Node

Overview

Node is a library to create nested data models and structures.

These data models are described as trees of nodes, optionally with attributes and schema definitions.

They utilize:

• Python's mapping and sequence API's for accessing node members.
• The contract of zope.location.interfaces.ILocation for hierarchy information.

One purpose of this package is to provide a unified API to different backend storages. Specific storage related implementations are for example:

• node.ext.fs
• node.ext.ldap
• node.ext.yaml
• node.ext.zodb

Another usecase is providing interfaces for specific application domains.

E.g. for user and group management, node.ext.ugm defines the interfaces. Additional it implements a file based default implementation. Then there are specific implementations of those interfaces in node.ext.ldap and cone.sql, to access users and groups in LDAP and SQL databases.

This package is also used to build in-memory models of all sorts.

E.g. yafowil is a HTML form processing and rendering library. It uses node trees for declarative description of the form model.

Another one to mention is cone.app, a Pyramid based development environment for web applications, which uses node trees to describe the application model.

Basic Usage

There are two basic node types. Mapping nodes and sequence nodes. This package provides some basic nodes to start from.

Mapping Nodes

Mapping nodes implement node.interfaces.IMappingNode. A mapping in python is a container object that supports arbitrary key lookups and implements the methods specified in the MutableMapping of pythons abstract base classes respective zope.interface.common.mapping.IFullMapping.

An unordered node. This can be used as base for trees where order of items doesn't matter:

from node.base import BaseNode

root = BaseNode(name='root')
root['child'] = BaseNode()

An ordered node. The order of items is preserved:

from node.base import OrderedNode

root = OrderedNode(name='orderedroot')
root['foo'] = OrderedNode()
root['bar'] = OrderedNode()

With printtree we can do a quick inspection of our node tree:

>>> root.printtree()
<class 'node.base.OrderedNode'>: orderedroot
  <class 'node.base.OrderedNode'>: foo
  <class 'node.base.OrderedNode'>: bar

Sequence Nodes

Sequence nodes implement node.interfaces.ISequenceNode. In the context of this library, a sequence is an implementation of the methods specified in the MutableSequence of pythons abstract base classes respective zope.interface.common.collections.IMutableSequence.

Using a list node:

from node.base import BaseNode
from node.base import ListNode

root = ListNode(name='listroot')
root.insert(0, BaseNode())
root.insert(1, BaseNode())

Check tree structure with printtree:

>>> root.printtree()
<class 'node.base.ListNode'>: listnode
  <class 'node.base.BaseNode'>: 0
  <class 'node.base.BaseNode'>: 1

Behaviors

node utilizes the plumber package.

The different functionalities of nodes are provided as plumbing behaviors:

from node.behaviors import DefaultInit
from node.behaviors import MappingNode
from node.behaviors import OdictStorage
from plumber import plumbing

@plumbing(
    DefaultInit,
    MappingNode,
    OdictStorage)
class CustomNode:
    pass

When inspecting the CustomNode class, we can see it was plumbed using given behaviors, now representing a complete node implementation:

>>> dir(CustomNode)
['__bool__', '__class__', '__contains__', '__delattr__', '__delitem__',
'__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__',
'__getattribute__', '__getitem__', '__gt__', '__hash__', '__implemented__',
'__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__',
'__module__', '__name__', '__ne__', '__new__', '__nonzero__', '__parent__',
'__plumbing__', '__plumbing_stacks__', '__provides__', '__reduce__',
'__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__sizeof__',
'__str__', '__subclasshook__', '__weakref__', 'acquire', 'clear', 'copy',
'deepcopy', 'detach', 'filtereditems', 'filtereditervalues', 'filteredvalues',
'get', 'has_key', 'items', 'iteritems', 'iterkeys', 'itervalues', 'keys',
'name', 'noderepr', 'parent', 'path', 'pop', 'popitem', 'printtree', 'root',
'setdefault', 'storage', 'treerepr', 'update', 'values']

Please read the documentation of plumber for detailed information about the plumbing system.

