nltk /nltk/tree.py

Language Python Lines 1527
MD5 Hash e9ec6a3bcfa7a76dc005b632e60cf528
Repository https://github.com/BrucePHill/nltk.git View Raw File
   1
   2
   3
   4
   5
   6
   7
   8
   9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22
  23
  24
  25
  26
  27
  28
  29
  30
  31
  32
  33
  34
  35
  36
  37
  38
  39
  40
  41
  42
  43
  44
  45
  46
  47
  48
  49
  50
  51
  52
  53
  54
  55
  56
  57
  58
  59
  60
  61
  62
  63
  64
  65
  66
  67
  68
  69
  70
  71
  72
  73
  74
  75
  76
  77
  78
  79
  80
  81
  82
  83
  84
  85
  86
  87
  88
  89
  90
  91
  92
  93
  94
  95
  96
  97
  98
  99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
# -*- coding: utf-8 -*-
# Natural Language Toolkit: Text Trees
#
# Copyright (C) 2001-2013 NLTK Project
# Author: Edward Loper <edloper@gradient.cis.upenn.edu>
#         Steven Bird <stevenbird1@gmail.com>
#         Peter Ljunglรถf <peter.ljunglof@gu.se>
#         Nathan Bodenstab <bodenstab@cslu.ogi.edu> (tree transforms)
# URL: <http://www.nltk.org/>
# For license information, see LICENSE.TXT

"""
Class for representing hierarchical language structures, such as
syntax trees and morphological trees.
"""
from __future__ import print_function, unicode_literals

# TODO: add LabelledTree (can be used for dependency trees)

import re

from nltk.grammar import Production, Nonterminal
from nltk.probability import ProbabilisticMixIn
from nltk.util import slice_bounds
from nltk.compat import string_types, python_2_unicode_compatible, unicode_repr
from nltk.internals import raise_unorderable_types

######################################################################
## Trees
######################################################################

@python_2_unicode_compatible
class Tree(list):
    """
    A Tree represents a hierarchical grouping of leaves and subtrees.
    For example, each constituent in a syntax tree is represented by a single Tree.

    A tree's children are encoded as a list of leaves and subtrees,
    where a leaf is a basic (non-tree) value; and a subtree is a
    nested Tree.

        >>> from nltk.tree import Tree
        >>> print(Tree(1, [2, Tree(3, [4]), 5]))
        (1 2 (3 4) 5)
        >>> vp = Tree('VP', [Tree('V', ['saw']),
        ...                  Tree('NP', ['him'])])
        >>> s = Tree('S', [Tree('NP', ['I']), vp])
        >>> print(s)
        (S (NP I) (VP (V saw) (NP him)))
        >>> print(s[1])
        (VP (V saw) (NP him))
        >>> print(s[1,1])
        (NP him)
        >>> t = Tree("(S (NP I) (VP (V saw) (NP him)))")
        >>> s == t
        True
        >>> t[1][1].node = "X"
        >>> print(t)
        (S (NP I) (VP (V saw) (X him)))
        >>> t[0], t[1,1] = t[1,1], t[0]
        >>> print(t)
        (S (X him) (VP (V saw) (NP I)))

    The length of a tree is the number of children it has.

        >>> len(t)
        2

    Any other properties that a Tree defines are known as node
    properties, and are used to add information about individual
    hierarchical groupings.  For example, syntax trees use a NODE
    property to label syntactic constituents with phrase tags, such as
    "NP" and "VP".

    Several Tree methods use "tree positions" to specify
    children or descendants of a tree.  Tree positions are defined as
    follows:

      - The tree position *i* specifies a Tree's *i*\ th child.
      - The tree position ``()`` specifies the Tree itself.
      - If *p* is the tree position of descendant *d*, then
        *p+i* specifies the *i*\ th child of *d*.

    I.e., every tree position is either a single index *i*,
    specifying ``tree[i]``; or a sequence *i1, i2, ..., iN*,
    specifying ``tree[i1][i2]...[iN]``.

    Construct a new tree.  This constructor can be called in one
    of two ways:

    - ``Tree(node, children)`` constructs a new tree with the
        specified node value and list of children.

    - ``Tree(s)`` constructs a new tree by parsing the string ``s``.
        It is equivalent to calling the class method ``Tree.parse(s)``.
    """
    def __init__(self, node_or_str, children=None):
        if children is None:
            if not isinstance(node_or_str, string_types):
                raise TypeError("%s: Expected a node value and child list "
                                "or a single string" % type(self).__name__)
            tree = type(self).parse(node_or_str)
            list.__init__(self, tree)
            self.node = tree.node
        elif isinstance(children, string_types):
            raise TypeError("%s() argument 2 should be a list, not a "
                            "string" % type(self).__name__)
        else:
            list.__init__(self, children)
            self.node = node_or_str

    #////////////////////////////////////////////////////////////
    # Comparison operators
    #////////////////////////////////////////////////////////////

    def __eq__(self, other):
        return (self.__class__ is other.__class__ and
                (self.node, list(self)) == (other.node, list(other)))

    def __lt__(self, other):
        if not isinstance(other, Tree):
            # raise_unorderable_types("<", self, other)
            # Sometimes children can be pure strings,
            # so we need to be able to compare with non-trees:
            return self.__class__.__name__ < other.__class__.__name__
        elif self.__class__ is other.__class__:
            return (self.node, list(self)) < (other.node, list(other))
        else:
            return self.__class__.__name__ < other.__class__.__name__

    # @total_ordering doesn't work here, since the class inherits from a builtin class
    __ne__ = lambda self, other: not self == other
    __gt__ = lambda self, other: not (self < other or self == other)
    __le__ = lambda self, other: self < other or self == other
    __ge__ = lambda self, other: not self < other

    #////////////////////////////////////////////////////////////
    # Disabled list operations
    #////////////////////////////////////////////////////////////

    def __mul__(self, v):
        raise TypeError('Tree does not support multiplication')
    def __rmul__(self, v):
        raise TypeError('Tree does not support multiplication')
    def __add__(self, v):
        raise TypeError('Tree does not support addition')
    def __radd__(self, v):
        raise TypeError('Tree does not support addition')

