foo.py

class Foo(object):
  bar = [1,2,3]
  
  def __init__(self, idx):
    self.idx = idx
  
  def my_bar(self):
    return self.bar[self.idx]

foo.rb

class Foo
  class << self
    attr_accessor :bar
    bar = [1,2,3]
  end
  
  def initialize(idx)
    @idx = idx
  end
  
  def my_bar
    self.class.bar[@idx]
  end
end

terminal

irb(main):001:0> require './foo'
=> true
irb(main):002:0> f = Foo.new(1)
=> #<Foo:0x007f89560ce1f8 @idx=1>
irb(main):003:0> f.my_bar
NoMethodError: undefined method `[]' for nil:NilClass
	from /Users/tmoney/foo.rb:12:in `my_bar'
	from (irb):3
	from /usr/bin/irb:12:in `<main>'