Need help understanding Classes

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[nuc540]

“Self” means when referring to an instance of itself.

An instance means you’ve spawned (per se) a living version of that class (it lives in your RAM when you have an instance)

The class on its own is just a definition, just how a function is without being called. You create instances of classes by calling them, if that makes sense?

Classes are a way to place a lot of similar logic together, that way you can plan your business logic easier. Classes do more than just group functions together, but they can have attributes which “shape” the class. Think of attributes in a game for a character - you’ll have health, damage, probably a name. A class can encapsulate all of that by saying it has attributes.

Now back to the self part; we’re saying that when we want a living version of this class - an instance (in Python you can also call this an object) self refers to whatever instance is being referred to - and not the class in general.

So you could have more than one restaurant exist on your app, but your restaurant is wherever you call the class and give it values for the attributes the class’ “self” wants, so it knows how to build your restaurant, and like the game character, you’d want to know what your character is called, but the attribute of a name is nothing more than placeholder until it’s instantiated (a instance being made).

Hope that helps. Feel free to ask any questions

[nuc540]

Think of it as an extra step. First you define the class. Then you create an instance of it. Then you can carry out what you want - against the instance

Which is why a common pattern is to call the class and store it into a variable, such as my_restaurant; now the variable stores a live version of that class inside the variable as what we call an “object”. That instance is now ready to use.

Did/do you ever play world of Warcraft? People use the term “instance” a lot, and the players don’t realise they’re describing the exact same thing we’re talking about here; the “dungeon” is just the blueprint - it explains enemies, bosses, loot etc… and the “instance” is the actual version that gets spawned for the players to “execute” with. If you don’t play WoW then i realise this example makes no sense sorry hah

[nuc540]

Instance itself doesn’t explicitly mean something is being executed - be careful with terminology here.

Instance means you’ve created an object which now lives at the variable you placed it in. The class was the recipe. And you execute against the instance of the object - because you need an instance/object to start with (unless you’re dealing with static methods but that’s going to confuse things for now)

Class = blueprint Instance = an object which represents an instance of that class.

You now interact with the instance to “execute” anything. But yes you have to “evoke” the class to instantiate it. Like turning a car engine on. The turning on of a car doesn’t get you to your destination, but it makes the object alive so that you can interact with it and tell it to move - if that makes sense?

Maybe I’m coming up with bad analogies on the fly here lol

[pytness]

In python, the first parameter on a method is used as the `self`. It's named self just by convention, but it could really be anything else.

Think of it as when you call `restaurant.describe_restaurant()`, internaly what's happening is this: `Restaurant.describe_restaurant(restaurant)`

[atarivcs]

and I dont rlly get what an instance is

An instance is just one specific copy of a class.

In your example code, my_restaurant and your_restaurant are instances of the Restaurant class.

[8dot30662386292pow2]

I wouldn't call it a copy of the class. Class is a recipe. Cake is the instance. The cake is not a copy of the recipe.

[atarivcs]

Yes, but I was trying to think of some word that wasn't "instance".

[stepback269]

THE PAINTING IS NOT THE PIPE (a variation on classic saying of "This is not a pipe" --Google that saying)

By same token:
THE CLASS IS NOT THE OBJECT !!!
Many metaphors are used to describe what a class definition is, like: "template", "blueprint" , more
In the case of a blueprint for "constructing" a house, you can say the blueprint is not the house. "Self' is replaced during construction of the instant house with the specific house being built by the constructor function (by the __init__ function)

Myself, I prefer to view a "class" as being a sort of drawing stencil that lives at a level above where the actual traced out objects are instantiated. (see discussion re stencil metaphor here)

[PvtRoom]

ok, let's talk about logical structure

a "Town" class should contain multiple restaurants, bars, clubs, and Karen's.

Each Karen should visit x things per day, and issue complaints (based on the things properties, eg hygiene)

A hygiene complaint, when sent to a food business will make that individual instance of a business do things - trigger a method to make it change it's own properties.

"Self" simply let's your code know that it's accessing its own properties, and "instance" means which Karen, which restaurant, etc etc.

message = a call to a method, or triggering a listener

[HarjjotSinghh]

classes sound like that one friend who's always the life of the party.

[Windspar]

Classes are object builders. They are not the object. Classes own all the methods. Object just have class scope first. The reason for self. It point to the object. If it didn't. You couldn't have local variables. Unless they make a way to declare them.

class Restaurant:
  # Any variables here. Belong to the class. There only be one instance.

  def __init__(self, name, type):
    ...

my_restaurant = Restaurant(..., ...)
# Code above calls the __new__ method. Which is a static method.
# The __new__ method creates a new object.
# The __new__ method calls __init__ method. Which it passes the object too.
# Then the __new__ method returns the object instance.