| Defining a new class creates a new context nested in the enclosing class (class Object by default). Use the :: scope operator to access nested classes/modules and their constants or class methods.
class MyClass # name must be capitalized C = "constant in MyClass" class Inner C = "in MyClass::Inner" end end p MyClass::C # constant in MyClass p MyClass::Inner::C # in MyClass::Inner
Existing classes, both built-in and newly defined, may be reopened and altered or extended at any time (even from within a C extension). Any existing instances will be updated with the new class definitions.
class String # existing built-in class def my_method puts "length: #{ length} " # implicit self end end "abc123".my_method # length: 6
Any class may be subclassed; use the < symbol instead of parentheses. Ruby classes support single inheritance only. Modules are used to add shared functionality among classes (and objects) in place of using multiple inheritance. Use initialize instead of __init__ for the "constructor." You do not need to pass in self for method definitions, since Ruby automatically binds it for you. Remember dot notation is for method calls only, so prepend instance variables with the @ symbol instead of self. Note Prepending a method name with self within a class definition is optional, since it will be called from within the context of the actual instance by default.
Use super to call the same-named method from the superclass (or an ancestor thereof).
class MyString < String def initialize(str) @str = str # instance variable super # call String#initialize(str) end end
Instantiate an object via the Class#new method. It will call any defined initialize method after allocating the object.
mystr = MyString.new("text here")
There is no equivalent to Python's __del__ hook, since Ruby does not use reference counting (see the section "Garbage Collection"). This may require you to organize your code differently. Ruby has both class variables and class methods.
class MyClass @@class_variable = "Accessible to this class and subclasses" MyConstant = "Globally accessible via MyClass::MyConstant" def MyClass.class_method # dot notation used in definition puts "Available as method of the class object itself" end end
Ruby has no internal-use, class-private, or magic-attribute naming like Python's _*, __*, and __*__ forms. Ruby uses the __*__ form sometimes, but only to accommodate certain naming conflicts. For example, Object#__send__ is an alias for send. | 
