Difference Between Class Variables and Class Instance Variables

Difference between class variables and class instance variables?

A class variable (@@) is shared among the class and all of its descendants. A class instance variable (@) is not shared by the class's descendants.

Class variable (@@)

Let's have a class Foo with a class variable @@i, and accessors for reading and writing @@i:

class Foo

@@i = 1

def self.i

def self.i=(value)
@@i = value


And a derived class:

class Bar < Foo

We see that Foo and Bar have the same value for @@i:

p Foo.i    # => 1
p Bar.i # => 1

And changing @@i in one changes it in both:

Bar.i = 2
p Foo.i # => 2
p Bar.i # => 2

Class instance variable (@)

Let's make a simple class with a class instance variable @i and accessors for reading and writing @i:

class Foo

@i = 1

def self.i

def self.i=(value)
@i = value


And a derived class:

class Bar < Foo

We see that although Bar inherits the accessors for @i, it does not inherit @i itself:

p Foo.i    # => 1
p Bar.i # => nil

We can set Bar's @i without affecting Foo's @i:

Bar.i = 2
p Foo.i # => 1
p Bar.i # => 2

Ruby class instance variable vs. class variable

Instance variable on a class:

class Parent
@things = []
def self.things
def things

class Child < Parent
@things = []

Parent.things << :car
Child.things << :doll
mom = Parent.new
dad = Parent.new

p Parent.things #=> [:car]
p Child.things #=> [:doll]
p mom.things #=> [:car]
p dad.things #=> [:car]

Class variable:

class Parent
@@things = []
def self.things
def things

class Child < Parent

Parent.things << :car
Child.things << :doll

p Parent.things #=> [:car,:doll]
p Child.things #=> [:car,:doll]

mom = Parent.new
dad = Parent.new
son1 = Child.new
son2 = Child.new
daughter = Child.new

[ mom, dad, son1, son2, daughter ].each{ |person| p person.things }
#=> [:car, :doll]
#=> [:car, :doll]
#=> [:car, :doll]
#=> [:car, :doll]
#=> [:car, :doll]

With an instance variable on a class (not on an instance of that class) you can store something common to that class without having sub-classes automatically also get them (and vice-versa). With class variables, you have the convenience of not having to write self.class from an instance object, and (when desirable) you also get automatic sharing throughout the class hierarchy.

Merging these together into a single example that also covers instance variables on instances:

class Parent
@@family_things = [] # Shared between class and subclasses
@shared_things = [] # Specific to this class

def self.family_things
def self.shared_things

attr_accessor :my_things
def initialize
@my_things = [] # Just for me
def family_things
def shared_things

class Child < Parent
@shared_things = []

And then in action:

mama = Parent.new
papa = Parent.new
joey = Child.new
suzy = Child.new

Parent.family_things << :house
papa.family_things << :vacuum
mama.shared_things << :car
papa.shared_things << :blender
papa.my_things << :quadcopter
joey.my_things << :bike
suzy.my_things << :doll
joey.shared_things << :puzzle
suzy.shared_things << :blocks

p Parent.family_things #=> [:house, :vacuum]
p Child.family_things #=> [:house, :vacuum]
p papa.family_things #=> [:house, :vacuum]
p mama.family_things #=> [:house, :vacuum]
p joey.family_things #=> [:house, :vacuum]
p suzy.family_things #=> [:house, :vacuum]

p Parent.shared_things #=> [:car, :blender]
p papa.shared_things #=> [:car, :blender]
p mama.shared_things #=> [:car, :blender]
p Child.shared_things #=> [:puzzle, :blocks]
p joey.shared_things #=> [:puzzle, :blocks]
p suzy.shared_things #=> [:puzzle, :blocks]

p papa.my_things #=> [:quadcopter]
p mama.my_things #=> []
p joey.my_things #=> [:bike]
p suzy.my_things #=> [:doll]

What is the difference between instance Variable Names and class Variable Names, in pharo?

I have an impression that this question was already asked and answered, but I was not able to find it, probably it was on a mailing list.

Instance variables are just the variables that are personal to an instance, so each instance of the class that you are defining will have it's own set of variables.

Instance variables of the class side. You can define that when you switch to the class side, then you will se a code like this:

MyClass class
instanceVariableNames: ''

In Pharo (and Smalltalks in general) each class is an instance of a meta class. The variables can be accessed by class-side methods, and as there is only a single instance representing class object, there will be only one set of these variables of that instance.

