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A simple and lightweight database library in python.

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# EasyData

EasyData is a lightweight database library for handling complex graphical data seemlessly in python.

## Databases

__I. Schemas__

Databases are dictionaries with additional structure built on top.

A database consists of schemas, objects, and attributes, which are all related symbolically.

A schema defines a label and a set of attributes, and is used to create new objects.

![Schema diagram](https://github.com/CarsonScott/easydata/blob/master/img/4927232E-8D0E-4211-9546-594CF7557A5B.jpeg)

We can define a schema to represent positions in space:
    
    db = Database()
    db.create_schema('point', ['x', 'y'])
    
It's useful to have some control over the acceptable values that can be assigned to the attributes of a schema.
 
Constraints are proposition functions that are assigned with respect to a particular attribute of a given schema.

We can define constraints on the x and y attributes of the point schema defined above.

These constraints ensure that any value assigned to either x or y will be a non-negative integer.

db.create_constraint('point', 'x', lambda val: isinstance(val, int) and val >= 0)
    db.create_constraint('point', 'y', lambda val: isinstance(val, int) and val >= 0)
    
If a value is given for a particular attribute while creating an object, which fails to satisfy the corresponding constraints of that attribute, then an exception is raised.

In other words, all constraints must be satisfied in order to create an instance of a particular schema.

Attributes can be added to existing schemas as well:

db.create_attribute('point', 'z', [lambda val: isinstance(val, int) and val >= 0])

__II. Objects__

As you can see the schema labeled 'point' defines two attributes: 'x', 'y', and 'z'.

Objects created using the point schema are automatically assigned those attributes, and therefore require the same number of values to be defined when they are created.

![Object diagram](https://github.com/CarsonScott/easydata/blob/master/img/D5C8A2F0-94B4-46C2-A9D7-BE7A3FE94563.jpeg)

We can create a new object using the point schema:

db.create_object('point', 'p1', [5, 6, 7], 0)
    
The database now contains an object called 'p1', equal to 0 (defaults to None), which has attributes 'x', 'y' and 'z', equal to 5, 6, and 7 respectively.

The value of an object is stored in a database as a typical dictionary element:

db['p1']
    >>> 0
    
__III. Attributes__

The individual attributes of an object are accessible through the following method:

db.get_attr('p1', 'x)
    >>> 5
    
    db.get_attr('p1', 'y')
    >>> 6

db.get_attr('p1', 'z')
    >>> 7

...or through the following short-hand:

db['p1', 'x']
    >>> 5
    
    db['p1', 'y']   
    >>> 6

db['p1', 'z']
    >>> 7

All attributes of an object are accessible as a dictionary:

db.get_attrs('p1')
    >>> {'x': 5, 'y': 6, 'z': 7}

***

## Graphs

__I. Nodes__

Graphs are databases with a preset schema for defining links between objects.

Graphs also come with a few additional methods for dealing with the link objects.

graph = Graph()
    graph.create_schema('point', ['x', 'y'])
    graph.create_object('point', 'p1', [5, 6])
    graph.create_object('point', 'p2', [3, 9])

Every object in a graph is assigned two additional attributes:
    
    graph.get_attrs('p1')
    >>> {'x': 5, 'y': 6, 'sources': [], 'targets': [])

__II. Links__

Links are created using a 'source' and 'target' key, each pointing to an existing object:

>>> graph.create_link('p1', 'p2')

The resulting link is an object with 'source' and 'target' attributes.
    
Every link is assigned a generated key based on the source and target keys used to define it:
    
    graph.get_key('p1', 'p2')
    >>> '(p1 p2)'

The attributes of the source and target objects are updated when a new link is created:

graph.get_attrs('p1')
    >>> {'x': 5, 'y': 6, 'sources': [], 'targets': ['p2'])
   
    graph.get_attrs('p2')
    >>> {'x': 3, 'y': 9, 'sources': ['p1'], 'targets': [])

Additional attributes can be added to the link schema:

graph.create_attribute('link', 'weight', [lambda val: isinstance(val, int) or isinstance(val, float)])

The additional attributes are assigned when a link is created:

graph.create_link('p1', 'p2', [4])

The link between 'p1' and 'p2' now has an attribute called 'weight', equal to 4.

__III. Hierarchies__

Every link is an object that may be the source or target of another link.

Links between links may be defined in an infinite hierarchy.

graph.get_attrs(graph.get_key('p1', 'p2')) 
    >>> {'source': 'p1', 'target': 'p2', 'weight': 6, 'sources': [], 'targets': [])

As you can see, the link between 'p1' and 'p2' also has attributes 'sources' and 'targets'.

graph.create_object('point', 'p3', [1, 2])
    graph.create_link('p2', 'p3', [2])

Now there is a new point called 'p3' that is connected with 'p2'.

We can create a link between links since there are now two links in the graph:

graph.create_link(graph.get_key('p1', 'p2'), graph.get_key('p1', 'p2'), [5])

The links from 'p1' to 'p2' and from 'p2' to 'p3' are now connected to one another.

The key assigned to this link reflects the hierarchical nature:

graph.get_key(graph.get_key('p1', 'p2'), graph.get_key('p1', 'p2'))
    >>> '((p1 p2) (p2 p3))'

***

## Notes

The set of instances for a particular schema are accessible in list form:

graph.get_instances('point')
    >>> ['p1', 'p2', 'p3']

Objects can be removed from a graph or database:

graph.remove_object('p1')
    
    graph.get_instances('point')
    >>> ['p2', 'p3']

An object can be tested to see if it's an instance of a particular schema:

graph.is_instance('p2', 'point')
    >>> True

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