    #////////////////////////////////////////////////////////////
    # Indexing (with support for tree positions)
    #////////////////////////////////////////////////////////////

    def __getitem__(self, index):
        if isinstance(index, (int, slice)):
            return list.__getitem__(self, index)
        elif isinstance(index, (list, tuple)):
            if len(index) == 0:
                return self
            elif len(index) == 1:
                return self[index[0]]
            else:
                return self[index[0]][index[1:]]
        else:
            raise TypeError("%s indices must be integers, not %s" %
                            (type(self).__name__, type(index).__name__))

    def __setitem__(self, index, value):
        if isinstance(index, (int, slice)):
            return list.__setitem__(self, index, value)
        elif isinstance(index, (list, tuple)):
            if len(index) == 0:
                raise IndexError('The tree position () may not be '
                                 'assigned to.')
            elif len(index) == 1:
                self[index[0]] = value
            else:
                self[index[0]][index[1:]] = value
        else:
            raise TypeError("%s indices must be integers, not %s" %
                            (type(self).__name__, type(index).__name__))

    def __delitem__(self, index):
        if isinstance(index, (int, slice)):
            return list.__delitem__(self, index)
        elif isinstance(index, (list, tuple)):
            if len(index) == 0:
                raise IndexError('The tree position () may not be deleted.')
            elif len(index) == 1:
                del self[index[0]]
            else:
                del self[index[0]][index[1:]]
        else:
            raise TypeError("%s indices must be integers, not %s" %
                            (type(self).__name__, type(index).__name__))

    #////////////////////////////////////////////////////////////
    # Basic tree operations
    #////////////////////////////////////////////////////////////

    def leaves(self):
        """
        Return the leaves of the tree.

            >>> t = Tree("(S (NP (D the) (N dog)) (VP (V chased) (NP (D the) (N cat))))")
            >>> t.leaves()
            ['the', 'dog', 'chased', 'the', 'cat']

        :return: a list containing this tree's leaves.
            The order reflects the order of the
            leaves in the tree's hierarchical structure.
        :rtype: list
        """
        leaves = []
        for child in self:
            if isinstance(child, Tree):
                leaves.extend(child.leaves())
            else:
                leaves.append(child)
        return leaves

    def flatten(self):
        """
        Return a flat version of the tree, with all non-root non-terminals removed.

            >>> t = Tree("(S (NP (D the) (N dog)) (VP (V chased) (NP (D the) (N cat))))")
            >>> print(t.flatten())
            (S the dog chased the cat)

        :return: a tree consisting of this tree's root connected directly to
            its leaves, omitting all intervening non-terminal nodes.
        :rtype: Tree
        """
        return Tree(self.node, self.leaves())

    def height(self):
        """
        Return the height of the tree.

            >>> t = Tree("(S (NP (D the) (N dog)) (VP (V chased) (NP (D the) (N cat))))")
            >>> t.height()
            5
            >>> print(t[0,0])
            (D the)
            >>> t[0,0].height()
            2

        :return: The height of this tree.  The height of a tree
            containing no children is 1; the height of a tree
            containing only leaves is 2; and the height of any other
            tree is one plus the maximum of its children's
            heights.
        :rtype: int
        """
        max_child_height = 0
        for child in self:
            if isinstance(child, Tree):
                max_child_height = max(max_child_height, child.height())
            else:
                max_child_height = max(max_child_height, 1)
        return 1 + max_child_height

    def treepositions(self, order='preorder'):
        """
            >>> t = Tree("(S (NP (D the) (N dog)) (VP (V chased) (NP (D the) (N cat))))")
            >>> t.treepositions() # doctest: +ELLIPSIS
            [(), (0,), (0, 0), (0, 0, 0), (0, 1), (0, 1, 0), (1,), (1, 0), (1, 0, 0), ...]
            >>> for pos in t.treepositions('leaves'):
            ...     t[pos] = t[pos][::-1].upper()
            >>> print(t)
            (S (NP (D EHT) (N GOD)) (VP (V DESAHC) (NP (D EHT) (N TAC))))

        :param order: One of: ``preorder``, ``postorder``, ``bothorder``,
            ``leaves``.
        """
        positions = []
        if order in ('preorder', 'bothorder'): positions.append( () )
        for i, child in enumerate(self):
            if isinstance(child, Tree):
                childpos = child.treepositions(order)
                positions.extend((i,)+p for p in childpos)
            else:
                positions.append( (i,) )
        if order in ('postorder', 'bothorder'): positions.append( () )
        return positions

    def subtrees(self, filter=None):
        """
        Generate all the subtrees of this tree, optionally restricted
        to trees matching the filter function.

            >>> t = Tree("(S (NP (D the) (N dog)) (VP (V chased) (NP (D the) (N cat))))")
            >>> for s in t.subtrees(lambda t: t.height() == 2):
            ...     print(s)
            (D the)
            (N dog)
            (V chased)
            (D the)
            (N cat)

        :type filter: function
        :param filter: the function to filter all local trees
        """
        if not filter or filter(self):
            yield self
        for child in self:
            if isinstance(child, Tree):
                for subtree in child.subtrees(filter):
                    yield subtree

    def productions(self):
        """
        Generate the productions that correspond to the non-terminal nodes of the tree.
        For each subtree of the form (P: C1 C2 ... Cn) this produces a production of the
        form P -> C1 C2 ... Cn.

            >>> t = Tree("(S (NP (D the) (N dog)) (VP (V chased) (NP (D the) (N cat))))")
            >>> t.productions()
            [S -> NP VP, NP -> D N, D -> 'the', N -> 'dog', VP -> V NP, V -> 'chased',
            NP -> D N, D -> 'the', N -> 'cat']

        :rtype: list(Production)
        """

        if not isinstance(self.node, string_types):
            raise TypeError('Productions can only be generated from trees having node labels that are strings')

        prods = [Production(Nonterminal(self.node), _child_names(self))]
        for child in self:
            if isinstance(child, Tree):
                prods += child.productions()
        return prods

    def pos(self):
        """
        Return a sequence of pos-tagged words extracted from the tree.

            >>> t = Tree("(S (NP (D the) (N dog)) (VP (V chased) (NP (D the) (N cat))))")
            >>> t.pos()
            [('the', 'D'), ('dog', 'N'), ('chased', 'V'), ('the', 'D'), ('cat', 'N')]

        :return: a list of tuples containing leaves and pre-terminals (part-of-speech tags).
            The order reflects the order of the leaves in the tree's hierarchical structure.
        :rtype: list(tuple)
        """
        pos = []
        for child in self:
            if isinstance(child, Tree):
                pos.extend(child.pos())
            else:
                pos.append((child, self.node))
        return pos

    def leaf_treeposition(self, index):
        """
        :return: The tree position of the ``index``-th leaf in this
            tree.  I.e., if ``tp=self.leaf_treeposition(i)``, then
            ``self[tp]==self.leaves()[i]``.

