Using "local" variables in Puppet templates is a Bad Idea

puppet.template.vars.01.txt

It turns out that using local variables (like "hostname") in templates
instead of instance variables (like "@hostname") is a really bad idea:

  # Ruby treats variables like methods, so we used to expose variables
  # within scope to the ERB code via method_missing.  As per RedMine #1427,
  # though, this means that conflicts between methods in our inheritance
  # tree (Kernel#fork) and variable names (fork => "yes/no") could arise.
  #
  # Worse, /new/ conflicts could pop up when a new kernel or object method
  # was added to Ruby, causing templates to suddenly fail mysteriously when
  # Ruby was upgraded.
  #
  # To ensure that legacy templates using unqualified names work we retain
  # the missing_method definition here until we declare the syntax finally
  # dead.
  def method_missing(name, *args)
    if scope.include?(name.to_s)
      return scope[name.to_s, {:file => file,:line => script_line}]
    else
      # Just throw an error immediately, instead of searching for
      # other missingmethod things or whatever.
      raise Puppet::ParseError.new("Could not find value for '#{name}'",@file,script_line)
    end
  end

That's totally crazy, right?

Source: https://github.com/puppetlabs/puppet/blob/master/lib/puppet/parser/templatewrapper.rb