仅用500行Python代码实现一个英文解析器的教程

>>> parser = parser.Parser()
>>> tokens = "Set the volume to zero when I 'm in a meeting unless John 's school calls".split()
>>> tags, heads = parser.parse(tokens)
>>> heads
[-1, 2, 0, 0, 3, 0, 7, 5, 7, 10, 8, 0, 13, 15, 15, 11]
>>> for i, h in enumerate(heads):
...  head = tokens[heads[h]] if h >= 1 else 'None'
...  print(tokens[i] + ' <-- ' + head])
Set <-- None
the <-- volume
volume <-- Set
to <-- Set
zero <-- to
when <-- Set
I <-- 'm
'm <-- when
in <-- 'm
a <-- meeting
meeting <-- in
unless <-- Set
John <-- 's
's  <-- calls
school <-- calls
calls <-- unless

class Parse(object):
  def __init__(self, n):
    self.n = n
    self.heads = [None] * (n-1)
    self.lefts = []
    self.rights = []
    for i in range(n+1):
      self.lefts.append(DefaultList(0))
      self.rights.append(DefaultList(0))
 
  def add_arc(self, head, child):
    self.heads[child] = head
    if child < head:
      self.lefts[head].append(child)
    else:
      self.rights[head].append(child)

SHIFT = 0; RIGHT = 1; LEFT = 2
MOVES = [SHIFT, RIGHT, LEFT]
 
def transition(move, i, stack, parse):
  global SHIFT, RIGHT, LEFT
  if move == SHIFT:
    stack.append(i)
    return i + 1
  elif move == RIGHT:
    parse.add_arc(stack[-2], stack.pop())
    return i
  elif move == LEFT:
    parse.add_arc(i, stack.pop())
    return i
  raise GrammarError("Unknown move: %d" % move)

class Parser(object):
  ...
  def parse(self, words):
    tags = self.tagger(words)
    n = len(words)
    idx = 1
    stack = [0]
    deps = Parse(n)
    while stack or idx < n:
      features = extract_features(words, tags, idx, n, stack, deps)
      scores = self.model.score(features)
      valid_moves = get_valid_moves(i, n, len(stack))
      next_move = max(valid_moves, key=lambda move: scores[move])
      idx = transition(next_move, idx, stack, parse)
    return tags, parse
 
def get_valid_moves(i, n, stack_depth):
  moves = []
  if i < n:
    moves.append(SHIFT)
  if stack_depth >= 2:
    moves.append(RIGHT)
  if stack_depth >= 1:
    moves.append(LEFT)
  return moves

class Perceptron(object)
  ...
  def score(self, features):
    all_weights = self.weights
    scores = dict((clas, 0) for clas in self.classes)
    for feat, value in features.items():
      if value == 0:
        continue
      if feat not in all_weights:
        continue
      weights = all_weights[feat]
      for clas, weight in weights.items():
        scores[clas] += value * weight
    return scores

def extract_features(words, tags, n0, n, stack, parse):
  def get_stack_context(depth, stack, data):
    if depth >;= 3:
      return data[stack[-1]], data[stack[-2]], data[stack[-3]]
    elif depth >= 2:
      return data[stack[-1]], data[stack[-2]], ''
    elif depth == 1:
      return data[stack[-1]], '', ''
    else:
      return '', '', ''
 
  def get_buffer_context(i, n, data):
    if i + 1 >= n:
      return data[i], '', ''
    elif i + 2 >= n:
      return data[i], data[i + 1], ''
    else:
      return data[i], data[i + 1], data[i + 2]
 
  def get_parse_context(word, deps, data):
    if word == -1:
      return 0, '', ''
    deps = deps[word]
    valency = len(deps)
    if not valency:
      return 0, '', ''
    elif valency == 1:
      return 1, data[deps[-1]], ''
    else:
      return valency, data[deps[-1]], data[deps[-2]]
 
  features = {}
  # Set up the context pieces --- the word, W, and tag, T, of:
  # S0-2: Top three words on the stack
  # N0-2: First three words of the buffer
  # n0b1, n0b2: Two leftmost children of the first word of the buffer
  # s0b1, s0b2: Two leftmost children of the top word of the stack
  # s0f1, s0f2: Two rightmost children of the top word of the stack
 
  depth = len(stack)
  s0 = stack[-1] if depth else -1
 
  Ws0, Ws1, Ws2 = get_stack_context(depth, stack, words)
  Ts0, Ts1, Ts2 = get_stack_context(depth, stack, tags)
 
