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# Natural Language Toolkit: Text Trees
#
# Copyright (C) 2001-2025 NLTK Project
# Author: Edward Loper <edloper@gmail.com>
# Steven Bird <stevenbird1@gmail.com>
# Peter Ljunglöf <peter.ljunglof@gu.se>
# Tom Aarsen <>
# URL: <https://www.nltk.org/>
# For license information, see LICENSE.TXT
import warnings
from abc import ABCMeta, abstractmethod
from nltk.tree.tree import Tree
from nltk.util import slice_bounds
######################################################################
## Parented trees
######################################################################
class AbstractParentedTree(Tree, metaclass=ABCMeta):
"""
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, children=None):
super().__init__(node, children)
# If children is None, the tree is read from node, 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
# ////////////////////////////////////////////////////////////
@abstractmethod
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.
"""
@abstractmethod
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``.
"""
# ////////////////////////////////////////////////////////////
# 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().__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().__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().__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().__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().append(child)
def extend(self, children):
for child in children:
if isinstance(child, Tree):
self._setparent(child, len(self))
super().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().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().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().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)
def __getnewargs__(self):
"""Method used by the pickle module when un-pickling.
This method provides the arguments passed to ``__new__``
upon un-pickling. Without this method, ParentedTree instances
cannot be pickled and unpickled in Python 3.7+ onwards.
:return: Tuple of arguments for ``__new__``, i.e. the label
and the children of this node.
:rtype: Tuple[Any, List[AbstractParentedTree]]
"""
return (self._label, list(self))
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, children=None):
self._parent = None
"""The parent of this Tree, or None if it has no parent."""
super().__init__(node, children)
if children is None:
# If children is None, the tree is read from node.
# 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):
from nltk.tree.immutable import ImmutableParentedTree
return ImmutableParentedTree
def copy(self, deep=False):
if not deep:
warnings.warn(
f"{self.__class__.__name__} objects do not support shallow copies. Defaulting to a deep copy."
)
return super().copy(deep=True)
# /////////////////////////////////////////////////////////////////
# 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 hasattr(child, "_parent") and 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, 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().__init__(node, children)
if children is None:
# If children is None, the tree is read from node.
# 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):
from nltk.tree.immutable import ImmutableMultiParentedTree
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)
__all__ = [
"ParentedTree",
"MultiParentedTree",
]