Is Using a God Class with Mixins Still Bad Practice?

Having written and then refactored a python "God object", I sympathize. What I did is break the original object down into sub sections based upon methods. For instance, the original looked like this pseudo code:

method A():
    self.bla += 1

method B():
    self.bla += 1

do stuff():
    self.bla = 1
    method A()
    method B()
    print self.bla

The stuff method is a self contained "unit" of work. I migrated it out to a new class which the original instantiates. This pulled out the necessary properties as well. Some were used only by the sub class and could move straight across. Others were shared, and got moved into a shared class.

The "God object" creates a new copy of the shared class at start up, and each of the new sub classes accepts a pointer as part of their init method. For example, here's a stripped version of the mailer:

#!/usr/bin/env python
# -*- coding: ascii -*-
'''Functions for emailing with dirMon.'''

from email.MIMEMultipart import MIMEMultipart
from email.MIMEBase import MIMEBase
from email.MIMEText import MIMEText
from email.Utils import COMMASPACE, formatdate
from email import Encoders
import os
import smtplib
import datetime
import logging

class mailer:
    def __init__(self,SERVER="mail.server.com",FROM="support@server.com"):
        self.server = SERVER
        self.send_from = FROM
        self.logger = logging.getLogger('dirMon.mailer')

    def send_mail(self, send_to, subject, text, files=[]):
        assert type(send_to)==list
        assert type(files)==list
        if self.logger.isEnabledFor(logging.DEBUG):
            self.logger.debug(' '.join(("Sending email to:",' '.join(send_to))))
            self.logger.debug(' '.join(("Subject:",subject)))
            self.logger.debug(' '.join(("Text:",text)))
            self.logger.debug(' '.join(("Files:",' '.join(files))))
        msg = MIMEMultipart()
        msg['From'] = self.send_from
        msg['To'] = COMMASPACE.join(send_to)
        msg['Date'] = formatdate(localtime=True)
        msg['Subject'] = subject
        msg.attach( MIMEText(text) )
        for f in files:
            part = MIMEBase('application', "octet-stream")
            part.set_payload( open(f,"rb").read() )
            Encoders.encode_base64(part)
            part.add_header('Content-Disposition', 'attachment; filename="%s"' % os.path.basename(f))
            msg.attach(part)
        smtp = smtplib.SMTP(self.server)
        mydict = smtp.sendmail(self.send_from, send_to, msg.as_string())
        if self.logger.isEnabledFor(logging.DEBUG):
            self.logger.debug("Email Successfully Sent!")
        smtp.close()
        return mydict

It is created once and shared between the different classes that need mailing capabilities.

So for you, create a class larry with the properties and methods you need. Everywhere the client says larry = blah replace it with larryObj.larry = blah. This migrates things to sub projects without breaking the current interface.

The only other thing to do is look for "units of work". If you were going to turn part of the "God Object" into it's own method, do so. But, put the method outside it. This forces you to create an interface between the components.

Laying that groundwork allows everything else to follow it. For example, a piece of the helper object demonstrating how it interfaces with the mailer:

#!/usr/bin/env python
'''This module holds a class to spawn various subprocesses'''
import logging, os, subprocess, time, dateAdditionLib, datetime, re

class spawner:
    def __init__(self, mailer):
        self.logger = logging.getLogger('dirMon.spawner')
        self.myMailer = mailer

Concentrate on the smallest individual unit of work possible, and move it out. This is easier to do, and lets you play with the setup quickly. Don't look at properties for moving stuff, they are ancillary to the tasks being done with them in most cases. Whatever is left over after you have dealt with the methods should probably stay in the original object, since it is part of the shared state.

But, the new objects should now accept the properties they need as init variables, not touching the caller objects property. They then return any necessary values, which the caller can use to update the shared properties as necessary. This helps to decouple the objects and makes for a more robust system.

Yes, these are mixins. They aren't as powerful as Scala traits (no ability to have a reference to the parent "self" object and its properties or methods), but the default methods can implement meaningful behaviour and so can be more more than sticking plasters to stop existing code breaking when an interface is extended.

The limited power of default methods seems to be deliberate, judging by the various informed discussions of the feature. The designers' advice is that default methods should generally not contain the concrete implementation of a method but accept a lambda or reference to a concrete class, either of which can provide the implementation. Example here