root/trunk/json.py

"""JSON conversion support for Dejavu Units."""

import datetime
import decimal
import time

import dejavu
from simplejson import JSONEncoder, JSONDecoder

__all__ = ["Encoder", "Decoder", "Converter"]

class Null(object):
    class meta(type):
        def __new__(cls, *args, **kwargs):
            if '_inst' not in vars(cls):
                cls._inst = type.__new__(cls, *args, **kwargs)
            return cls._inst
    __metaclass__ = meta
    def __init__(self, *args, **kwargs): pass
    def __call__(self, *args, **kwargs): return self
    def __repr__(self): return "Null()"
    def __nonzero__(self): return False
    def __getattr__(self, name): return self
    def __setattr__(self, name, value): return self
    def __delattr__(self, name): return self

class Encoder(JSONEncoder):
    """Extends the base simplejson JSONEncoder for Dejavu."""
    
    DATE_FMT = "%Y-%m-%d"
    TIME_FMT = "%H:%M:%S"
    DATETIME_FMT = "%s %s" % (DATE_FMT, TIME_FMT)
    
    def default(self, o):
        _special_types = (datetime.date, datetime.time, datetime.datetime,
                          decimal.Decimal)
        if isinstance(o, dejavu.Unit):
            cls = o.__class__
            d = {'__dejavu.Unit__': True,
                 '__class__': cls.__name__}
            for key in o.properties:
                val = getattr(o, key)
                type = getattr(cls, key).type
                if val is None and type in _special_types:
                    val = type
                d[key] = val
            return d
        
        # We MUST check for a datetime.datetime instance before datetime.date.
        # datetime.datetime is a subclass of datetime.date, and therefore
        # instances of it are also instances of datetime.date.
        if isinstance(o, datetime.datetime) or o is datetime.datetime:
            if o is datetime.datetime:
                o = Null()
            return {'__datetime__': True,
                    'value': o.strftime(self.DATETIME_FMT)}
        
        if isinstance(o, datetime.date) or o is datetime.date:
            if o is datetime.date:
                o = Null()
            return {'__date__': True,
                    'value': o.strftime(self.DATE_FMT)}
        
        if isinstance(o, datetime.time) or o is datetime.time:
            if o is datetime.time:
                o = Null()
            return {'__time__': True,
                    'value': o.strftime(self.TIME_FMT)}
        
        if isinstance(o, decimal.Decimal) or o is decimal.Decimal:
            if o is decimal.Decimal:
                value = None
            else:
                value = unicode(o)
            return {'__decimal__': True,
                    'value': value}
        
        if isinstance(o, Null):
            return None
        
        else:
            return JSONEncoder.default(self, o)

class Decoder(JSONDecoder):
    """Extends the base simplejson JSONDecoder for Dejavu."""
    
    DATE_FMT = "%Y-%m-%d"
    TIME_FMT = "%H:%M:%S"
    DATETIME_FMT = "%s %s" % (DATE_FMT, TIME_FMT)
    
    def __init__(self, store=None, encoding=None, object_hook=None):
        JSONDecoder.__init__(self, encoding, object_hook)
        if not self.object_hook:
            self.object_hook = self.json_to_python
        self.store = store
    
    def json_to_python(self, d):
        if '__datetime__' in d:
            if d['value'] is None:
                return None
            strp = time.strptime(d['value'], self.DATETIME_FMT)[:7]
            return datetime.datetime(*strp)
        if '__date__' in d:
            if d['value'] is None:
                return None
            strp = time.strptime(d['value'], self.DATE_FMT)[:3]
            return datetime.date(*strp)
        if '__time__' in d:
            if d['value'] is None:
                return None
            strp = time.strptime(d['value'], self.TIME_FMT)[3:6]
            return datetime.time(*strp)
        if '__decimal__' in d:
            if d['value'] is None:
                return None
            return decimal.Decimal(d['value'])
        if '__dejavu.Unit__' in d:
            # If d['ID'] != None, should we lookup the Unit in storage set
            # any changed attributes, and then return the Unit?  The approach
            # below is to return a new instance of the Unit with the values
            # from d.  It is then up to library user to explicitly set the
            # property values on any existing Unit.
            clsname = d['__class__']
            cls = self.store.class_by_name(clsname)
            params = dict([(str(k), v) for k,v in d.iteritems() if not k.startswith('__')])
            unit = cls(**params)
            return unit
        return d

class Converter(object):
    """Provides two-way conversion of Units/JSON via loads and dumps methods.
    
    Also converts datetime.date, datetime.time, datetime.datetime and
    decimal.Decimal to/from JSON.

    This is accomplished by the Encoder and Decoder classes, which are
    subclasses of their counterparts in simplejson.  If you wish to change
    the output of the converter at all, you should probably subclass the
    Encoder/Decoder and then make a cusom Converter subclass with your
    encoder/decoder as class attributes.
    """
    
    encoder = Encoder
    decoder = Decoder
    
    def __init__(self, store):
        self.store = store
    
    def loads(self, s, encoding=None, **kw):
        return self.decoder(encoding=encoding, store=self.store, **kw).decode(s)

    def dumps(self, obj, skipkeys=False, ensure_ascii=False,
              check_circular=True, allow_nan=True, indent=None, **kw):
        return self.encoder(skipkeys, ensure_ascii, check_circular,
                            allow_nan, indent, **kw).encode(obj)