objdict 0.1.0b3

=========
ObjDict
=========

Uses_.
------
Why an 'ObjDict'? The reasons include:
- `The structure/ad-hoc object 'swiss army knife' class`_.
- `Support for json message encoding and decoding`_.
- `ObjDict in place of dictionaries as convenient ad-hoc data structures`_
- mutable equivalent to nametuple (or namedlist)
- adding json serialization to classes

Background_.
------------
- `History and Acknowledgements`_.
- `jsonweb alternative to ObjDict JSON processing`_.
- `Multiple Uses of Dictionaries`_.
- `Introducing the ObjDict`_.
- `Multiple modes of Dictionary Use and JSON`_.
- `ObjDict json general`_.
- `ObjDict json load tools`_.
- `ObjDict json dump tools`_.

Instructions_.
--------------
- General Notes and Restrictions
- Instancing and json load
- str representation and json dumps
-

Uses
-----
The structure/ad-hoc object 'swiss army knife' class
+++++++++++++++++++++++++++++++++++++++++++++++++++++

As described in this 'uses' sections, the ObjDict class has many uses, and can
be used in place of namedtupples, namedlists, OrderedDict objects as well as
for processing Json data. One single import gives this flexibility.

The one trade-off for this flexibiliy, compared to using the individual specialised
classes is performance. If you have performance critical code that is used in
massively iterative loops then, for example, namedtuples are far better as long
namedtuples provide all the functionality you require. But every last nanosecond
is not of the essence and flexibility to adapt and simply code is desired, then
'ObjDict' can be a replacement for serveral other classes, plus provide best tools
for working with json data.

Support for json message encoding and decoding
++++++++++++++++++++++++++++++++++++++++++++++

Where an application has the need to build json data to save or transmit, or
to decode and process json data loaded or recieved, the ObjDict structure provides all
the tools to achive this, with clear Object oriented code. This usage has different
requirements than json serialisation (as disccussed below), as it is necessary
to be able to produce not just a json representation of an object, but create
objects that can desribe any required possible
arbitary json data to produce or decode specific messages.
For example, the order of fields may be signifcant in a
json message, although field order may not be significant for object
serialisation. The ObjDict class has
the tools to produce exactly the json data required by any application, and to decode
any possible incomming json messages for processing. It was for this usage that
ObjDict was intially developed.

ObjDict in place of dictionaries as convenient ad-hoc data structures
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
See the text below on 'mulitple uses of dictionaries' for background.
There is a significant amount of code where dictionaries have been used for
ad-hoc structures. The use case often arises where it can become useful if
these data structure can have elements accessed in the simpler are many

Mutable Equivalent To NamedTuple
++++++++++++++++++++++++++++++++
There are occasions where a 'namedtuple' cannot be used due to the need for
mutable objects. The ObjDict also fulfills this need and can be intialised
from list data. There are many other classes that also fill this need, but
the ObjDict combines this functionality with json processing, with dictionary
access to data and other functions.

Json Serialisable Objects.
++++++++++++++++++++++++++
Applications that have a need to serialise objects in order to restore those
objects either within the same application, or in an application connected
through a datalink, may desire json as the format for object storage or object
message format. The ObjDict class and module provides the tools for this,
serialising the state of an object in order for that state to be later
loaded, either by an identical class, or a different class which has use
for some or all of the same 'object state' information.

OrderedDict alternative.
++++++++++++++++++++++++
OrderedDicts do everything dictionaries can, and in some applications it can
be useful to simply move to OrderedDict classes for all dictionaries. 'ObjDict'
is another alternative, with a shorter name, even more flexibiliy and power,
and a much more readable 'str' representation that can also be used for clearer
initialisation. See instructions for details on 'str' and initialisation
flexibility.

Background
----------
History and Acknowledgements
++++++++++++++++++++++++++++
The project was emerged from a need for code to generate and decode json
messages. Originally the package `jsonweb <http://www.jsonweb.net/>`_. was
selected for the task, but it became clear the use case differed. 'jsonweb' is
ideal for representing classes as json, and reloading classes from that json
and provides validation and tests and schema that are not reproduced in ObjDict.
However ObjDict provides specifically for classes created to generate or process
json as data, as
opposed to json as a representation of the class, and now the ObjDict
class with a wider range of uses. The whole issue of json data which ambiguously
may correspond to either a dictionary collection, or an object, arises from
general processing of json data and gives rise to the ObjDict. The ObjDict
project started out to add more control
over json as a fork of jsonweb, but evolved over time to the different use cases.

jsonweb alternative to ObjDict JSON processing
++++++++++++++++++++++++++++++++++++++++++++++++
The project 'jsonweb' overlaps is use cases with this project. The focus of
'jsonweb' is to provide for serializing python object structures and instancing
python objects from the serialized form. ObjDict can be used for this role also,
but currently lacks the validation logic used by 'jsonweb' to ensure JSON data
matches exactly the required format.