        :raise IndexError: If this tree contains fewer than ``index+1``
            leaves, or if ``index<0``.
        """
        if index < 0: raise IndexError('index must be non-negative')

        stack = [(self, ())]
        while stack:
            value, treepos = stack.pop()
            if not isinstance(value, Tree):
                if index == 0: return treepos
                else: index -= 1
            else:
                for i in range(len(value)-1, -1, -1):
                    stack.append( (value[i], treepos+(i,)) )

        raise IndexError('index must be less than or equal to len(self)')

    def treeposition_spanning_leaves(self, start, end):
        """
        :return: The tree position of the lowest descendant of this
            tree that dominates ``self.leaves()[start:end]``.
        :raise ValueError: if ``end <= start``
        """
        if end <= start:
            raise ValueError('end must be greater than start')
        # Find the tree positions of the start & end leaves, and
        # take the longest common subsequence.
        start_treepos = self.leaf_treeposition(start)
        end_treepos = self.leaf_treeposition(end-1)
        # Find the first index where they mismatch:
        for i in range(len(start_treepos)):
            if i == len(end_treepos) or start_treepos[i] != end_treepos[i]:
                return start_treepos[:i]
        return start_treepos

    #////////////////////////////////////////////////////////////
    # Transforms
    #////////////////////////////////////////////////////////////

    def chomsky_normal_form(self, factor = "right", horzMarkov = None, vertMarkov = 0, childChar = "|", parentChar = "^"):
        """
        This method can modify a tree in three ways:

          1. Convert a tree into its Chomsky Normal Form (CNF)
             equivalent -- Every subtree has either two non-terminals
             or one terminal as its children.  This process requires
             the creation of more"artificial" non-terminal nodes.
          2. Markov (vertical) smoothing of children in new artificial
             nodes
          3. Horizontal (parent) annotation of nodes

        :param factor: Right or left factoring method (default = "right")
        :type  factor: str = [left|right]
        :param horzMarkov: Markov order for sibling smoothing in artificial nodes (None (default) = include all siblings)
        :type  horzMarkov: int | None
        :param vertMarkov: Markov order for parent smoothing (0 (default) = no vertical annotation)
        :type  vertMarkov: int | None
        :param childChar: A string used in construction of the artificial nodes, separating the head of the
                          original subtree from the child nodes that have yet to be expanded (default = "|")
        :type  childChar: str
        :param parentChar: A string used to separate the node representation from its vertical annotation
        :type  parentChar: str
        """
        from .treetransforms import chomsky_normal_form
        chomsky_normal_form(self, factor, horzMarkov, vertMarkov, childChar, parentChar)

    def un_chomsky_normal_form(self, expandUnary = True, childChar = "|", parentChar = "^", unaryChar = "+"):
        """
        This method modifies the tree in three ways:

          1. Transforms a tree in Chomsky Normal Form back to its
             original structure (branching greater than two)
          2. Removes any parent annotation (if it exists)
          3. (optional) expands unary subtrees (if previously
             collapsed with collapseUnary(...) )

        :param expandUnary: Flag to expand unary or not (default = True)
        :type  expandUnary: bool
        :param childChar: A string separating the head node from its children in an artificial node (default = "|")
        :type  childChar: str
        :param parentChar: A sting separating the node label from its parent annotation (default = "^")
        :type  parentChar: str
        :param unaryChar: A string joining two non-terminals in a unary production (default = "+")
        :type  unaryChar: str
        """
        from .treetransforms import un_chomsky_normal_form
        un_chomsky_normal_form(self, expandUnary, childChar, parentChar, unaryChar)

    def collapse_unary(self, collapsePOS = False, collapseRoot = False, joinChar = "+"):
        """
        Collapse subtrees with a single child (ie. unary productions)
        into a new non-terminal (Tree node) joined by 'joinChar'.
        This is useful when working with algorithms that do not allow
        unary productions, and completely removing the unary productions
        would require loss of useful information.  The Tree is modified
        directly (since it is passed by reference) and no value is returned.

        :param collapsePOS: 'False' (default) will not collapse the parent of leaf nodes (ie.
                            Part-of-Speech tags) since they are always unary productions
        :type  collapsePOS: bool
        :param collapseRoot: 'False' (default) will not modify the root production
                             if it is unary.  For the Penn WSJ treebank corpus, this corresponds
                             to the TOP -> productions.
        :type collapseRoot: bool
        :param joinChar: A string used to connect collapsed node values (default = "+")
        :type  joinChar: str
        """
        from .treetransforms import collapse_unary
        collapse_unary(self, collapsePOS, collapseRoot, joinChar)

    #////////////////////////////////////////////////////////////
    # Convert, copy
    #////////////////////////////////////////////////////////////

    @classmethod
    def convert(cls, tree):
        """
        Convert a tree between different subtypes of Tree.  ``cls`` determines
        which class will be used to encode the new tree.

        :type tree: Tree
        :param tree: The tree that should be converted.
        :return: The new Tree.
        """
        if isinstance(tree, Tree):
            children = [cls.convert(child) for child in tree]
            return cls(tree.node, children)
        else:
            return tree

    def copy(self, deep=False):
        if not deep: return type(self)(self.node, self)
        else: return type(self).convert(self)

    def _frozen_class(self): return ImmutableTree
    def freeze(self, leaf_freezer=None):
        frozen_class = self._frozen_class()
        if leaf_freezer is None:
            newcopy = frozen_class.convert(self)
        else:
            newcopy = self.copy(deep=True)
            for pos in newcopy.treepositions('leaves'):
                newcopy[pos] = leaf_freezer(newcopy[pos])
            newcopy = frozen_class.convert(newcopy)
        hash(newcopy) # Make sure the leaves are hashable.
        return newcopy

    #////////////////////////////////////////////////////////////
    # Parsing
    #////////////////////////////////////////////////////////////

    @classmethod
    def parse(cls, s, brackets='()', parse_node=None, parse_leaf=None,
              node_pattern=None, leaf_pattern=None,
              remove_empty_top_bracketing=False):
        """
        Parse a bracketed tree string and return the resulting tree.
        Trees are represented as nested brackettings, such as::

          (S (NP (NNP John)) (VP (V runs)))

        :type s: str
        :param s: The string to parse

        :type brackets: str (length=2)
        :param brackets: The bracket characters used to mark the
            beginning and end of trees and subtrees.

        :type parse_node: function
        :type parse_leaf: function
        :param parse_node, parse_leaf: If specified, these functions
            are applied to the substrings of ``s`` corresponding to
            nodes and leaves (respectively) to obtain the values for
            those nodes and leaves.  They should have the following
            signature:

               parse_node(str) -> value

            For example, these functions could be used to parse nodes
            and leaves whose values should be some type other than
            string (such as ``FeatStruct``).
            Note that by default, node strings and leaf strings are
            delimited by whitespace and brackets; to override this
            default, use the ``node_pattern`` and ``leaf_pattern``
            arguments.

