[Python-ideas] Serialization of custom classes.

[Jakob Bowyer]

"""
Currently json dumping an object is sketchy because of the need to serialise.
E.g.
"""
import json

class Something(object):
    def __init__(self, arg1):
        self.arg1 = arg1

    def __str__(self):
        return str(self.arg1)

    def __repr__(self):
        return str(self.arg1)

    def double(self):
        return self.arg1 * 2

test = Something(42)

#json.dumps(test)

"""This ofc raises a TypeError as shown here
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
  File "C:\Python26\Lib\site-packages\spyderlib\widgets\externalshell\startup.py",
line 122, in runfile
    execfile(filename, glbs)
  File "C:\infobarb\test.py", line 22, in <module>
    json.dumps(test)
  File "C:\Python26\lib\json\__init__.py", line 230, in dumps
    return _default_encoder.encode(obj)
  File "C:\Python26\lib\json\encoder.py", line 367, in encode
    chunks = list(self.iterencode(o))
  File "C:\Python26\lib\json\encoder.py", line 317, in _iterencode
    for chunk in self._iterencode_default(o, markers):
  File "C:\Python26\lib\json\encoder.py", line 323, in _iterencode_default
    newobj = self.default(o)
  File "C:\Python26\lib\json\encoder.py", line 344, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: 42 is not JSON serializable

My suggested fix and an idea used by several others could be to add a __json__
method to objects, this method would be tried by json.dumps() before it tries to
serialise the argument. It should return all of the instance variables in a json
format and any other infomation considered to be correct for the dump.
This could be extended to provide a __json__ and a __jsons__ format where the
latter returns a string format of the __json__ serialzation an example of which
(crudely constructed ofc) is below. But for now I consider __json__ to return a
string serialized for json dumping.
"""

import json

def customjsondumps(obj):
    try:
        return json.dumps(obj.__json__())
    except AttributeError:
        raise AttributeError('Object has no __json__ method.')

"""There are ofc several ideas for the __json__ method e.g."""

def __json__(self):
    """This method returns a serialized string for json.dumps"""
    return self.__dict__

"""Or returning some fancy constructed dict, list or other serialised form for
dumping into json."""

class Something(object):
    def __init__(self, arg1):
        self.arg1 = arg1

    def __str__(self):
        return str(self.arg1)

    def __repr__(self):
        return str(self.arg1)

    def double(self):
        return self.arg1 * 2

    def __json__(self):
        ret = self.__dict__.copy()
        for key, item in ret.iteritems():
            ret[key] = item.__repr__()
            return ret

test = Something(41)