[SOLVED] What happens if I assign to a variable inside a Ruby class definition?

What happens if I assign to a variable inside a Ruby class definition?

class A
    a = 1
end

In the above code, what happens to a? For example in Python, it becomes a class variable that can be accessed as A.a. However in Ruby, I don't know how to access it - attempts to do so in the enclosing scope as a, A.a and A.new.a all give errors:

undefined local variable or method `a` for main (NameError)
undefined method `a` for class A (NoMethodError)
undefined method `a` for an instance of A (NoMethodError)

Solution

The local variable a is in scope for the duration of the scope, in this case while the class definition is executing until the end of the class.

This is because in Ruby, all code is executed from top to bottom, including class definitions which allows you to do things like

class Foo
  var = :hello
  attr_accessor var
end

foo = Foo.new
foo.hello = "world"
foo.hello
# => "world"

Ruby knows class variables which are prefixed by double-@ characters, e.g. @@var. These are valid for a class, all of the class's subclasses, and their objects. Generally, the use of class variables is discouraged due to the rather broad scope and the often unexpected semantics of value sharing in sub-classes.

Instead, you would often use an instance variable of a class's singleton class (which is sometimes also called eigenclass). This could be used like this:

class Foo
  @foo = "foo"
  
  def self.foo
    @foo
  end

  def self.foo=(value)
    @foo
  end
end

class Bar < Foo
end

Foo.foo = "hello"
Foo.foo
# => "hello"

Bar.foo
# => nil