[Fixed]-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 ;).

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

Had same issue when creating new model from django admin. Overriding ModelAdmin.save_model method to manage transaction manually worked.

def save_model(self, request, obj, form, change):
    from django.db import transaction
    with transaction.commit_on_success():
       super(ModelAdmin, self).save_model(request, obj, form, change)

    # write your code here