Why can I access an object during it's post_save Signal, but not when I trigger code within that signal that calls it on another process

We ran into a similar issue and we ended up using on_commit callback (NOTE: This is only possible with Django >= 1.9). So, you could possible do something like:

from django.db import transaction

class A(models.Model):
    stuff...

def trigger_on_post_save( sender, instance, create, raw, **keywords):
    def on_commit():
        urlopen(r'http://127.0.0.1:[port]' + 
                 reverse(some_view_url, args(instance_pk) ).read()
    transaction.on_commit(on_commit)

post_save.connect( trigger_on_post_save, A )

The idea here is that you wil be calling your endpoint after the transaction has been committed, so the instance involved in the transaction will be already saved ;).

I believe post_save fires after the save occurs, but before the transaction is commited to the database. By default, Django only commits changes to the database after the request has been completed.

Two possible solutions to your problem:

1. Manage your transactions manually, and fire a custom signal after you commit.
2. Have your second process wait a little while for the request to go through.

To be honest though, your whole setup seems a little bit nasty. You should probably look into Celery for asynchronous task queuing.

It's nice place to use decorators. There is slightly extended version of yoanis-gil's answer:

from django.db import transaction
from django.db.models.signals import post_save

def on_transaction_commit(func):
    def inner(*args, **kwargs):
        transaction.on_commit(lambda: func(*args, **kwargs))
    return inner


@receiver(post_save, sender=A)
@on_transaction_commit
def trigger_on_post_save(sender, **kwargs):
    # Do things here