Attributes

While it is not strictly necessary, it's a good idea to separate the hierarchical structure of a model from the node related attributes to avoid naming conflicts. Attributes are provided via node.behaviors.Attributes plumbing behavior:

from node.behaviors import Attributes
from node.behaviors import DefaultInit
from node.behaviors import DictStorage
from node.behaviors import MappingNode
from plumber import plumbing

@plumbing(
    Attributes,
    DefaultInit,
    MappingNode,
    DictStorage)
class NodeWithAttributes:
    pass

The node now provides an attrs attribute. Node attributes are itself just a node:

>>> node = NodeWithAttributes()
>>> attrs = node.attrs
>>> attrs
<NodeAttributes object 'None' at ...>

>>> attrs['foo'] = 'foo'

If it's desired to access attribute members via python attribute access, attribute_access_for_attrs must be set on node:

>>> node.attribute_access_for_attrs = True
>>> attrs = node.attrs
>>> attrs.foo = 'bar'
>>> attrs.foo
'bar'

A custom attributes implementation can be set by defining attributes_factory on the node:

from node.behaviors import NodeAttributes

class CustomAttributes(NodeAttributes):
    pass

class CustomAttributesNode(NodeWithAttributes):
    attributes_factory = CustomAttributes

This factory is then used to instantiate the attributes:

>>> node = CustomAttributesNode()
>>> node.attrs
<CustomAttributes object 'None' at ...>

Schema

To describe the data types of node members, this package provides a mechanism for defining schemata.

This can happen in different ways. One is to define the schema for node members directly. This is useful for nodes representing a leaf in the hierarchy or for node attribute nodes:

from node import schema
from node.base import BaseNode
from node.behaviors import DefaultInit
from node.behaviors import DictStorage
from node.behaviors import MappingNode
from node.behaviors import Schema
from plumber import plumbing

@plumbing(
    DefaultInit,
    MappingNode,
    DictStorage,
    Schema)
class SchemaNode:
    schema = {
        'int': schema.Int(),
        'float': schema.Float(default=1.),
        'str': schema.Str(),
        'bool': schema.Bool(default=False),
        'node': schema.Node(BaseNode)
    }

Children defined in the schema provide a default value. If not explicitely defined, the default value is always node.utils.UNSET:

>>> node = SchemaNode()
>>> node['int']
<UNSET>

>>> node['float']
1.0

>>> node['bool']
False

Children defined in the schema are validated against the defined type when setting it's value:

>>> node = SchemaNode()
>>> node['int'] = 'A'
Traceback (most recent call last):
  ...
ValueError: A is no <class 'int'> type

For accessing members defined in the schema as node attributes, SchemaAsAttributes plumbing behavior can be used:

from node.behaviors import SchemaAsAttributes

@plumbing(SchemaAsAttributes)
class SchemaAsAttributesNode(BaseNode):
    schema = {
        'int': schema.Int(default=1),
    }

Node attrs now provides access to the schema members:

>>> node = SchemaAsAttributesNode()
>>> node.attrs['int']
1

Schema members can also be defined as class attributes. This is syntactically the most elegant way, but comes with the tradeoff of possible naming conflicts:

from node.behaviors import SchemaProperties

@plumbing(
    DefaultInit,
    MappingNode,
    DictStorage,
    SchemaProperties)
class SchemaPropertiesNode:
    text = schema.Str(default='Text')

Here we access text as class attribute:

>>> node = SchemaPropertiesNode()
>>> node.text
'Text'

>>> node.text = 1
Traceback (most recent call last):
  ...
ValueError: 1 is no <class 'str'> type

Plumbing Behaviors

General Behaviors

node.behaviors.DefaultInit

Plumbing behavior providing default __init__ function on node. This behavior is going to be deprecated in future versions. Use node.behaviors.NodeInit instead. See node.interfaces.IDefaultInit.

node.behaviors.NodeInit

Plumbing behavior for transparent setting of __name__ and __parent__ at object initialization time. See node.interfaces.INodeInit.

node.behaviors.Node

Fill in gaps for full INode API. See node.interfaces.INode.

node.behaviors.ContentishNode

A node which can contain children. See node.interfaces.IContentishNode. Concrete implementations are node.behaviors.MappingNode and node.behaviors.SequenceNode.

node.behaviors.Attributes

Provide attributes on node. See node.interfaces.IAttributes. If node.behaviors.Nodespaces is applied on node, the attributes instance gets stored internally in __attrs__ nodespace, otherwise its set on __attrs__ attribute.

node.behaviors.Events

Provide an event registration and dispatching mechanism. See node.interfaces.IEvents.

node.behaviors.BoundContext

Mechanism for scoping objects to interfaces and classes. See node.interfaces.IBoundContext.

node.behaviors.NodeReference

Plumbing behavior holding an index of nodes contained in the tree. See node.interfaces.INodeReference.

node.behaviors.WildcardFactory

Plumbing behavior providing factories by wildcard patterns. See node.interfaces.IWildcardFactory.

