Karim Hamidou
Using Django lazily translated strings in JSON — 20 February 2013

Last week I had to add support for i18n to a django application. Everything went swiftly, except for one thing : I needed to output a translated JSON structure in my HTML and doing this simple with json.dumps triggered a TypeError exception, with the message : “<django.utils.functional.__proxy__ object at 0x987a86c> is not JSON serializable”.

The solution to this problem is to define a custom serializer to force the conversion of lazy translated strings (also known as Promise objects) to a string. To do this define the following class :

class LazyEncoder(json.JSONEncoder):
"""Encodes django's lazy i18n strings.
Used to serialize translated strings to JSON, because
simplejson chokes on it otherwise.
"""
    def default(self, obj):
        if isinstance(obj, Promise):
            return force_unicode(obj)
        return obj

and call simplejson this way : json.dumps(object, cls=LazyEncoder)