Question

I am using a custom Django model field to and widget to render a GoogleMap widget in my admin, i also want to use South with my project to handle database migrations. However after much effort i am unable to generate a custom South rule that fits, this are my custom model and the last of the many instrospection rules that i’ve tried.

class GoogleMapMarkerField(models.CharField):
    __metaclass__ = models.SubfieldBase
    description = _('Un marcador de Google Maps')
    widget = GoogleMapMarkerWidget

    def __init__(self, center, *args, **kwargs):
        kwargs['max_length'] = 100
        kwargs['help_text'] = _('Arrastre el cursor en el mapa para seleccionar el punto')
        self.center = center

        super(GoogleMapMarkerField, self).__init__(*args, **kwargs)

    def formfield(self, **kwargs):
        defaults = {
            'center': self.center,
            'form_class':GoogleMapMarkerFormField
        }
        defaults.update(kwargs)
        return super(GoogleMapMarkerField, self).formfield(**defaults)

    def to_python(self, value):
        if isinstance(value, GoogleMapMarker):
            return value
        if isinstance(value, list):
            return GoogleMapMarker(*map(float, value))

        elif isinstance(value, basestring):
            try:
                return GoogleMapMarker(*map(float, value.split(',')))
            except ValueError:
                pass

    def get_prep_value(self, value):
        return '%f,%f' % (value.latitude, value.longitude)


add_introspection_rules([
            (
                (GoogleMapMarkerField, ),
            [],
            {
                'center': ('center', {}),
            }
            )

    ],  ["^website\.fields\.GoogleMapMarkerField"])

And this is the traceback that i’m getting

Traceback (most recent call last):
  File "manage.py", line 14, in <module>
    execute_manager(settings)
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/django/core/management/__init__.py", line 438, in execute_manager
    utility.execute()
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/django/core/management/__init__.py", line 379, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/django/core/management/base.py", line 191, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/django/core/management/base.py", line 220, in execute
    output = self.handle(*args, **options)
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/south/management/commands/schemamigration.py", line 97, in handle
    old_orm = last_migration.orm(),
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/south/utils.py", line 62, in method
    value = function(self)
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/south/migration/base.py", line 422, in orm
    return FakeORM(self.migration_class(), self.app_label())
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/south/orm.py", line 46, in FakeORM
    _orm_cache[args] = _FakeORM(*args)  
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/south/orm.py", line 125, in __init__
    self.models[name] = self.make_model(app_label, model_name, data)
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/south/orm.py", line 321, in make_model
    field = self.eval_in_context(code, app, extra_imports)
  File "/home/armonge/workspace/env/lib/python2.7/site-packages/south/orm.py", line 236, in eval_in_context
    return eval(code, globals(), fake_locals)
  File "<string>", line 1, in <module>
TypeError: __init__() takes at least 2 arguments (1 given)

Answer 1

center isn’t a keyword argument, it’s a positional argument. You shouldn’t use positional arguments with South, it doesn’t understand them. (See Custom Fields: Keyword Arguments). You could solve this by providing center with a default value( center=None would be fine) and then following the example code at the link for defining the keyword name as passed to __init__, the name as stored in the database, and a dictionary of options (may be blank, but setting the default value there too helps).