models.py

def onPreSave(sender, **kwargs):
    import logging as logger
    model_instance = kwargs['instance']
    logger.debug('before the changes is like this')
    from django.core import serializers
    logger.warning(serializers.serialize('json', [model_instance,], indent=4))
    logger.warning("==============")
    logger.warning("pre_save done ;)")
    logger.warning("==============")

def onPostSave(sender, **kwargs):
    import  logging as logger
    model_instance = kwargs['instance']
    logger.debug("Start post_save")
    from django.core import serializers
    logger.warning(serializers.serialize('json', [model_instance,], indent=4))
    logger.warning("==============")
    logger.warning("post_save done ;)")
    logger.warning("==============")

pre_save.connect(onPreSave, sender=Poll)
post_save.connect(onPostSave, sender=Poll)