In fact, rather than an emphasis on validation, the original primary usecase of
ObjDict is to allow maxium flexibility
for the JSON data representing an object. The ObjDict object itself is a generic
object to enable working with free format data objects. Beyond the ObjDict
class, the entire JSON processing philosophy is to provide for information sent
between
computer systems where , the message specification will tend to evolve from version to
version. This requires flexible interpretation of data, and the ability to
easily ignore additional data that may have been added in later versions,
providing easy backward compatibility.

The structure for JSON dump and load is a very flexible framework, and any feature
including more rigid validation could easily be added.

Multiple Uses of Dictionaries
++++++++++++++++++++++++++++++
In python, dictionaries are designed as 'collections' but are often used as
ad-hoc structures or objects. In a true collection, the key for an entry does
not indicate properties
of the value associated with the key. For example, a collection of people,
keyed by names,
would not normally infer the significance or type of data for each entry
(or in this case person) by the key. The data has the same implications regardless
of whether the key is 'bob' or 'jane'. The data associate with 'bob' or 'jane'
is of the same type and is interpreted the same way.
For an 'ad-hoc' structure the keys **do** signal both the nature of the data and
even the type of data.
Consider for each entry for a person we have a full name, and age.
A dictionary could be used to hold this information, but this time it is an
ad-hoc structure. As a dictionary we always expect the same two keys, and each
is specific to the information and different keys even have different types of data.
This is not a dictionary as a collection, but as an ad-hoc structure. These are two
very different uses of a dictionary, the collection the dictionary was designed for,
and the ad-hoc structure or ad-hoc object as a second use.

Introducing the ObjDict
++++++++++++++++++++++++
An ObjDict is a subclass of dictionary designed to support this second
'ad-hoc object' mode of use. An ObjDict supports all normal dict operations, but
add support for accessing and setting entries as attributes.
So::

bob['full_name']= 'Robert Roberts'
is equivalent to
bob.full_name = 'Robert Roberts'

Either form can be used. ObjDicts also have further uses.

Multiple modes of Dictionary Use and JSON
++++++++++++++++++++++++++++++++++++++++++
The standard json dump and load map json 'objects' to python dictionaries.
JSON objects even look like python dictionaries (using {}
braces and a ':'). In javascript, objects can also
be treated as similar to dictionaries in python. The reality is some json
objects are best represented in python as objects, yet others are best
represented as dictionaries.

Consider::

{ "name": {"first":"fred", "last":"blogs"}
"colour_codes":{"red":100,"green":010, "yellow":110, "white":111 }
}

In this data, the 'name' is really an object but the 'color_codes' is a
true dictionary. Name is not a true dictionary because it is not a collection
of similar objects, but rather something with two specific properties.
Iterating through name does not really make sense, however iterating through
our colours does make sense. Adding to the collection of colours and their
being a variable number of colours in the collection is all consistent.
Treating 'name' is not ideal as the 'keys' rather than being entries in a collections
each have specific meaning. Keys should not really have meaning, and these keys
are really 'attributes' of name, and name better represented as an object.

So two types of information are represented in the same way in json.

Another limitation of working with python dictionaries and JSON is that in messages
order can be significant and but dictionaries are not ordered.

The solution provided here is to map JSON 'objects' to a new python ObjDict
(Object Dictionaries). These act like OrderedDictionaries, but can also be treated
as python objects.

So 'dump' or '__json__()' or 'str() / __str__()' of the 'names' and
'colour_codes' example above produces an
outer ObjDict containing two inner 'ObjDict's, 'name' and 'colour_codes'.
Assume the outer ObjDict is assigned to a variable 'data'
Each obj dict can be treated as either an object or a dictionay, so all the code
below is valid::

data= ObjDict(string_from_above)
name = data['name'] #works, but as 'data' is not a real 'dict' not ideal
name = data.name #better
first_name = data.name.first
first_name = data["name"]["first"] #works but again not ideal

red_code = data.colour_codes["red"]
#as colour codes is a true collection it will be unlikely to set
#members to individual variables, but the code is valid

ObjDict items also 'str' or 'dump' back to the original JSON as above.
However if the original string was changed to::

{ "name": {"first":"fred", "last":"blogs", "__type__": "Name"}
"colour_codes":{"red":100,"green":010, "yellow":110, "white":111 }
}

The json 'load' used to load or intitalise ObjDict uses an object_pairs_hook
that checks a table of registered class names and corresponding classes.

If there is an entry in the table, then that class will be used for embeded objects.
Entries with no __type__ result in ObjDict objects, and if the 'DefaultType' is
set then a class derived from the default type, with the name from the value
of '__type__' will be returned. If 'DefaultType' is None, then an exception will
be generated.
See the instructions section for further information.