Mapping Behaviors

node.behaviors.MappingNode

Turn an object into a mapping node. Extends node.behaviors.Node. See node.interfaces.IMappingNode.

node.behaviors.MappingAdopt

Plumbing behavior that provides __name__ and __parent__ attribute adoption on child nodes of mapping. See node.interfaces.IMappingAdopt.

node.behaviors.MappingConstraints

Plumbing behavior for constraints on mapping nodes. See node.interfaces.IMappingConstraints.

node.behaviors.UnicodeAware

Plumbing behavior to ensure unicode for keys and string values. See node.interfaces.IUnicodeAware.

node.behaviors.Alias

Plumbing behavior that provides aliasing of child keys. See node.interfaces.IAlias.

node.behaviors.AsAttrAccess

Plumbing behavior to get node as IAttributeAccess implementation. See node.interfaces.IAsAttrAccess.

node.behaviors.ChildFactory

Plumbing behavior providing child factories. See node.interfaces.IChildFactory.

node.behaviors.FixedChildren

Plumbing Behavior that initializes a fixed dictionary as children. See node.interfaces.IFixedChildren.

node.behaviors.Nodespaces

Plumbing behavior for providing nodespaces on node. See node.interfaces.INodespaces.

node.behaviors.Lifecycle

Plumbing behavior taking care of lifecycle events. See node.interfaces.ILifecycle.

node.behaviors.AttributesLifecycle

Plumbing behavior for handling lifecycle events on attribute manipulation. See node.interfaces.IAttributesLifecycle.

node.behaviors.Invalidate

Plumbing behavior for node invalidation. See node.interfaces.Invalidate.

node.behaviors.VolatileStorageInvalidate

Plumbing behavior for invalidating nodes using a volatile storage. See node.interfaces.Invalidate.

node.behaviors.Cache

Plumbing behavior for caching. See node.interfaces.ICache.

node.behaviors.MappingOrder

Plumbing behavior for ordering support on mapping nodes. See node.interfaces.IMappingOrder.

node.behaviors.UUIDAware

Plumbing behavior providing a uuid on nodes. See node.interfaces.IUUIDAware.

node.behaviors.MappingReference

Plumbing behavior to provide node.interfaces.INodeReference on mapping nodes. See node.interfaces.IMappingReference.

node.behaviors.MappingStorage

Provide abstract mapping storage access. See node.interfaces.IMappingStorage.

node.behaviors.DictStorage

Provide dictionary storage. Extends node.behaviors.MappingStorage. See node.interfaces.IMappingStorage.

node.behaviors.OdictStorage

Provide ordered dictionary storage. Extends node.behaviors.MappingStorage. See node.interfaces.IMappingStorage.

node.behaviors.Fallback

Provide a way to fall back to values by subpath stored on another node. See node.interfaces.IFallback.

node.behaviors.Schema

Provide schema validation and value serialization on node values. See node.interfaces.ISchema.

node.behaviors.SchemaAsAttributes

Provide schema validation and value serialization on node values via dedicated attributes object. See node.interfaces.ISchemaAsAttributes.

node.behaviors.SchemaProperties

Provide schema fields as class properties. See node.interfaces.ISchemaProperties.

node.behaviors.MappingFilter

Filter mapping children by class or interface. See node.interfaces.IChildFilter.

Sequence Behaviors

node.behaviors.SequenceNode

Turn an object into a sequence node. Extends node.behaviors.Node. See node.interfaces.IMappingNode.

node.behaviors.SequenceAdopt

Plumbing behavior that provides __name__ and __parent__ attribute adoption on child nodes of sequence. See node.interfaces.ISequenceAdopt.

node.behaviors.SequenceConstraints

Plumbing behavior for constraints on sequence nodes. See node.interfaces.ISequenceConstraints.

node.behaviors.SequenceStorage

Provide abstract sequence storage access. See node.interfaces.ISequenceStorage.

node.behaviors.ListStorage

Provide list storage. See node.interfaces.ISequenceStorage.

node.behaviors.SequenceReference

Plumbing behavior to provide node.interfaces.INodeReference on sequence nodes. See node.interfaces.ISequenceReference.

node.behaviors.SequenceFilter

Filter sequence children by class or interface. See node.interfaces.IChildFilter.

node.behaviors.SequenceOrder

Plumbing behavior for ordering support on sequence nodes. See node.interfaces.ISequenceOrder.

JSON Serialization

Nodes can be serialized to and deserialized from JSON:

>>> from node.serializer import serialize
>>> json_dump = serialize(BaseNode(name='node'))

>>> from node.serializer import deserialize
>>> deserialize(json_dump)
<BaseNode object 'node' at ...>

For details on serialization API please read file in docs/archive/serializer.rst.

Python Versions

• Python 2.7, 3.7+
• May work with other versions (untested)

Contributors

• Robert Niederreiter
• Florian Friesdorf
• Jens Klein