base.py

from signal import Signal
from model import Model

pre_save = Signal()
post_save = Signal()

def post_save_method():
    print('post save method run')

def pre_save_method():
    print('pre save method run')

post_save.connect(post_save_method, sender=Model)
pre_save.connect(pre_save_method, sender=Model)

model = Model()
model.save()

model.py

class Model:
    def save(self):
        pre_save.send(self)
        # blabla
        post_save.send(self)

signal.py

class Signal:
    def __init__(self):
        self._receivers = []
    def connect(self, receiver, name=None, sender=None):
        if callable(receiver):
            name = name or receiver.__name__
            receiver_data = {
                'function': receiver,
                'name': name,
                'sender': sender
            }
            self._receivers.append(receiver_data)
        else:
            raise BaseException('Receiver must be callable')

    def disconnect(self, receiver=None, name=None):
        if receiver:
            name = name or receiver.__name__
        for item in self._receivers:
            if item['name'] == name:
                del item
                break

    def send(self, instance):
        for item in self._receivers:
            if isinstance(instance, item['sender']) or item['sender'] is None:
                item['function'].__call__()