1.5 - Parent reference and backwards paths

We said that each item is enveloped into a bag node, and that can be contained by several bags in different places, this means that a Bag knows its children but ignores who is its father, in fact it may have many fathers.

We could set stricter hypotesis about the structure of a bag, making it more similar to a tree-leaf model, this would happen if a bag had a back reference to the bag that contains it. This feature is implemented by the method setBackRef() If we call the method setBackRef on a bag instance, that bag becomes the root of a tree structure in which each leaf (BagNode) knows its father. This means that we can traverse a bag backward using the property parent of bag's nodes.

family = Bag()
family['grandpa'] = Bag() 
family['grandpa'].setBackRef()
family['grandpa.father.son.nephew']=Bag()

nephew = family['grandpa.father.son.nephew']      
son = family['grandpa.father.son']
father = family['grandpa.father']

>>> son.parent == father
True
>>> nephew.parent.parent == father
True
>>> nephew.parent == son
True

A bag with back reference can be traversed with special back-paths that use a new syntax. The symbol '../' in a path is equivalent to the property parent.

>>> nephew['../../'] == father
True

When the backreference is set, it is possible to get from the bag its own BagNode:

>>> father.node
BagNode : father at 432464

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