attr_checker.rb

#! /Users/matt/.rvm/rubies/ruby-2.2.1/bin/ruby

class Foo
  attr_accessor :bar

  def initialize
    puts "initialized"
    @bar = 5
  end

  def bar_divider
    @bar /= 5
  end

  def broken_bar_divider
    bar /= 5
  end

  def other_divider
    bar = 4/2
  end

  def checker
    puts bar
  end
end

foo = Foo.new   # => "initialized"
foo.checker     # => 5

p foo.bar_divider # => 1

p foo.other_divider # doesn't actually assign bar

p foo.broken_bar_divider # undefined method '/' for nil