ObjDict json general
+++++++++++++++++++++
Consider loading an object properties from json. A simple loop to use each json field
to set each attribute, and the class to be set is simply one class. However, what if
some of those fields are themselves objects, and possibly fields within those
again objects? Within the single 'top-level' object, there may be many embedded
objects and identifying and processing these embedded objects is the actuall challenge.

In general, handling embedded objects is achieved through the '__from_json__' class method
within each class for the 'json.load', or the '__json__' method within each
object for the 'json.dump'.

Standard routines to perform these methods are available, together with the tools
to easily decorate classes and other utilities.

ObjDict json load tools
++++++++++++++++++++++++
The three main tools for loading json objects are an 'object_pairs_hook' method to
be passed to the standard 'json.load' fuction, the '__from_json__' classmethod that
can be added to any class to control instancing the class from json and
the 'from_json' decorator.

The philosophy is the use of simple, flexible building blocks.

object_pairs_hook
~~~~~~~~~~~~~~~~~~
A class within the objdict module, 'ObjPairHook', is a wrapper tool to provide
a function for the standard library json.load() function. Simply instance an ObjPairHook
and pass the 'from_json' method to json_load(). eg::
hook=ObjPairHook().from_json
json.load(object_pairs_hook=hook)

class ObjPairsHook()
def __init__(classes_list=[],BaseHook=None,BaseType=None):

The 'from_json' method will check all json objects for a '__type__' entry, or use
'default' processing. For objects with a '__type__', both the entries in the
'classses_list' parameter, and the default_classes_list maintained within
the objdict module and added to through
the 'from_json' decorator, can be instanced if there is a name match.

For objects with __type__ entries but no name match with either source of classes
then the a dynamic class based on 'BaseClass' is generated and selected as the 'class'.

For objects with no __type__ entry, then the 'BaseHook' is selected as the
'class' (although in practice is it also
possible to use a method rather than a class).

Once a class is selected, then if this class has a __from_json__ attribute, then
this classmethod is called to instance an object, otherwise the normal init methods
for the class is called.

__from_json__ class method
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Providing a __from_json__ classmethod is called to instance an the object
by the 'object_pairs_hook' if an attribute of this name is present.

from_json decorator
~~~~~~~~~~~~~~~~~~~~
the from_json decorator, when used to decorate a class, adds the class to
default_class list used by the object_pairs_hook

ObjDict json dump tools
++++++++++++++++++++++++
The __json__ method, JsonEncoder class, the @to_json decorator and the
json_encoder_table of
to_json converters are the main
tools for encoding json. Whereas jsonweb takes an approach of decorating classes
with configuration information to allow the encoder class to produce the json
output, ObjDict uses a JsonEncoder that delegates the encoding to __json__
method within each object, or from a table of class/converter pairs.

JsonEncoder class
~~~~~~~~~~~~~~~~~~~
The json_encoder class does the actual encoding, and for each object it first
checks for a __json__ method and class that method if present. For objects
defined outside of scope (eg. Decimal() ), the encoder checks the encoder_table
for a matching entry and if present calls that encoder.

to_json decorator
~~~~~~~~~~~~~~~~~~~
this decorator checks if the class has a __json__ method, and if not decorates
the class with a default __json__ method. The __json__ method itself is then
decorated with any configuration data

__json__ method
~~~~~~~~~~~~~~~~
For any object this is either a function or a bound method to be called with
the object to be encoded as a parameter. The method should return either a
string or a dictionary to be included included in the json output.

json_encoder_table
~~~~~~~~~~~~~~~~~~~
this is an object which can be imported from the objdict module to access the
'add' method (json_encoder.add(<class>,<method/function>). By default, the
table contains entries for Decimal, date and time.


.. _Instructions:

Instructions.
-------------
- `General Notes and Restrictions`_.
- `Initialisation and json load`_.
- str and json dumps
- custom classes and json


General Notes and Restrictions
+++++++++++++++++++++++++++++++
Since ObjDict keys do not have to be valid attribute names (for example an
integer can be a dictionary key but not an attribute name, and dictionary keys
can contain spaces), so not all
key entries can be accessed as attributes. Similarly, there are attributes
which are not considered to be key data, and these attributes have an underscore
preceding the name. Some attributes are part of the scaffolding of the ObjDict
class and these all have a leading underscore, as well as a trailing underscore.
It is recommened to use a leading undercore for all class 'scaffolding' added as
extentions to the ObjDict class or to derived classes, where this scaffolding
is not to be included as also dictionary data.


Initialisation and json load
+++++++++++++++++++++++++++++
ObjDict can be intialised from lists, from json strings, from dictionaries,
from parameter lists or from keyword parameter lists.

