signals.md

Maneira 1

# A lógica é mantida dentro de um método da classe
# arquivo 'models.py'
from django.db.models.signals import post_save

class Product(TimestampableMixin):
    # ...

    @classmethod
    def increment_stock(cls, sender, instance, **kwargs):
        product = instance.product
        product.stock += instance.amount
        product.save()


class StockEntry(TimestampableMixin):
    # ...


post_save.connect(Product.increment_stock, sender=StockEntry)

Maneira 2

# A lógica é mantida num método fora da classe
# arquivo 'models.py'
from django.db.models.signals import pre_delete
from django.dispatch import receiver


@receiver(pre_delete, sender=Banner) # Banner é o model
def handler_pre_delete(sender, instance, **kwargs):
    os.remove(instance.imagem.path)
    print('sender', sender)

Maneira ~3

Doc auxiliar: https://docs.djangoproject.com/en/2.2/ref/applications/#configuring-applications\ A lógica é mantida dentro do arquivo de configuração em 'apps.py'. E deve ser chamada a partir do settings puxando pela classe que herda de AppConfig.

# 'stock/signals.py'
def increment_stock(sender, instance, **kwargs):
    product = instance.product
    product.stock += instance.amount
    product.save()

# Maneira ~1
# post_save.connect(increment_stock, sender=StockEntry)

# Maneira ~1
# lógica em 'app.py' no método 'ready'

No 'apps.py' podemos configurar de 2 maneiras

# 'apps.py'
from django.apps import AppConfig


class StockConfig(AppConfig):
    name = 'stock'

    def ready(self):
        # Maneira ~1
        # para manter essa maneira veja os comentários do arquivo 'signals.py'
        # from stock import signals

        # Maneira ~2
        from stock.signals import increment_stock
        from django.db.models.signals import post_save
        stock_entry_model = self.get_model(model_name='StockEntry')
        post_save.connect(increment_stock, sender=stock_entry_model)

Agora em settings.py, chame a app de forma correta.

# ...
INSTALLED_APPS = [
    # ...
    'stock.apps.StockConfig',
]

Signal personalizado

Em 'signals.py'

from django.dispatch.dispatcher import Signal

signal_personalizado_product_stock_changed = Signal()`

def increment_stock(sender, instance, created, **kwargs):
    if created is True:
        product = instance.product
        product.stock += instance.amount
        product.save()
        
        # Adiciona
        signal_personalizado_product_stock_changed.send(sender=None, instance=product)  

def send_mail_stock_max_overload(sender, instance, **kwargs):
    if instance.stock > instance.stock_max:
        # Enviar email
        pass
        
post_save.connect(increment_stock, sender=StockEntry)
# Adiciona
signal_personalizado_product_stock_changed.connect(send_mail_stock_max_overload, sender=None)

https://docs.djangoproject.com/en/2.2/topics/signals/
https://docs.djangoproject.com/en/2.2/ref/signals/