        :type node_pattern: str
        :type leaf_pattern: str
        :param node_pattern, leaf_pattern: Regular expression patterns
            used to find node and leaf substrings in ``s``.  By
            default, both nodes patterns are defined to match any
            sequence of non-whitespace non-bracket characters.

        :type remove_empty_top_bracketing: bool
        :param remove_empty_top_bracketing: If the resulting tree has
            an empty node label, and is length one, then return its
            single child instead.  This is useful for treebank trees,
            which sometimes contain an extra level of bracketing.

        :return: A tree corresponding to the string representation ``s``.
            If this class method is called using a subclass of Tree,
            then it will return a tree of that type.
        :rtype: Tree
        """
        if not isinstance(brackets, string_types) or len(brackets) != 2:
            raise TypeError('brackets must be a length-2 string')
        if re.search('\s', brackets):
            raise TypeError('whitespace brackets not allowed')
        # Construct a regexp that will tokenize the string.
        open_b, close_b = brackets
        open_pattern, close_pattern = (re.escape(open_b), re.escape(close_b))
        if node_pattern is None:
            node_pattern = '[^\s%s%s]+' % (open_pattern, close_pattern)
        if leaf_pattern is None:
            leaf_pattern = '[^\s%s%s]+' % (open_pattern, close_pattern)
        token_re = re.compile('%s\s*(%s)?|%s|(%s)' % (
            open_pattern, node_pattern, close_pattern, leaf_pattern))
        # Walk through each token, updating a stack of trees.
        stack = [(None, [])] # list of (node, children) tuples
        for match in token_re.finditer(s):
            token = match.group()
            # Beginning of a tree/subtree
            if token[0] == open_b:
                if len(stack) == 1 and len(stack[0][1]) > 0:
                    cls._parse_error(s, match, 'end-of-string')
                node = token[1:].lstrip()
                if parse_node is not None: node = parse_node(node)
                stack.append((node, []))
            # End of a tree/subtree
            elif token == close_b:
                if len(stack) == 1:
                    if len(stack[0][1]) == 0:
                        cls._parse_error(s, match, open_b)
                    else:
                        cls._parse_error(s, match, 'end-of-string')
                node, children = stack.pop()
                stack[-1][1].append(cls(node, children))
            # Leaf node
            else:
                if len(stack) == 1:
                    cls._parse_error(s, match, open_b)
                if parse_leaf is not None: token = parse_leaf(token)
                stack[-1][1].append(token)

        # check that we got exactly one complete tree.
        if len(stack) > 1:
            cls._parse_error(s, 'end-of-string', close_b)
        elif len(stack[0][1]) == 0:
            cls._parse_error(s, 'end-of-string', open_b)
        else:
            assert stack[0][0] is None
            assert len(stack[0][1]) == 1
        tree = stack[0][1][0]

        # If the tree has an extra level with node='', then get rid of
        # it.  E.g.: "((S (NP ...) (VP ...)))"
        if remove_empty_top_bracketing and tree.node == '' and len(tree) == 1:
            tree = tree[0]
        # return the tree.
        return tree

    @classmethod
    def _parse_error(cls, s, match, expecting):
        """
        Display a friendly error message when parsing a tree string fails.
        :param s: The string we're parsing.
        :param match: regexp match of the problem token.
        :param expecting: what we expected to see instead.
        """
        # Construct a basic error message
        if match == 'end-of-string':
            pos, token = len(s), 'end-of-string'
        else:
            pos, token = match.start(), match.group()
        msg = '%s.parse(): expected %r but got %r\n%sat index %d.' % (
            cls.__name__, expecting, token, ' '*12, pos)
        # Add a display showing the error token itsels:
        s = s.replace('\n', ' ').replace('\t', ' ')
        offset = pos
        if len(s) > pos+10:
            s = s[:pos+10]+'...'
        if pos > 10:
            s = '...'+s[pos-10:]
            offset = 13
        msg += '\n%s"%s"\n%s^' % (' '*16, s, ' '*(17+offset))
        raise ValueError(msg)

    #////////////////////////////////////////////////////////////
    # Visualization & String Representation
    #////////////////////////////////////////////////////////////

    def draw(self):
        """
        Open a new window containing a graphical diagram of this tree.
        """
        from nltk.draw.tree import draw_trees
        draw_trees(self)

    def __repr__(self):
        childstr = ", ".join(unicode_repr(c) for c in self)
        return '%s(%s, [%s])' % (type(self).__name__, unicode_repr(self.node), childstr)

    def __str__(self):
        return self.pprint()

    def pprint(self, margin=70, indent=0, nodesep='', parens='()', quotes=False):
        """
        :return: A pretty-printed string representation of this tree.
        :rtype: str
        :param margin: The right margin at which to do line-wrapping.
        :type margin: int
        :param indent: The indentation level at which printing
            begins.  This number is used to decide how far to indent
            subsequent lines.
        :type indent: int
        :param nodesep: A string that is used to separate the node
            from the children.  E.g., the default value ``':'`` gives
            trees like ``(S: (NP: I) (VP: (V: saw) (NP: it)))``.
        """

        # Try writing it on one line.
        s = self._pprint_flat(nodesep, parens, quotes)
        if len(s)+indent < margin:
            return s

        # If it doesn't fit on one line, then write it on multi-lines.
        if isinstance(self.node, string_types):
            s = '%s%s%s' % (parens[0], self.node, nodesep)
        else:
            s = '%s%s%s' % (parens[0], unicode_repr(self.node), nodesep)
        for child in self:
            if isinstance(child, Tree):
                s += '\n'+' '*(indent+2)+child.pprint(margin, indent+2,
                                                  nodesep, parens, quotes)
            elif isinstance(child, tuple):
                s += '\n'+' '*(indent+2)+ "/".join(child)
            elif isinstance(child, string_types) and not quotes:
                s += '\n'+' '*(indent+2)+ '%s' % child
            else:
                s += '\n'+' '*(indent+2)+ unicode_repr(child)
        return s+parens[1]

    def pprint_latex_qtree(self):
        r"""
        Returns a representation of the tree compatible with the
        LaTeX qtree package. This consists of the string ``\Tree``
        followed by the parse tree represented in bracketed notation.

        For example, the following result was generated from a parse tree of
        the sentence ``The announcement astounded us``::

          \Tree [.I'' [.N'' [.D The ] [.N' [.N announcement ] ] ]
              [.I' [.V'' [.V' [.V astounded ] [.N'' [.N' [.N us ] ] ] ] ] ] ]

        See http://www.ling.upenn.edu/advice/latex.html for the LaTeX
        style file for the qtree package.