Initialisation From Lists or Parameter Lists.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Intitialisation from a list of key value pairs, as with OrderedDict class is
supported. Beyond key value pairs, there is also support for direct initialisation
from lists. The _keys parameter must be included for initialisation from lists.
Also, Classes
derived from ObjDict can have _keys as a class attribute, providing an similar
use pattern to the 'namedtuple'. '_keys' can be either
a list of strings, or a string with space or comma separated values. When
initialising from a list or parameter list, the list size must match the number
of keys created through '_keys', however other items can be added after
initialisation.

So this code produces True::

class XY(ObjDict):
_keys='x y'

sample = XY(1,3)
sample.x,sample.y == 1,3

Alternatively form to produce a similar result but with the SubClass would be::

sample= ObjDict(1,3,_keys='x y')
sample= ObjDict([1,3],_keys='x y')

Initialisation from Json Strings.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For more complex intitialisation, json strings can provide an ideal solution.
This allows for complex structures with nested embeded 'ObjDict' or other objects

The background section ``

Initialisation from dict, OrderedDict, or key word arguments.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

str and json dumps.
+++++++++++++++++++
A limitation with OrderDict objects is that 'str' represenation can be clumsy
when the structure is nested.

Both the 'str' and 'json' methods of the ObjDict class produce json output which
remains clear regardless of nested structures.

Custom classes
++++++++++++++
Custom classes allow for json data to result in instantiating objects other
than ObjDict from json data. These custom classes can be subclassed from ObjDict
or built from first principles.


eading data directly into a class with appropriate
methods to manipulate data, and can also customise how data is written back as JSON.

Such classes can be subclassed from ObjDict but do not need to be.

For a 'dummy' class which is just a dict use::

@decode.from_object()
class Sample(ObjDict):
pass

A simple introduction/migration is to leave 'combiParse' still treating
objects as dictionaries by using the no__type__=True parameter.
This allows an app to use its own code to convert dictionaries into object,
but still benefit from unParse being able to generate JSON directly from objects.

E.g. if you have::

{ "name":{
"first": "joe",
"last": "foo"
}
}
#now code
@objdict.from_json()
class Name:
def __init__(self,first=None,last=None,**kwargs):
self.first=first
self.last=last


Read with::

combiParse(string)

then convert the name
dictionary into an object and put that object back in the original tree::

tree=combiParse(string)
tree['name'] = Name(**tree['name']) # kwargs!!! i.e. "**" required :-)

The result would be 'unParsed' ::

{ "name":{
__type__: "Name"
"first": "joe",
"last": "foo"
}
}


Decoding automatically to objects can then be added at a later time.

Note: using '@decode.from_object()' instead of '@decode.from_object()'
results in all of the json being passed as a single dict paramter,
not just parameters listed in the 'init',
being in the call to instance the object.
This means the 'JSONSimpleHandler' needs a \*args in the signature. We also
need the same solution when decoding manually as in the migration example.

Maintaining Order With Custom Classes and Defaults.
+++++++++++++++++++++++++++++++++++++++++++++++++++
ObjDict classes and automatically created classes currently maintain key order,
but of course cannot provide for default values for attributes.

Custom classes can specify default values for attributes, but currently custom
classes do not automatically maintain order, even if based on ObjDict classes.

Maintaining order and supporting default values are available with an __init__
method. Note, the order attributes are set will be their order in a message.
Classes subclasses from ObjDict will have '__type__' at the end of json output.

If a custom class is decorated with @decode.from_object(JSONSimpleHandler),
then all fields in the raw JSON will be sent in a single dict. Of course, as
a dict order is lost and also there are no default values.
The recommended code for the init is something like this::

@objdict.from_json()
class Custom(ObjDict):
def __init__(self,*args,**kwargs):
super(Custom,self).__init__()
if args:
arg0=args[0]
assert len(args)==0, "unexpected argument"
self.arg1=arg0.pop('arg1',default)
self.arg2=arg0.pop('arg2',default)
........
self.update(arg0)
self.update(**kwargs)

Life is much simpler with @decode.from_object(), but at the expense of ignoring
any unexpected arguments. Currently \*\*kwargs will always be empty in this case
but a future update will likely address this. Example::

@decode.from_object()
class Custom(ObjDict):
def __init__(self,arg1=None,arg2=None ....,**kwargs):
super(Custom,self).__init__()
self.arg1=arg1
self.arg2=arg1
........
self.update(**kwargs) #currently kwargs will be empty


All that is needed as imports is above.

This system supports both 'ObjDict's and custom classes. In JSON representation
a __type__ field is used to indicate actual type. For your own classes use::

@encode.to_object()
@decode.from_object()
class Sample:
def __init(self,p1,p2,...):
self.p1=p1
self.p2=p2
....

to map between::

{ "p1": 1, "p2":2, "__type__": "Sample"}
and
Sample(1,2)

However simple examples such as this could also use the default 'ObjDict' objects.