Django signal handlers are stored as weak refs and should *not* be added inside closures.

signal-handlers-example.py

# BAD: Signal receivers will not be executed because, when the function
#      returns, all references to them will be lost and they will be GC'd.

from django.db.models.signals import pre_delete
from django.dispatch import receiver

def add_signal_receivers_unsafe():
    @receiver(pre_delete)
    def receive_pre_delete(sender, **kwargs):
        print "pre_delete received!"

    print "receviers inside add_signal_receivers_unsafe:", pre_delete.receivers

print "receivers before:", pre_delete.receivers
add_signal_receivers_unsafe()
print "receivers after:", pre_delete.receivers

# Prints:
#     $ python manage.py script signal-handlers-example.py
#     receivers before: []
#     receviers inside add_signal_receivers_unsafe: [((4396463960, 4371124552), <weakref at 0x1096cfba8; to 'function' at 0x1060cb758 (receive_pre_delete)>)]
#     receivers after: []

# GOOD: store a reference to the receiver before connecting it:

_strongref_receivers = []
def receiver_strongref(*args, **kwargs):
    def receiver_strongref_helper(f):
        _strongref_receivers.append(f)
        return receiver(*args, **kwargs)(f)
    return receiver_strongref_helper

def add_signal_receivers_safe():
    @receiver_strongref(pre_delete)
    def receive_pre_delete(sender, **kwargs):
        print "pre_delete received!"

    print "receviers inside add_signal_receivers:", pre_delete.receivers

print "receivers before:", pre_delete.receivers
add_signal_receivers_safe()
print "receivers after:", pre_delete.receivers

# Prints:
#     $ python manage.py script signal-handlers-example.py
#     receivers before: []
#     receviers inside add_signal_receivers_safe: [((4396463960, 4371124552), <weakref at 0x1096cfba8; to 'function' at 0x1060cb758 (receive_pre_delete)>)]
#     receivers after: [[((4396463960, 4371124552), <weakref at 0x1096cfba8; to 'function' at 0x1060cb758 (receive_pre_delete)>)]]