I am trying to teach myself programming, but I have always been very confused by the object oriented approach.

Recently, I have been reading about SOLID development principles.

When I create Classes, using Python, it seems I add the application logic there. However, this appears to defy the single responsibility principle, and injection principles.

For a simple example of my problem, let's assume that my ultimate goal is to change the TV to channel 10, turn the connected (but separate) surround sound system to max, and then turn the TV off.

I would do something like this:

from SpeakerSystem import DolbyDigital

Class Tv(object):

def __init__(self):
    self.speakers = DolbyDigital()
    self.volume = 10
    self.channel = 1
    self.power = 'Off'

def turnOn(self):
    self.power = 'On'

def turnOff(self):
    self.power = 'Off'

def channelUp(self):
    self.channel += 1

def channelDown(self):
    self.channel -= 1

def volumeUp(self):

def volumeDown(self):

def setChannel(self, station):
    self.channel = station

def process(self):
    self.channel = 10
    while self.volume != self.max_volume:

Ignore for a minute that this is a contrived example!

Then, to solve my problem i will simply instantiate Tv, and run process().

The problem is that I always create many methods like process() here; I essentially bake in my application code in with the class.

I think this is wrong? The vast majority of methods in my class actually go about solving my problem directly; they clearly will not be useful to anyone, or even myself, outside of my specific problem domain.

In the example, process is not useful to anyone (only useful for my niche problem). Imagine 10 more methods like that, which are totally useless.

Also, I have read that baking in self.speakers = DolbyDigital() is also bad design, because now Tv depends on it. And this gets messy when unit-testing; having to constantly mock out other objects. Many books suggest that objects should not even be responsible for initializing other objects....

The more I read about OOP and OODesign, it seems the class itself should be general, it should provide only the basic methods (it shouldn't actually do the whole job), and that the application code should somehow be separate. Is this true? If so, where should the application code go?

I thought about creating a plain old main() file, however it seems that if I did that, the programming paradigm then shifts to procedural. And the whole point of learning OOP is to stick with OOP!?

I am truly utterly confused about it. Again, sorry for the seemingly dumb question, but I have no formal training, so just trying to improve with your help!

I have read several object oriented books, including those by Gamma et. al, Uncle Bob, and several other seminal books. It seems my question is so stupid/novice that they do not actually cover it.


You process method is indeed in a wrong place.

What is the context where you will switch to channel 10? Why channel 10, and not channel 9?

  • Maybe the user asked specifically the channel 10. In this case, 10 should be the input, not a value within the class.

  • Maybe this is the default channel, the one which will be shown every time the TV is turned on. In this case, this should be either a constant or, often, a configuration entry.

    By making it a constant, you can be sure that later, when requirements change and the default channel becomes the channel 9, you have to make the change in one location only.

    By moving it to configuration, you acquire the benefit of letting the system administrator change the value later, without having to modify and redeploy the application.

As for the self.speakers = DolbyDigital() part, you should let the caller the ability to set the speakers. This is useful in two cases:

  • When the caller doesn't want to use DolbyDigital, but wants to use something else.

  • When you are testing your class. You can then set the speakers to SpeakersMock or SpeakersStub in order to focus your testing on the class itself and isolate those tests to avoid side effects.

This doesn't prevent you from having a default value for speakers field. The benefit of having one is when you initialize Tv in different places in your code. Instead of having to think every time about the initialization of speakers, you just let the class apply the default value.

The lack of such default values can sometimes lead to constructors requesting a dozen of parameters. Not inherently bad, but still difficult to use.

| improve this answer | |
  • What do you mean by the 'caller'? Do you mean to pass DolbyDigital() as an argument in init()? Also, where explicitly should 'process()' go? In this example, the user just wants an app that executes process(). – BBedit Apr 6 '15 at 22:14
  • @BBedit: (1) the caller is the code which uses your Tv class; it may be the main method of the application, but also may be buried down somewhere in libraries. (2) DolbyDigital can be passed to __init__, but can also be accessed through a property. (3) The location of your business logic depends on the application; there is no single answer which covers an infinite number of situations. It's all about OOP: the logic should be where it makes sense for it to be. – Arseni Mourzenko Apr 7 '15 at 11:34

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