  Wn0, Wn1, Wn2 = get_buffer_context(n0, n, words)
  Tn0, Tn1, Tn2 = get_buffer_context(n0, n, tags)
 
  Vn0b, Wn0b1, Wn0b2 = get_parse_context(n0, parse.lefts, words)
  Vn0b, Tn0b1, Tn0b2 = get_parse_context(n0, parse.lefts, tags)
 
  Vn0f, Wn0f1, Wn0f2 = get_parse_context(n0, parse.rights, words)
  _, Tn0f1, Tn0f2 = get_parse_context(n0, parse.rights, tags)
 
  Vs0b, Ws0b1, Ws0b2 = get_parse_context(s0, parse.lefts, words)
  _, Ts0b1, Ts0b2 = get_parse_context(s0, parse.lefts, tags)
 
  Vs0f, Ws0f1, Ws0f2 = get_parse_context(s0, parse.rights, words)
  _, Ts0f1, Ts0f2 = get_parse_context(s0, parse.rights, tags)
 
  # Cap numeric features at 5?
  # String-distance
  Ds0n0 = min((n0 - s0, 5)) if s0 != 0 else 0
 
  features['bias'] = 1
  # Add word and tag unigrams
  for w in (Wn0, Wn1, Wn2, Ws0, Ws1, Ws2, Wn0b1, Wn0b2, Ws0b1, Ws0b2, Ws0f1, Ws0f2):
    if w:
      features['w=%s' % w] = 1
  for t in (Tn0, Tn1, Tn2, Ts0, Ts1, Ts2, Tn0b1, Tn0b2, Ts0b1, Ts0b2, Ts0f1, Ts0f2):
    if t:
      features['t=%s' % t] = 1
 
  # Add word/tag pairs
  for i, (w, t) in enumerate(((Wn0, Tn0), (Wn1, Tn1), (Wn2, Tn2), (Ws0, Ts0))):
    if w or t:
      features['%d w=%s, t=%s' % (i, w, t)] = 1
 
  # Add some bigrams
  features['s0w=%s, n0w=%s' % (Ws0, Wn0)] = 1
  features['wn0tn0-ws0 %s/%s %s' % (Wn0, Tn0, Ws0)] = 1
  features['wn0tn0-ts0 %s/%s %s' % (Wn0, Tn0, Ts0)] = 1
  features['ws0ts0-wn0 %s/%s %s' % (Ws0, Ts0, Wn0)] = 1
  features['ws0-ts0 tn0 %s/%s %s' % (Ws0, Ts0, Tn0)] = 1
  features['wt-wt %s/%s %s/%s' % (Ws0, Ts0, Wn0, Tn0)] = 1
  features['tt s0=%s n0=%s' % (Ts0, Tn0)] = 1
  features['tt n0=%s n1=%s' % (Tn0, Tn1)] = 1
 
  # Add some tag trigrams
  trigrams = ((Tn0, Tn1, Tn2), (Ts0, Tn0, Tn1), (Ts0, Ts1, Tn0),
        (Ts0, Ts0f1, Tn0), (Ts0, Ts0f1, Tn0), (Ts0, Tn0, Tn0b1),
        (Ts0, Ts0b1, Ts0b2), (Ts0, Ts0f1, Ts0f2), (Tn0, Tn0b1, Tn0b2),
        (Ts0, Ts1, Ts1))
  for i, (t1, t2, t3) in enumerate(trigrams):
    if t1 or t2 or t3:
      features['ttt-%d %s %s %s' % (i, t1, t2, t3)] = 1
 
