Add a custom widget to all bigIntegerfields on a django_import_export Model Admin, or otherwise override the modelresource_factory

gistfile1.py

from import_export.resources import *

from import_export.admin import *
from import_export.widgets import Widget

class NullWidget(Widget):
    def render(self, value):
        if value == "":
            return None
        return value

from django.db.models.fields import BigIntegerField
import import_export.fields as ieFields

def modelresource_factory(model, resource_class=ModelResource):
    """
    Factory for creating ``ModelResource`` class for given Django model.
    """
    attrs = {'model': model}
    Meta = type(str('Meta'), (object,), attrs)

    class_name = model.__name__ + str('Resource')

    class_attrs = {
        'Meta': Meta,
        }
    metaclass = ModelDeclarativeMetaclass
    resourceClass=metaclass(class_name, (resource_class,), class_attrs)

    for i in model._meta.fields:
        if type(i) == BigIntegerField:
            resourceClass.fields[i.name].widget = NullWidget()

    return resourceClass

class ImportExportModelAdmin(ImportExportModelAdmin):
    def __init__(self, *args, **kwargs):
        self.ExportMixin=ExportMixin
        super(ImportExportModelAdmin, self).__init__(*args, **kwargs)

    def get_resource_class(self):
        if not self.resource_class:
            return modelresource_factory(self.model)
        else:
            return self.resource_class