Modifying the test environment temporarily

environment_helper.rb

module EnvironmentHelper
  def with_environment(replacement_env)
    original_env = ENV.to_hash
    ENV.update(replacement_env)
    yield
  ensure
    ENV.replace(original_env)
  end
end

usage.rb

include EnvironmentHelper

puts "Before: #{ENV.fetch("FOO")}"

with_environment("FOO" => "baz") do
  puts "During: #{ENV.fetch("FOO")}"
end

puts "After: #{ENV.fetch("FOO")}"