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File name : tree.py

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"""
A class for storing a tree graph. Primarily used for filter constructs in the
ORM.
"""

import copy

from django.utils.hashable import make_hashable

class Node:
    """
    A single internal node in the tree graph. A Node should be viewed as a
    connection (the root) with the children being either leaf nodes or other
    Node instances.
    """
    # Standard connector type. Clients usually won't use this at all and
    # subclasses will usually override the value.
    default = 'DEFAULT'

def __init__(self, children=None, connector=None, negated=False):
        """Construct a new Node. If no connector is given, use the default."""
        self.children = children[:] if children else []
        self.connector = connector or self.default
        self.negated = negated

# Required because django.db.models.query_utils.Q. Q. __init__() is
    # problematic, but it is a natural Node subclass in all other respects.
    @classmethod
    def _new_instance(cls, children=None, connector=None, negated=False):
        """
        Create a new instance of this class when new Nodes (or subclasses) are
        needed in the internal code in this class. Normally, it just shadows
        __init__(). However, subclasses with an __init__ signature that aren't
        an extension of Node.__init__ might need to implement this method to
        allow a Node to create a new instance of them (if they have any extra
        setting up to do).
        """
        obj = Node(children, connector, negated)
        obj.__class__ = cls
        return obj

def __str__(self):
        template = '(NOT (%s: %s))' if self.negated else '(%s: %s)'
        return template % (self.connector, ', '.join(str(c) for c in self.children))

def __repr__(self):
        return "<%s: %s>" % (self.__class__.__name__, self)

def __deepcopy__(self, memodict):
        obj = Node(connector=self.connector, negated=self.negated)
        obj.__class__ = self.__class__
        obj.children = copy.deepcopy(self.children, memodict)
        return obj

def __len__(self):
        """Return the number of children this node has."""
        return len(self.children)

def __bool__(self):
        """Return whether or not this node has children."""
        return bool(self.children)

def __contains__(self, other):
        """Return True if 'other' is a direct child of this instance."""
        return other in self.children

def __eq__(self, other):
        return (
            self.__class__ == other.__class__ and
            (self.connector, self.negated) == (other.connector, other.negated) and
            self.children == other.children
        )

def __hash__(self):
        return hash((self.__class__, self.connector, self.negated, *make_hashable(self.children)))

def add(self, data, conn_type, squash=True):
        """
        Combine this tree and the data represented by data using the
        connector conn_type. The combine is done by squashing the node other
        away if possible.

This tree (self) will never be pushed to a child node of the
        combined tree, nor will the connector or negated properties change.

Return a node which can be used in place of data regardless if the
        node other got squashed or not.

If `squash` is False the data is prepared and added as a child to
        this tree without further logic.
        """
        if data in self.children:
            return data
        if not squash:
            self.children.append(data)
            return data
        if self.connector == conn_type:
            # We can reuse self.children to append or squash the node other.
            if (isinstance(data, Node) and not data.negated and
                    (data.connector == conn_type or len(data) == 1)):
                # We can squash the other node's children directly into this
                # node. We are just doing (AB)(CD) == (ABCD) here, with the
                # addition that if the length of the other node is 1 the
                # connector doesn't matter. However, for the len(self) == 1
                # case we don't want to do the squashing, as it would alter
                # self.connector.
                self.children.extend(data.children)
                return self
            else:
                # We could use perhaps additional logic here to see if some
                # children could be used for pushdown here.
                self.children.append(data)
                return data
        else:
            obj = self._new_instance(self.children, self.connector,
                                     self.negated)
            self.connector = conn_type
            self.children = [obj, data]
            return data

def negate(self):
        """Negate the sense of the root connector."""
        self.negated = not self.negated