  # Add some valency and distance features
  vw = ((Ws0, Vs0f), (Ws0, Vs0b), (Wn0, Vn0b))
  vt = ((Ts0, Vs0f), (Ts0, Vs0b), (Tn0, Vn0b))
  d = ((Ws0, Ds0n0), (Wn0, Ds0n0), (Ts0, Ds0n0), (Tn0, Ds0n0),
    ('t' + Tn0+Ts0, Ds0n0), ('w' + Wn0+Ws0, Ds0n0))
  for i, (w_t, v_d) in enumerate(vw + vt + d):
    if w_t or v_d:
      features['val/d-%d %s %d' % (i, w_t, v_d)] = 1
  return features

class Parser(object):
  ...
  def train_one(self, itn, words, gold_tags, gold_heads):
    n = len(words)
    i = 2; stack = [1]; parse = Parse(n)
    tags = self.tagger.tag(words)
    while stack or (i + 1) < n:
      features = extract_features(words, tags, i, n, stack, parse)
      scores = self.model.score(features)
      valid_moves = get_valid_moves(i, n, len(stack))
      guess = max(valid_moves, key=lambda move: scores[move])
      gold_moves = get_gold_moves(i, n, stack, parse.heads, gold_heads)
      best = max(gold_moves, key=lambda move: scores[move])
    self.model.update(best, guess, features)
    i = transition(guess, i, stack, parse)
  # Return number correct
  return len([i for i in range(n-1) if parse.heads[i] == gold_heads[i]])

Oracle(state) = | gold_arcs ∩ reachable_arcs(state) |

  shift_cost = Oracle(state) – Oracle(shift(state))
  right_cost = Oracle(state) – Oracle(right(state))
  left_cost = Oracle(state) – Oracle(left(state))

def get_gold_moves(n0, n, stack, heads, gold):
  def deps_between(target, others, gold):
    for word in others:
      if gold[word] == target or gold[target] == word:
        return True
    return False
 
  valid = get_valid_moves(n0, n, len(stack))
  if not stack or (SHIFT in valid and gold[n0] == stack[-1]):
    return [SHIFT]
  if gold[stack[-1]] == n0:
    return [LEFT]
  costly = set([m for m in MOVES if m not in valid])
  # If the word behind s0 is its gold head, Left is incorrect
  if len(stack) >= 2 and gold[stack[-1]] == stack[-2]:
    costly.add(LEFT)
  # If there are any dependencies between n0 and the stack,
  # pushing n0 will lose them.
  if SHIFT not in costly and deps_between(n0, stack, gold):
    costly.add(SHIFT)
  # If there are any dependencies between s0 and the buffer, popping
  # s0 will lose them.
  if deps_between(stack[-1], range(n0+1, n-1), gold):
    costly.add(LEFT)
    costly.add(RIGHT)
  return [m for m in MOVES if m not in costly]

java -mx10000m -cp "$scriptdir/*:" edu.stanford.nlp.parser.lexparser.LexicalizedParser \
-outputFormat "penn" edu/stanford/nlp/models/lexparser/englishFactored.ser.gz $*

"""Stanford parser retokenises numbers. Split them."""
import sys
import re
 
qp_re = re.compile('\xc2\xa0')
for line in sys.stdin:
  line = line.rstrip()
  if qp_re.search(line):
    line = line.replace('(CD', '(QP (CD', 1) + ')'
    line = line.replace('\xc2\xa0', ') (CD ')
  print line

for f in $1/*.mrg; do
 echo $f
 grep -v CODE $f > "$f.2"
 out="$f.dep"
 java -mx800m -cp "$scriptdir/*:" edu.stanford.nlp.trees.EnglishGrammaticalStructure \
  -treeFile "$f.2" -basic -makeCopulaHead -conllx > $out
done

./scripts/train.py -x zhang+stack -k 8 -p ~/data/stanford/train.conll ~/data/parsers/tmp
./scripts/parse.py ~/data/parsers/tmp ~/data/stanford/devi.txt /tmp/parse/
./scripts/evaluate.py /tmp/parse/parses ~/data/stanford/dev.conll