.
|
|
|
|
:rtype: None
|
|
"""
|
|
productions = self._grammar.productions()
|
|
|
|
# Any production whose RHS is an extension of another production's RHS
|
|
# will never be used.
|
|
for i in range(len(productions)):
|
|
for j in range(i + 1, len(productions)):
|
|
rhs1 = productions[i].rhs()
|
|
rhs2 = productions[j].rhs()
|
|
if rhs1[: len(rhs2)] == rhs2:
|
|
print("Warning: %r will never be used" % productions[i])
|
|
|
|
|
|
##//////////////////////////////////////////////////////
|
|
## Stepping Shift/Reduce Parser
|
|
##//////////////////////////////////////////////////////
|
|
class SteppingShiftReduceParser(ShiftReduceParser):
|
|
"""
|
|
A ``ShiftReduceParser`` that allows you to setp through the parsing
|
|
process, performing a single operation at a time. It also allows
|
|
you to change the parser's grammar midway through parsing a text.
|
|
|
|
The ``initialize`` method is used to start parsing a text.
|
|
``shift`` performs a single shift operation, and ``reduce`` performs
|
|
a single reduce operation. ``step`` will perform a single reduce
|
|
operation if possible; otherwise, it will perform a single shift
|
|
operation. ``parses`` returns the set of parses that have been
|
|
found by the parser.
|
|
|
|
:ivar _history: A list of ``(stack, remaining_text)`` pairs,
|
|
containing all of the previous states of the parser. This
|
|
history is used to implement the ``undo`` operation.
|
|
:see: ``nltk.grammar``
|
|
"""
|
|
|
|
def __init__(self, grammar, trace=0):
|
|
super(SteppingShiftReduceParser, self).__init__(grammar, trace)
|
|
self._stack = None
|
|
self._remaining_text = None
|
|
self._history = []
|
|
|
|
def parse(self, tokens):
|
|
tokens = list(tokens)
|
|
self.initialize(tokens)
|
|
while self.step():
|
|
pass
|
|
return self.parses()
|
|
|
|
def stack(self):
|
|
"""
|
|
:return: The parser's stack.
|
|
:rtype: list(str and Tree)
|
|
"""
|
|
return self._stack
|
|
|
|
def remaining_text(self):
|
|
"""
|
|
:return: The portion of the text that is not yet covered by the
|
|
stack.
|
|
:rtype: list(str)
|
|
"""
|
|
return self._remaining_text
|
|
|
|
def initialize(self, tokens):
|
|
"""
|
|
Start parsing a given text. This sets the parser's stack to
|
|
``[]`` and sets its remaining text to ``tokens``.
|
|
"""
|
|
self._stack = []
|
|
self._remaining_text = tokens
|
|
self._history = []
|
|
|
|
def step(self):
|
|
"""
|
|
Perform a single parsing operation. If a reduction is
|
|
possible, then perform that reduction, and return the
|
|
production that it is based on. Otherwise, if a shift is
|
|
possible, then perform it, and return True. Otherwise,
|
|
return False.
|
|
|
|
:return: False if no operation was performed; True if a shift was
|
|
performed; and the CFG production used to reduce if a
|
|
reduction was performed.
|
|
:rtype: Production or bool
|
|
"""
|
|
return self.reduce() or self.shift()
|
|
|
|
def shift(self):
|
|
"""
|
|
Move a token from the beginning of the remaining text to the
|
|
end of the stack. If there are no more tokens in the
|
|
remaining text, then do nothing.
|
|
|
|
:return: True if the shift operation was successful.
|
|
:rtype: bool
|
|
"""
|
|
if len(self._remaining_text) == 0:
|
|
return False
|
|
self._history.append((self._stack[:], self._remaining_text[:]))
|
|
self._shift(self._stack, self._remaining_text)
|
|
return True
|
|
|
|
def reduce(self, production=None):
|
|
"""
|
|
Use ``production`` to combine the rightmost stack elements into
|
|
a single Tree. If ``production`` does not match the
|
|
rightmost stack elements, then do nothing.
|
|
|
|
:return: The production used to reduce the stack, if a
|
|
reduction was performed. If no reduction was performed,
|
|
return None.
|
|
|
|
:rtype: Production or None
|
|
"""
|
|
self._history.append((self._stack[:], self._remaining_text[:]))
|
|
return_val = self._reduce(self._stack, self._remaining_text, production)
|
|
|
|
if not return_val:
|
|
self._history.pop()
|
|
return return_val
|
|
|
|
def undo(self):
|
|
"""
|
|
Return the parser to its state before the most recent
|
|
shift or reduce operation. Calling ``undo`` repeatedly return
|
|
the parser to successively earlier states. If no shift or
|
|
reduce operations have been performed, ``undo`` will make no
|
|
changes.
|
|
|
|
:return: true if an operation was successfully undone.
|
|
:rtype: bool
|
|
"""
|
|
if len(self._history) == 0:
|
|
return False
|
|
(self._stack, self._remaining_text) = self._history.pop()
|
|
return True
|
|
|
|
def reducible_productions(self):
|
|
"""
|
|
:return: A list of the productions for which reductions are
|
|
available for the current parser state.
|
|
:rtype: list(Production)
|
|
"""
|
|
productions = []
|
|
for production in self._grammar.productions():
|
|
rhslen = len(production.rhs())
|
|
if self._match_rhs(production.rhs(), self._stack[-rhslen:]):
|
|
productions.append(production)
|
|
return productions
|
|
|
|
def parses(self):
|
|
"""
|
|
:return: An iterator of the parses that have been found by this
|
|
parser so far.
|
|
:rtype: iter(Tree)
|
|
"""
|
|
if (
|
|
len(self._remaining_text) == 0
|
|
and len(self._stack) == 1
|
|
and self._stack[0].label() == self._grammar.start().symbol()
|
|
):
|
|
yield self._stack[0]
|
|
|
|
# copied from nltk.parser
|
|
|
|
def set_grammar(self, grammar):
|
|
"""
|
|
Change the grammar used to parse texts.
|
|
|
|
:param grammar: The new grammar.
|
|
:type grammar: CFG
|
|
"""
|
|
self._grammar = grammar
|
|
|
|
|
|
##//////////////////////////////////////////////////////
|
|
## Demonstration Code
|
|
##//////////////////////////////////////////////////////
|
|
|
|
|
|
def demo():
|
|
"""
|
|
A demonstration of the shift-reduce parser.
|
|
"""
|
|
|
|
from nltk import parse, CFG
|
|
|
|
grammar = CFG.fromstring(
|
|
"""
|
|
S -> NP VP
|
|
NP -> Det N | Det N PP
|
|
VP -> V NP | V NP PP
|
|
PP -> P NP
|
|
NP -> 'I'
|
|
N -> 'man' | 'park' | 'telescope' | 'dog'
|
|
Det -> 'the' | 'a'
|
|
P -> 'in' | 'with'
|
|
V -> 'saw'
|
|
"""
|
|
)
|
|
|
|
sent = "I saw a man in the park".split()
|
|
|
|
parser = parse.ShiftReduceParser(grammar, trace=2)
|
|
for p in parser.parse(sent):
|
|
print(p)
|
|
|
|
|
|
if __name__ == "__main__":
|
|
demo()
|