        :return: A latex qtree representation of this tree.
        :rtype: str
        """
        reserved_chars = re.compile('([#\$%&~_\{\}])')

        pprint = self.pprint(indent=6, nodesep='', parens=('[.', ' ]'))
        return r'\Tree ' + re.sub(reserved_chars, r'\\\1', pprint)

    def _pprint_flat(self, nodesep, parens, quotes):
        childstrs = []
        for child in self:
            if isinstance(child, Tree):
                childstrs.append(child._pprint_flat(nodesep, parens, quotes))
            elif isinstance(child, tuple):
                childstrs.append("/".join(child))
            elif isinstance(child, string_types) and not quotes:
                childstrs.append('%s' % child)
            else:
                childstrs.append(unicode_repr(child))
        if isinstance(self.node, string_types):
            return '%s%s%s %s%s' % (parens[0], self.node, nodesep,
                                    " ".join(childstrs), parens[1])
        else:
            return '%s%s%s %s%s' % (parens[0], unicode_repr(self.node), nodesep,
                                    " ".join(childstrs), parens[1])


class ImmutableTree(Tree):
    def __init__(self, node_or_str, children=None):
        super(ImmutableTree, self).__init__(node_or_str, children)
        # Precompute our hash value.  This ensures that we're really
        # immutable.  It also means we only have to calculate it once.
        try:
            self._hash = hash((self.node, tuple(self)))
        except (TypeError, ValueError):
            raise ValueError("%s: node value and children "
                             "must be immutable" % type(self).__name__)

    def __setitem__(self, index, value):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def __setslice__(self, i, j, value):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def __delitem__(self, index):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def __delslice__(self, i, j):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def __iadd__(self, other):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def __imul__(self, other):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def append(self, v):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def extend(self, v):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def pop(self, v=None):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def remove(self, v):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def reverse(self):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def sort(self):
        raise ValueError('%s may not be modified' % type(self).__name__)
    def __hash__(self):
        return self._hash

    def _get_node(self):
        """Get the node value"""
        return self._node
    def _set_node(self, value):
        """
        Set the node value.  This will only succeed the first time the
        node value is set, which should occur in ImmutableTree.__init__().
        """
        if hasattr(self, 'node'):
            raise ValueError('%s may not be modified' % type(self).__name__)
        self._node = value
    node = property(_get_node, _set_node)


######################################################################
## Parented trees
######################################################################

class AbstractParentedTree(Tree):
    """
    An abstract base class for a ``Tree`` that automatically maintains
    pointers to parent nodes.  These parent pointers are updated
    whenever any change is made to a tree's structure.  Two subclasses
    are currently defined:

      - ``ParentedTree`` is used for tree structures where each subtree
        has at most one parent.  This class should be used in cases
        where there is no"sharing" of subtrees.

      - ``MultiParentedTree`` is used for tree structures where a
        subtree may have zero or more parents.  This class should be
        used in cases where subtrees may be shared.

    Subclassing
    ===========
    The ``AbstractParentedTree`` class redefines all operations that
    modify a tree's structure to call two methods, which are used by
    subclasses to update parent information:

      - ``_setparent()`` is called whenever a new child is added.
      - ``_delparent()`` is called whenever a child is removed.
    """

    def __init__(self, node_or_str, children=None):
        super(AbstractParentedTree, self).__init__(node_or_str, children)
        # If children is None, the tree is parsed from node_or_str, and
        # all parents will be set during parsing.
        if children is not None:
            # Otherwise we have to set the parent of the children.
            # Iterate over self, and *not* children, because children
            # might be an iterator.
            for i, child in enumerate(self):
                if isinstance(child, Tree):
                    self._setparent(child, i, dry_run=True)
            for i, child in enumerate(self):
                if isinstance(child, Tree):
                    self._setparent(child, i)

    #////////////////////////////////////////////////////////////
    # Parent management
    #////////////////////////////////////////////////////////////

    def _setparent(self, child, index, dry_run=False):
        """
        Update the parent pointer of ``child`` to point to ``self``.  This
        method is only called if the type of ``child`` is ``Tree``;
        i.e., it is not called when adding a leaf to a tree.  This method
        is always called before the child is actually added to the
        child list of ``self``.

        :type child: Tree
        :type index: int
        :param index: The index of ``child`` in ``self``.
        :raise TypeError: If ``child`` is a tree with an impropriate
            type.  Typically, if ``child`` is a tree, then its type needs
            to match the type of ``self``.  This prevents mixing of
            different tree types (single-parented, multi-parented, and
            non-parented).
        :param dry_run: If true, the don't actually set the child's
            parent pointer; just check for any error conditions, and
            raise an exception if one is found.
        """
        raise NotImplementedError()

    def _delparent(self, child, index):
        """
        Update the parent pointer of ``child`` to not point to self.  This
        method is only called if the type of ``child`` is ``Tree``; i.e., it
        is not called when removing a leaf from a tree.  This method
        is always called before the child is actually removed from the
        child list of ``self``.

        :type child: Tree
        :type index: int
        :param index: The index of ``child`` in ``self``.
        """
        raise NotImplementedError()

    #////////////////////////////////////////////////////////////
    # Methods that add/remove children
    #////////////////////////////////////////////////////////////
    # Every method that adds or removes a child must make
    # appropriate calls to _setparent() and _delparent().

    def __delitem__(self, index):
        # del ptree[start:stop]
        if isinstance(index, slice):
            start, stop, step = slice_bounds(self, index, allow_step=True)
            # Clear all the children pointers.
            for i in range(start, stop, step):
                if isinstance(self[i], Tree):
                    self._delparent(self[i], i)
            # Delete the children from our child list.
            super(AbstractParentedTree, self).__delitem__(index)

