lazr.json 0.2

JSON serialisation utilities.

Importable

The lazr.json package is importable, and has a version number.

>>> import lazr.json
>>> print 'VERSION:', lazr.json.__version__
VERSION: ...

Object Serialisation

Import the methods and classes relevant to this package.

>>> import json
>>> from lazr.json import (
...     custom_type_decoder,
...     CustomTypeEncoder,
...     register_serialisable_type,
...     )

Define a complex object class we want to serialise.

>>> class ComplexObject:
...     def __init__(self, str='string_value', i=5):
...         self.f_string = str
...         self.f_int = i
...     def __eq__(self, other):
...         return (self.f_int == other.f_int and
...             self.f_string == other.f_string)

By default an instance of the complex object cannot be serialised.

>>> obj = ComplexObject()
>>> try:
...     json.dumps(obj)
... except TypeError as e:
...      print e
<...> is not JSON serializable

The CustomTypeEncoder is used to serialise complex objects. However, until the class of object is registered as being serialisable, an exception will still be raised.

>>> try:
...     json.dumps(obj, cls=CustomTypeEncoder)
... except TypeError as e:
...      print e
<...> is not JSON serializable

We can register the object's class as being serialisable. If we don't specify an encoder then the object's __dict__ is used.

>>> register_serialisable_type("complex", ComplexObject)
>>> print json.dumps(obj, cls=CustomTypeEncoder)
{"%complex,__builtin__.ComplexObject%":
{"f_int": 5, "f_string": "string_value"}}

We can register an encoder for a complex object.

>>> class ComplexObjectEncoder(json.JSONEncoder):
...     def default(self, obj):
...         if isinstance(obj, ComplexObject):
...             return {
...                 'int_value': obj.f_int,
...                 'str_value': obj.f_string}
...         return json.JSONEncoder.default(self, obj)

>>> register_serialisable_type(
...     "complex", ComplexObject, ComplexObjectEncoder)

The json encoding of the object is changed accordingly.

>>> print json.dumps(obj, cls=CustomTypeEncoder)
{"%complex,__builtin__.ComplexObject%":
"{\"int_value\": 5, \"str_value\": \"string_value\"}"}

Arbitrarily complex objects are supported, including those with complex attributes.

>>> class AnotherComplexObject:
...     def __init__(self):
...         self.obj = ComplexObject()

>>> register_serialisable_type("another", AnotherComplexObject)
>>> another = AnotherComplexObject()
>>> print json.dumps(another, cls=CustomTypeEncoder)
{"%another,__builtin__.AnotherComplexObject%":
{"obj": {"%complex,__builtin__.ComplexObject%":
"{\"int_value\": 5, \"str_value\": \"string_value\"}"}}}

Collections of complex objects can be serialised.

>>> complex_collection = [ComplexObject() for x in range(4)]
>>> json_collection = json.dumps(complex_collection, cls=CustomTypeEncoder)
>>> print json_collection
[{"%complex,__builtin__.ComplexObject%":
"{\"int_value\": 5, \"str_value\": \"string_value\"}"}, ...]

Serialising Built In Types

Support for some object types is included out-of-the-box.

1. lazr.enum

   >>> from lazr.enum import (
   ...     EnumeratedType,
   ...     Item,
   ...     )
   >>> class Fruit(EnumeratedType):
   ...     "A choice of fruit."
   ...     APPLE = Item('Apple')
   

   >>> an_apple = Fruit.APPLE
   >>> print json.dumps(an_apple, cls=CustomTypeEncoder)
   {"%BaseItem,lazr.enum._enum.Item%": "{\"type\": \"Fruit\", \"name\": \"APPLE\"}"}
   

Object Deserialisation

To deserialise a json string containing complex objects, the json.loads() object_hook is used. One of two requirements must be met for an object to be able to be deserialised:

1. The object class provides a from_dict() method.
2. A decoder for the object class is specified.

If none of the above is true, the object dict from the serialisation process is returned. There's been no decoder for ComplexObject registered yet so all that can be expected is the object's dict:

>>> obj = ComplexObject('complex', 6)
>>> json_data = json.dumps(obj, cls=CustomTypeEncoder)
>>> deserialised = json.loads(json_data, object_hook=custom_type_decoder)
>>> print deserialised
{"int_value": 6, "str_value": "complex"}

Create a decoder for ComplexObject:

>>> class ComplexObjectDecoder:
...     @classmethod
...     def from_dict(cls, class_, value_dict):
...         return class_(
...             value_dict['str_value'], value_dict['int_value'])

Register the decoder:

>>> register_serialisable_type(
...     "complex", ComplexObject, ComplexObjectEncoder,
...     ComplexObjectDecoder)

Now do the deserialisation again and we expect a ComplexObject back:

>>> deserialised = json.loads(json_data, object_hook=custom_type_decoder)
>>> print deserialised == obj
True

Collections of complex objects can be deserialised:

>>> deserialised_collection = json.loads(
...     json_collection, object_hook=custom_type_decoder)
>>> print len(deserialised_collection)
4
>>> print deserialised_collection[0] == complex_collection[0]
True

Now we'll use a from_dict() method on the object class itself. Remove the registered decoder:

>>> register_serialisable_type(
...     "complex", ComplexObject, ComplexObjectEncoder)

Add a from_dict method to ComplexObject:

>>> def from_dict(class_, value_dict):
...     return class_(
...         value_dict['str_value'], value_dict['int_value'])
>>> import types
>>> ComplexObject.from_dict = types.MethodType(from_dict, ComplexObject)

Do the deserialisation again and we expect a ComplexObject back:

>>> deserialised = json.loads(json_data, object_hook=custom_type_decoder)
>>> print deserialised == obj
True

Single value dicts which look like (but are not) serialised objects are correctly handled.

>>> dict_val = json.loads('{"a": 123}', object_hook=custom_type_decoder)
>>> print dict_val
{u'a': 123}
>>> dict_val = json.loads('{"a,b": 123}', object_hook=custom_type_decoder)
>>> print dict_val
{u'a,b': 123}
>>> dict_val = json.loads('{"%a,b%": 123}', object_hook=custom_type_decoder)
>>> print dict_val
{u'%a,b%': 123}

Deserialising Built In Types

Support for deserialising some types is supported out-of-the-box.

1. lazr.enum

   >>> json_apple = json.dumps(an_apple, cls=CustomTypeEncoder)
   >>> deserialised_apple = json.loads(
   ...     json_apple, object_hook=custom_type_decoder)
   >>> print deserialised_apple == an_apple
   True
   

Other Documents

.. toctree::
   :glob:

   *
   docs/*

NEWS for lazr.json