Class variables are the variables defined by classVariableNames: '' on the instance side template. I like to call them "pool variables", but in fact if you define such variable, all the instances from the hierarchy will be able to access it. Let's say that you have a class A and it's subclass B. If you have a "class variable" in A, you can access the same variable from both instances of A and B. It's like having a global variable for a hierarchy. I recommend not to use this type of variables.

Difference between Class variables and Instance variables

Class variables are shadowed by instance attribute. This means that when looking up an attribute, Python first looks in the instance, then in the class. Furthermore, setting a variable on an object (e.g. self) always creates an instance variable - it never changes the class variable.

This means that when, in your second example you do:

self.x += 1

which is (in this case, see footnote) equivalent to:

self.x = self.x + 1

what Python does is:

  1. Look up self.x. At that point, self doesn't have the instance attribute x, so the class attribute A.x is found, with the value 10.
  2. The RHS is evaluated, giving the result 11.
  3. This result is assigned to a new instance attribute x of self.

So below that, when you look up x.x, you get this new instance attribute that was created in add(). When looking up y.x, you still get the class attribute. To change the class attribute, you'd have to use A.x += 1 explicitly – the lookup only happens when reading the value of an attribute.

Your first example is a classical gotcha and the reason you shouldn't use class attributes as "default" values for instance attributes. When you call:


there is no assignment to self.x taking place. (Changing the contents of a mutable object, like a list, is not the same as assignment.) Thus, no new instance attribute is added to x that would shadow it, and looking up x.x and y.x later on gives you the same list from the class attribute.

Note: In Python, x += y is not always equivalent to x = x + y. Python allows you to override the in-place operators separately from the normal ones for a type. This mostly makes sense for mutable objects, where the in-place version will directly change the contents without a reassignment of the LHS of the expression. However, immutable objects (such as numbers in your second example) do not override in-place operators. In that case, the statement does get evaluated as a regular addition and a reassignment, explaining the behaviour you see.

(I lifted the above from this SO answer, see there for more details.)

Difference between Ruby's class variable and instance variable

I think "instance variable, class variable and the difference between them in ruby" has a good explanation of the difference between local, instance and class variables.

difference between class method , instance method , instance variable , class variable?

First take a look at this diagram:

from "Metaprogramming Ruby" book

You can rightly say that “obj has a method called my_method( ),” meaning that you’re able to call obj.my_method(). By contrast, you shouldn’t say that “MyClass has a method named my_method().” That would be confusing, because it would imply that you’re able to call MyClass.my_method() as if it were a class method.

To remove the ambiguity, you should say that my_method() is an instance method (not just “a method”) of MyClass, meaning that it’s defined in MyClass, and you actually need an instance of MyClass to call it. It’s the same method, but when you talk about the class, you call it an instance method, and when you talk about the object, you simply call it a method. Remember this distinction, and you won’t get confused when writing introspective code like this:

String.instance_methods == "abc".methods # => true String.methods == "abc".methods # => false

an object’s instance variables live in the object itself, and an object’s methods live in the object’s class. That’s why objects of the same class share methods but don’t share instance variables.

What is the difference between class and instance variables?

When you write a class block, you create class attributes (or class variables). All the names you assign in the class block, including methods you define with def become class attributes.

After a class instance is created, anything with a reference to the instance can create instance attributes on it. Inside methods, the "current" instance is almost always bound to the name self, which is why you are thinking of these as "self variables". Usually in object-oriented design, the code attached to a class is supposed to have control over the attributes of instances of that class, so almost all instance attribute assignment is done inside methods, using the reference to the instance received in the self parameter of the method.

Class attributes are often compared to static variables (or methods) as found in languages like Java, C#, or C++. However, if you want to aim for deeper understanding I would avoid thinking of class attributes as "the same" as static variables. While they are often used for the same purposes, the underlying concept is quite different. More on this in the "advanced" section below the line.

An example!

class SomeClass:
def __init__(self):
self.foo = 'I am an instance attribute called foo'
self.foo_list = []

bar = 'I am a class attribute called bar'
bar_list = []

After executing this block, there is a class SomeClass, with 3 class attributes: __init__, bar, and bar_list.

Then we'll create an instance:

instance = SomeClass()

When this happens, SomeClass's __init__ method is executed, receiving the new instance in its self parameter. This method creates two instance attributes: foo and foo_list. Then this instance is assigned into the instance variable, so it's bound to a thing with those two instance attributes: foo and foo_list.


print instance.bar


I am a class attribute called bar

How did this happen? When we try to retrieve an attribute through the dot syntax, and the attribute doesn't exist, Python goes through a bunch of steps to try and fulfill your request anyway. The next thing it will try is to look at the class attributes of the class of your instance. In this case, it found an attribute bar in SomeClass, so it returned that.

