python.rb

# Helper method for the common case of installing a Python application.
# Creates a virtualenv in `libexec`, installs all `resource`s defined
# on the formula, and then installs the formula.
  def virtualenv_install_with_resources(options = {})
    python = options[:using]
      if python.nil?
        wanted = %w[python python@2 python@3 python3].select { |py| needs_python?(py) }
        raise FormulaAmbiguousPythonError, self if wanted.size > 1
        python = wanted.first || "python2.7"
      end
      venv = virtualenv_create(libexec, python.delete("@"))
      venv.pip_install resources
      venv.pip_install_and_link buildpath
      venv
  end