        # del ptree[i]
        elif isinstance(index, int):
            if index < 0: index += len(self)
            if index < 0: raise IndexError('index out of range')
            # Clear the child's parent pointer.
            if isinstance(self[index], Tree):
                self._delparent(self[index], index)
            # Remove the child from our child list.
            super(AbstractParentedTree, self).__delitem__(index)

        elif isinstance(index, (list, tuple)):
            # del ptree[()]
            if len(index) == 0:
                raise IndexError('The tree position () may not be deleted.')
            # del ptree[(i,)]
            elif len(index) == 1:
                del self[index[0]]
            # del ptree[i1, i2, i3]
            else:
                del self[index[0]][index[1:]]

        else:
            raise TypeError("%s indices must be integers, not %s" %
                            (type(self).__name__, type(index).__name__))

    def __setitem__(self, index, value):
        # ptree[start:stop] = value
        if isinstance(index, slice):
            start, stop, step = slice_bounds(self, index, allow_step=True)
            # make a copy of value, in case it's an iterator
            if not isinstance(value, (list, tuple)):
                value = list(value)
            # Check for any error conditions, so we can avoid ending
            # up in an inconsistent state if an error does occur.
            for i, child in enumerate(value):
                if isinstance(child, Tree):
                    self._setparent(child, start + i*step, dry_run=True)
            # clear the child pointers of all parents we're removing
            for i in range(start, stop, step):
                if isinstance(self[i], Tree):
                    self._delparent(self[i], i)
            # set the child pointers of the new children.  We do this
            # after clearing *all* child pointers, in case we're e.g.
            # reversing the elements in a tree.
            for i, child in enumerate(value):
                if isinstance(child, Tree):
                    self._setparent(child, start + i*step)
            # finally, update the content of the child list itself.
            super(AbstractParentedTree, self).__setitem__(index, value)

        # ptree[i] = value
        elif isinstance(index, int):
            if index < 0: index += len(self)
            if index < 0: raise IndexError('index out of range')
            # if the value is not changing, do nothing.
            if value is self[index]:
                return
            # Set the new child's parent pointer.
            if isinstance(value, Tree):
                self._setparent(value, index)
            # Remove the old child's parent pointer
            if isinstance(self[index], Tree):
                self._delparent(self[index], index)
            # Update our child list.
            super(AbstractParentedTree, self).__setitem__(index, value)

        elif isinstance(index, (list, tuple)):
            # ptree[()] = value
            if len(index) == 0:
                raise IndexError('The tree position () may not be assigned to.')
            # ptree[(i,)] = value
            elif len(index) == 1:
                self[index[0]] = value
            # ptree[i1, i2, i3] = value
            else:
                self[index[0]][index[1:]] = value

        else:
            raise TypeError("%s indices must be integers, not %s" %
                            (type(self).__name__, type(index).__name__))

    def append(self, child):
        if isinstance(child, Tree):
            self._setparent(child, len(self))
        super(AbstractParentedTree, self).append(child)

    def extend(self, children):
        for child in children:
            if isinstance(child, Tree):
                self._setparent(child, len(self))
            super(AbstractParentedTree, self).append(child)

    def insert(self, index, child):
        # Handle negative indexes.  Note that if index < -len(self),
        # we do *not* raise an IndexError, unlike __getitem__.  This
        # is done for consistency with list.__getitem__ and list.index.
        if index < 0: index += len(self)
        if index < 0: index = 0
        # Set the child's parent, and update our child list.
        if isinstance(child, Tree):
            self._setparent(child, index)
        super(AbstractParentedTree, self).insert(index, child)

    def pop(self, index=-1):
        if index < 0: index += len(self)
        if index < 0: raise IndexError('index out of range')
        if isinstance(self[index], Tree):
            self._delparent(self[index], index)
        return super(AbstractParentedTree, self).pop(index)

    # n.b.: like `list`, this is done by equality, not identity!
    # To remove a specific child, use del ptree[i].
    def remove(self, child):
        index = self.index(child)
        if isinstance(self[index], Tree):
            self._delparent(self[index], index)
        super(AbstractParentedTree, self).remove(child)

    # We need to implement __getslice__ and friends, even though
    # they're deprecated, because otherwise list.__getslice__ will get
    # called (since we're subclassing from list).  Just delegate to
    # __getitem__ etc., but use max(0, start) and max(0, stop) because
    # because negative indices are already handled *before*
    # __getslice__ is called; and we don't want to double-count them.
    if hasattr(list, '__getslice__'):
        def __getslice__(self, start, stop):
            return self.__getitem__(slice(max(0, start), max(0, stop)))
        def __delslice__(self, start, stop):
            return self.__delitem__(slice(max(0, start), max(0, stop)))
        def __setslice__(self, start, stop, value):
            return self.__setitem__(slice(max(0, start), max(0, stop)), value)

class ParentedTree(AbstractParentedTree):
    """
    A ``Tree`` that automatically maintains parent pointers for
    single-parented trees.  The following are methods for querying
    the structure of a parented tree: ``parent``, ``parent_index``,
    ``left_sibling``, ``right_sibling``, ``root``, ``treeposition``.

    Each ``ParentedTree`` may have at most one parent.  In
    particular, subtrees may not be shared.  Any attempt to reuse a
    single ``ParentedTree`` as a child of more than one parent (or
    as multiple children of the same parent) will cause a
    ``ValueError`` exception to be raised.

    ``ParentedTrees`` should never be used in the same tree as ``Trees``
    or ``MultiParentedTrees``.  Mixing tree implementations may result
    in incorrect parent pointers and in ``TypeError`` exceptions.
    """
    def __init__(self, node_or_str, children=None):
        self._parent = None
        """The parent of this Tree, or None if it has no parent."""
        super(ParentedTree, self).__init__(node_or_str, children)
        if children is None:
            # If children is None, the tree is parsed from node_or_str.
            # After parsing, the parent of the immediate children
            # will point to an intermediate tree, not self.
            # We fix this by brute force:
            for i, child in enumerate(self):
                if isinstance(child, Tree):
                    child._parent = None
                    self._setparent(child, i)

    def _frozen_class(self): return ImmutableParentedTree

    #/////////////////////////////////////////////////////////////////
    # Methods
    #/////////////////////////////////////////////////////////////////

    def parent(self):
        """The parent of this tree, or None if it has no parent."""
        return self._parent

    def parent_index(self):
        """
        The index of this tree in its parent.  I.e.,
        ``ptree.parent()[ptree.parent_index()] is ptree``.  Note that
        ``ptree.parent_index()`` is not necessarily equal to
        ``ptree.parent.index(ptree)``, since the ``index()`` method
        returns the first child that is equal to its argument.
        """
        if self._parent is None: return None
        for i, child in enumerate(self._parent):
            if child is self: return i
        assert False, 'expected to find self in self._parent!'

    def left_sibling(self):
        """The left sibling of this tree, or None if it has none."""
        parent_index = self.parent_index()
        if self._parent and parent_index > 0:
            return self._parent[parent_index-1]
        return None # no left sibling

    def right_sibling(self):
        """The right sibling of this tree, or None if it has none."""
        parent_index = self.parent_index()
        if self._parent and parent_index < (len(self._parent)-1):
            return self._parent[parent_index+1]
        return None # no right sibling

    def root(self):
        """
        The root of this tree.  I.e., the unique ancestor of this tree
        whose parent is None.  If ``ptree.parent()`` is None, then
        ``ptree`` is its own root.
        """
        root = self
        while root.parent() is not None:
            root = root.parent()
        return root

    def treeposition(self):
        """
        The tree position of this tree, relative to the root of the
        tree.  I.e., ``ptree.root[ptree.treeposition] is ptree``.
        """
        if self.parent() is None:
            return ()
        else:
            return self.parent().treeposition() + (self.parent_index(),)