That's also how method calls work by the way. When you call mylist.append(5), for example, mylist doesn't have an attribute named append. But the class of mylist does, and it's bound to a method object. That method object is returned by the mylist.append bit, and then the (5) bit calls the method with the argument 5.

The way this is useful is that all instances of SomeClass will have access to the same bar attribute. We could create a million instances, but we only need to store that one string in memory, because they can all find it.

But you have to be a bit careful. Have a look at the following operations:

sc1 = SomeClass()

sc2 = SomeClass()

print sc1.foo_list
print sc1.bar_list

print sc2.foo_list
print sc2.bar_list

What do you think this prints?

[2, 20]
[2, 20]

This is because each instance has its own copy of foo_list, so they were appended to separately. But all instances share access to the same bar_list. So when we did sc1.bar_list.append(2) it affected sc2, even though sc2 didn't exist yet! And likewise sc2.bar_list.append(20) affected the bar_list retrieved through sc1. This is often not what you want.

Advanced study follows. :)

To really grok Python, coming from traditional statically typed OO-languages like Java and C#, you have to learn to rethink classes a little bit.

In Java, a class isn't really a thing in its own right. When you write a class you're more declaring a bunch of things that all instances of that class have in common. At runtime, there's only instances (and static methods/variables, but those are really just global variables and functions in a namespace associated with a class, nothing to do with OO really). Classes are the way you write down in your source code what the instances will be like at runtime; they only "exist" in your source code, not in the running program.

In Python, a class is nothing special. It's an object just like anything else. So "class attributes" are in fact exactly the same thing as "instance attributes"; in reality there's just "attributes". The only reason for drawing a distinction is that we tend to use objects which are classes differently from objects which are not classes. The underlying machinery is all the same. This is why I say it would be a mistake to think of class attributes as static variables from other languages.

But the thing that really makes Python classes different from Java-style classes is that just like any other object each class is an instance of some class!

In Python, most classes are instances of a builtin class called type. It is this class that controls the common behaviour of classes, and makes all the OO stuff the way it does. The default OO way of having instances of classes that have their own attributes, and have common methods/attributes defined by their class, is just a protocol in Python. You can change most aspects of it if you want. If you've ever heard of using a metaclass, all that is is defining a class that is an instance of a different class than type.

The only really "special" thing about classes (aside from all the builtin machinery to make them work they way they do by default), is the class block syntax, to make it easier for you to create instances of type. This:

class Foo(BaseFoo):
def __init__(self, foo):
self.foo = foo

z = 28

is roughly equivalent to the following:

def __init__(self, foo):
self.foo = foo

classdict = {'__init__': __init__, 'z': 28 }

Foo = type('Foo', (BaseFoo,) classdict)

And it will arrange for all the contents of classdict to become attributes of the object that gets created.

So then it becomes almost trivial to see that you can access a class attribute by Class.attribute just as easily as i = Class(); i.attribute. Both i and Class are objects, and objects have attributes. This also makes it easy to understand how you can modify a class after it's been created; just assign its attributes the same way you would with any other object!

In fact, instances have no particular special relationship with the class used to create them. The way Python knows which class to search for attributes that aren't found in the instance is by the hidden __class__ attribute. Which you can read to find out what class this is an instance of, just as with any other attribute: c = some_instance.__class__. Now you have a variable c bound to a class, even though it probably doesn't have the same name as the class. You can use this to access class attributes, or even call it to create more instances of it (even though you don't know what class it is!).

And you can even assign to i.__class__ to change what class it is an instance of! If you do this, nothing in particular happens immediately. It's not earth-shattering. All that it means is that when you look up attributes that don't exist in the instance, Python will go look at the new contents of __class__. Since that includes most methods, and methods usually expect the instance they're operating on to be in certain states, this usually results in errors if you do it at random, and it's very confusing, but it can be done. If you're very careful, the thing you store in __class__ doesn't even have to be a class object; all Python's going to do with it is look up attributes under certain circumstances, so all you need is an object that has the right kind of attributes (some caveats aside where Python does get picky about things being classes or instances of a particular class).

That's probably enough for now. Hopefully (if you've even read this far) I haven't confused you too much. Python is neat when you learn how it works. :)

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