Question

I’m using the code found here to convert a GAE model into JSON:

def to_dict(self):
    return dict([(p, unicode(getattr(self, p))) for p in self.properties()])

It works just fine, but if a property does not have a value, it puts a default string of “None”, and this is interpreted as a real value in my client device (Objective-C), even though it should be interpreted as a nil value.

How can I modify the code above while maintaining its brevity to skip over and not write properties to the dictionary that have None values?

Answer 1

def to_dict(self):
    return dict((p, unicode(getattr(self, p))) for p in self.properties()
                if getattr(self, p) is not None)

You don’t need to create a list first (the surrounding []), you can just use a generator expression to build the values on-the-fly.

It’s not quite brief, but if your model structure ever gets a bit more complex, you may want to look at this recursive variant:

# Define 'simple' types
SIMPLE_TYPES = (int, long, float, bool, dict, basestring, list)

def to_dict(model):
    output = {}

    for key, prop in model.properties().iteritems():
        value = getattr(model, key)

        if isinstance(value, SIMPLE_TYPES) and value is not None:
            output[key] = value
        elif isinstance(value, datetime.date):
            # Convert date/datetime to ms-since-epoch ("new Date()").
            ms = time.mktime(value.utctimetuple())
            ms += getattr(value, 'microseconds', 0) / 1000
            output[key] = int(ms)
        elif isinstance(value, db.GeoPt):
            output[key] = {'lat': value.lat, 'lon': value.lon}
        elif isinstance(value, db.Model):
            # Recurse
            output[key] = to_dict(value)
        else:
            raise ValueError('cannot encode ' + repr(prop))

    return output

This could be easily extended with other non-simple types by adding to the elif branches.