    #/////////////////////////////////////////////////////////////////
    # Parent Management
    #/////////////////////////////////////////////////////////////////

    def _delparent(self, child, index):
        # Sanity checks
        assert isinstance(child, ParentedTree)
        assert self[index] is child
        assert child._parent is self

        # Delete child's parent pointer.
        child._parent = None

    def _setparent(self, child, index, dry_run=False):
        # If the child's type is incorrect, then complain.
        if not isinstance(child, ParentedTree):
            raise TypeError('Can not insert a non-ParentedTree '+
                            'into a ParentedTree')

        # If child already has a parent, then complain.
        if child._parent is not None:
            raise ValueError('Can not insert a subtree that already '
                             'has a parent.')

        # Set child's parent pointer & index.
        if not dry_run:
            child._parent = self


class MultiParentedTree(AbstractParentedTree):
    """
    A ``Tree`` that automatically maintains parent pointers for
    multi-parented trees.  The following are methods for querying the
    structure of a multi-parented tree: ``parents()``, ``parent_indices()``,
    ``left_siblings()``, ``right_siblings()``, ``roots``, ``treepositions``.

    Each ``MultiParentedTree`` may have zero or more parents.  In
    particular, subtrees may be shared.  If a single
    ``MultiParentedTree`` is used as multiple children of the same
    parent, then that parent will appear multiple times in its
    ``parents()`` method.

    ``MultiParentedTrees`` should never be used in the same tree as
    ``Trees`` or ``ParentedTrees``.  Mixing tree implementations may
    result in incorrect parent pointers and in ``TypeError`` exceptions.
    """
    def __init__(self, node_or_str, children=None):
        self._parents = []
        """A list of this tree's parents.  This list should not
           contain duplicates, even if a parent contains this tree
           multiple times."""
        super(MultiParentedTree, self).__init__(node_or_str, children)
        if children is None:
            # If children is None, the tree is parsed from node_or_str.
            # After parsing, the parent(s) of the immediate children
            # will point to an intermediate tree, not self.
            # We fix this by brute force:
            for i, child in enumerate(self):
                if isinstance(child, Tree):
                    child._parents = []
                    self._setparent(child, i)

    def _frozen_class(self): return ImmutableMultiParentedTree

    #/////////////////////////////////////////////////////////////////
    # Methods
    #/////////////////////////////////////////////////////////////////

    def parents(self):
        """
        The set of parents of this tree.  If this tree has no parents,
        then ``parents`` is the empty set.  To check if a tree is used
        as multiple children of the same parent, use the
        ``parent_indices()`` method.

        :type: list(MultiParentedTree)
        """
        return list(self._parents)

    def left_siblings(self):
        """
        A list of all left siblings of this tree, in any of its parent
        trees.  A tree may be its own left sibling if it is used as
        multiple contiguous children of the same parent.  A tree may
        appear multiple times in this list if it is the left sibling
        of this tree with respect to multiple parents.

        :type: list(MultiParentedTree)
        """
        return [parent[index-1]
                for (parent, index) in self._get_parent_indices()
                if index > 0]

    def right_siblings(self):
        """
        A list of all right siblings of this tree, in any of its parent
        trees.  A tree may be its own right sibling if it is used as
        multiple contiguous children of the same parent.  A tree may
        appear multiple times in this list if it is the right sibling
        of this tree with respect to multiple parents.

        :type: list(MultiParentedTree)
        """
        return [parent[index+1]
                for (parent, index) in self._get_parent_indices()
                if index < (len(parent)-1)]

    def _get_parent_indices(self):
        return [(parent, index)
                for parent in self._parents
                for index, child in enumerate(parent)
                if child is self]

    def roots(self):
        """
        The set of all roots of this tree.  This set is formed by
        tracing all possible parent paths until trees with no parents
        are found.

        :type: list(MultiParentedTree)
        """
        return list(self._get_roots_helper({}).values())

    def _get_roots_helper(self, result):
        if self._parents:
            for parent in self._parents:
                parent._get_roots_helper(result)
        else:
            result[id(self)] = self
        return result

    def parent_indices(self, parent):
        """
        Return a list of the indices where this tree occurs as a child
        of ``parent``.  If this child does not occur as a child of
        ``parent``, then the empty list is returned.  The following is
        always true::

          for parent_index in ptree.parent_indices(parent):
              parent[parent_index] is ptree
        """
        if parent not in self._parents: return []
        else: return [index for (index, child) in enumerate(parent)
                      if child is self]

    def treepositions(self, root):
        """
        Return a list of all tree positions that can be used to reach
        this multi-parented tree starting from ``root``.  I.e., the
        following is always true::

          for treepos in ptree.treepositions(root):
              root[treepos] is ptree
        """
        if self is root:
            return [()]
        else:
            return [treepos+(index,)
                    for parent in self._parents
                    for treepos in parent.treepositions(root)
                    for (index, child) in enumerate(parent) if child is self]


    #/////////////////////////////////////////////////////////////////
    # Parent Management
    #/////////////////////////////////////////////////////////////////

    def _delparent(self, child, index):
        # Sanity checks
        assert isinstance(child, MultiParentedTree)
        assert self[index] is child
        assert len([p for p in child._parents if p is self]) == 1

        # If the only copy of child in self is at index, then delete
        # self from child's parent list.
        for i, c in enumerate(self):
            if c is child and i != index: break
        else:
            child._parents.remove(self)

    def _setparent(self, child, index, dry_run=False):
        # If the child's type is incorrect, then complain.
        if not isinstance(child, MultiParentedTree):
            raise TypeError('Can not insert a non-MultiParentedTree '+
                            'into a MultiParentedTree')

        # Add self as a parent pointer if it's not already listed.
        if not dry_run:
            for parent in child._parents:
                if parent is self: break
            else:
                child._parents.append(self)

class ImmutableParentedTree(ImmutableTree, ParentedTree):
    pass

class ImmutableMultiParentedTree(ImmutableTree, MultiParentedTree):
    pass


######################################################################
## Probabilistic trees
######################################################################

@python_2_unicode_compatible
class ProbabilisticTree(Tree, ProbabilisticMixIn):
    def __init__(self, node_or_str, children=None, **prob_kwargs):
        Tree.__init__(self, node_or_str, children)
        ProbabilisticMixIn.__init__(self, **prob_kwargs)

