1.4 - Bag nodes

We discovered in the previous chapter that we can associate a set of attributes to each item. We will now discuss a more advanced concept about a Bag, where we introduce the BagNode. A Bag is a collection of 'nodes'.

A 'BagNode' is an object composed of three things:

• label
• attributes
• value (or item)

In order to avoid confusion between the terms item and node, what we used to call an 'item' we will now call a value.

If you need to work with nodes, you may get them with the methods:

getNode(path)	returns a node
getNodes()	returns a list of nodes
getNodeByAttr(attribute, attr_value)	returns the node that has the passed couple attribute-value

mybag = Bag({'paper':1, 'scissors':2})
papernode = mybag.getNode('paper')
mybag.setItem('rock', 3 , color='grey')
rocknode=mybag.getNodeByAttr('color','grey')
nodes=mybag.getNodes()

The method getNodes() implements the bag's property nodes.

>>>mybag.getNodes() == mybag.nodes
True

If you have a node instance you may use one of the following methods:

hasAttr(attribute)	returns true if the node has a value for the passed attribute
setAttr(attribute=value)	set to the node one or more attributes passed as kwargs
getAttr(attribute)	returns the attribute's value
replaceAttr(attribute=value)	replaces the value of one or more attributes passed as kwargs
delAttr(attribute)	deletes the attribute with the passed name
getLabel()	returns the node's label
setLabel(label)	sets the node's label
getValue()	returns the node's value
setValue()	sets the node's value

>>> print papernode.hasAttr('color')
False

>>> papernode.setAttr(color='white')
>>> print papernode.getAttr('color')
white

>>> papernode.replaceAttr(color='yellow')

>>> papernode.delAttr('color')

>>> papernode.setLabel('sheet')
>>> print papernode.getLabel()
sheet

>>> papernode.setValue(8)
>>> papernode.getValue()
8

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