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django-db-signals

This app adds a set of signals to some of Django's database operations:

  • django.db.signals.pre_commit
  • django.db.signals.post_commit
  • django.db.signals.pre_rollback
  • django.db.signals.post_rollback
  • django.db.signals.pre_transaction_management
  • django.db.signals.post_transaction_management

Requirements

  • Python 2.6/2.7/3.2/3.3 (3.x requires Django >=1.5)
  • Django 1.2/1.3/1.4/1.5

Installation

  1. Install from PyPI: pip install django-db-signals.

  2. Add 'django_db_signals' to INSTALLED_APPS.

  3. Enable the signals by adding to your models.py:

    import django_db_signals
django_db_signals.enable()

Example

Let's assume you've installed django-db-signals, and now you'd like to log a message each time the database is rolled back:

from django.db import signals
from django.dispatch import receiver
import logging

logger = logging.getLogger(__name__)

@receiver(signals.post_rollback)
def log_rollbacks(sender, **kwargs):
    # sender is a DatabaseWrapper object
    logger.info("A rollback occurred on database %s" %
                sender.alias)

Design

In the same way that Django settings are available via django.conf.settings attributes, signals are available via django.db.signals attributes. As such, attempting to import individual signals will fail:

>>> import django_db_signals
>>> django_db_signals.enable()

>>> from django.db import signals  # GOOD
>>> signals.pre_commit
<django.dispatch.dispatcher.Signal object at 0x1089c8b90>

>>> from django.db.signals import pre_commit  # BAD
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named signals

pre_… vs post_… signals

pre_… signals are sent before an operation occurs. The signals are sent via .send(…). Exceptions raised in receivers are propagated to the application. This can be exploited to cancel the operation (e.g. to block a commit).

post_… signals are sent after an operation, and as such can't offer the same cancel the pending operation behaviour. The signal is sent via .send_robust(…) to ensure all receivers are called. Any exceptions raised are logged, but are not propagated to the application.

Logging

A logger named django.db.signals is used to log all exceptions raised in post_… receivers.

Signal senders

For all database signals, the sender of the signal is Django's database connection wrapper.

Troubleshooting

"I can import django.db.signals, but when I try to access a signal I get an AttributeError exception."

You need to enable the signals via django_db_signals.enable()