    # We have to patch up these methods to make them work right:
    def _frozen_class(self): return ImmutableProbabilisticTree
    def __repr__(self):
        return '%s (p=%r)' % (Tree.unicode_repr(self), self.prob())
    def __str__(self):
        return '%s (p=%.6g)' % (self.pprint(margin=60), self.prob())
    def copy(self, deep=False):
        if not deep: return type(self)(self.node, self, prob=self.prob())
        else: return type(self).convert(self)
    @classmethod
    def convert(cls, val):
        if isinstance(val, Tree):
            children = [cls.convert(child) for child in val]
            if isinstance(val, ProbabilisticMixIn):
                return cls(val.node, children, prob=val.prob())
            else:
                return cls(val.node, children, prob=1.0)
        else:
            return val

    def __eq__(self, other):
        return (self.__class__ is other.__class__ and
                (self.node, list(self), self.prob()) ==
                (other.node, list(other), other.prob()))

    def __lt__(self, other):
        if not isinstance(other, Tree):
            raise_unorderable_types("<", self, other)
        if self.__class__ is other.__class__:
            return ((self.node, list(self), self.prob()) <
                    (other.node, list(other), other.prob()))
        else:
            return self.__class__.__name__ < other.__class__.__name__


@python_2_unicode_compatible
class ImmutableProbabilisticTree(ImmutableTree, ProbabilisticMixIn):
    def __init__(self, node_or_str, children=None, **prob_kwargs):
        ImmutableTree.__init__(self, node_or_str, children)
        ProbabilisticMixIn.__init__(self, **prob_kwargs)
        self._hash = hash((self.node, tuple(self), self.prob()))

    # We have to patch up these methods to make them work right:
    def _frozen_class(self): return ImmutableProbabilisticTree
    def __repr__(self):
        return '%s [%s]' % (Tree.unicode_repr(self), self.prob())
    def __str__(self):
        return '%s [%s]' % (self.pprint(margin=60), self.prob())
    def copy(self, deep=False):
        if not deep: return type(self)(self.node, self, prob=self.prob())
        else: return type(self).convert(self)
    @classmethod
    def convert(cls, val):
        if isinstance(val, Tree):
            children = [cls.convert(child) for child in val]
            if isinstance(val, ProbabilisticMixIn):
                return cls(val.node, children, prob=val.prob())
            else:
                return cls(val.node, children, prob=1.0)
        else:
            return val


def _child_names(tree):
    names = []
    for child in tree:
        if isinstance(child, Tree):
            names.append(Nonterminal(child.node))
        else:
            names.append(child)
    return names

######################################################################
## Parsing
######################################################################

def bracket_parse(s):
    """
    Use Tree.parse(s, remove_empty_top_bracketing=True) instead.
    """
    raise NameError("Use Tree.parse(s, remove_empty_top_bracketing=True) instead.")

def sinica_parse(s):
    """
    Parse a Sinica Treebank string and return a tree.  Trees are represented as nested brackettings,
    as shown in the following example (X represents a Chinese character):
    S(goal:NP(Head:Nep:XX)|theme:NP(Head:Nhaa:X)|quantity:Dab:X|Head:VL2:X)#0(PERIODCATEGORY)

    :return: A tree corresponding to the string representation.
    :rtype: Tree
    :param s: The string to be converted
    :type s: str
    """
    tokens = re.split(r'([()| ])', s)
    for i in range(len(tokens)):
        if tokens[i] == '(':
            tokens[i-1], tokens[i] = tokens[i], tokens[i-1]     # pull nonterminal inside parens
        elif ':' in tokens[i]:
            fields = tokens[i].split(':')
            if len(fields) == 2:                                # non-terminal
                tokens[i] = fields[1]
            else:
                tokens[i] = "(" + fields[-2] + " " + fields[-1] + ")"
        elif tokens[i] == '|':
            tokens[i] = ''

    treebank_string = " ".join(tokens)
    return Tree.parse(treebank_string, remove_empty_top_bracketing=True)

#    s = re.sub(r'^#[^\s]*\s', '', s)  # remove leading identifier
#    s = re.sub(r'\w+:', '', s)       # remove role tags

#    return s

######################################################################
## Demonstration
######################################################################

def demo():
    """
    A demonstration showing how Trees and Trees can be
    used.  This demonstration creates a Tree, and loads a
    Tree from the Treebank corpus,
    and shows the results of calling several of their methods.
    """

    from nltk import tree

    # Demonstrate tree parsing.
    s = '(S (NP (DT the) (NN cat)) (VP (VBD ate) (NP (DT a) (NN cookie))))'
    t = Tree(s)
    print("Convert bracketed string into tree:")
    print(t)
    print(t.__repr__())

    print("Display tree properties:")
    print(t.node)           # tree's constituent type
    print(t[0])             # tree's first child
    print(t[1])             # tree's second child
    print(t.height())
    print(t.leaves())
    print(t[1])
    print(t[1,1])
    print(t[1,1,0])

    # Demonstrate tree modification.
    the_cat = t[0]
    the_cat.insert(1, tree.Tree.parse('(JJ big)'))
    print("Tree modification:")
    print(t)
    t[1,1,1] = tree.Tree.parse('(NN cake)')
    print(t)
    print()

    # Tree transforms
    print("Collapse unary:")
    t.collapse_unary()
    print(t)
    print("Chomsky normal form:")
    t.chomsky_normal_form()
    print(t)
    print()

    # Demonstrate probabilistic trees.
    pt = tree.ProbabilisticTree('x', ['y', 'z'], prob=0.5)
    print("Probabilistic Tree:")
    print(pt)
    print()

    # Demonstrate parsing of treebank output format.
    t = tree.Tree.parse(t.pprint())
    print("Convert tree to bracketed string and back again:")
    print(t)
    print()

    # Demonstrate LaTeX output
    print("LaTeX output:")
    print(t.pprint_latex_qtree())
    print()

    # Demonstrate Productions
    print("Production output:")
    print(t.productions())
    print()

    # Demonstrate tree nodes containing objects other than strings
    t.node = ('test', 3)
    print(t)

__all__ = ['ImmutableProbabilisticTree', 'ImmutableTree', 'ProbabilisticMixIn',
           'ProbabilisticTree', 'Tree', 'bracket_parse',
           'sinica_parse', 'ParentedTree', 'MultiParentedTree',
           'ImmutableParentedTree', 'ImmutableMultiParentedTree']

if __name__ == "__main__":
    import doctest
    doctest.testmod(optionflags=doctest.NORMALIZE_WHITESPACE)
Back to Top