code: def _setup_no_fallback(parser):
      
  
      cli_dest = 'tox_pyenv_fallback'
      halp = ('If `pyenv which {basepython}` exits non-zero when looking '
              'up the python executable, do not allow fallback to tox\'s '
              'built-in default logic.')
      # Add a command-line option.
      tox_pyenv_group = parser.argparser.add_argument_group(
          title='{0} plugin options'.format(__title__),
      )
      tox_pyenv_group.add_argument(
          '--tox-pyenv-no-fallback', '-F',
          dest=cli_dest,
          default=True,
          action='store_false',
          help=halp
      )
  
      def _pyenv_fallback(testenv_config, value):
          cli_says = getattr(testenv_config.config.option, cli_dest)
          return cli_says or value
  
      # Add an equivalent tox.ini [testenv] section option.
      parser.add_testenv_attribute(
          name=cli_dest,
          type="bool",
          postprocess=_pyenv_fallback,
          default=False,
          help=('If `pyenv which {basepython}` exits non-zero when looking '
                'up the python executable, allow fallback to tox\'s '
                'built-in default logic.'),
      )
docs: Add the option, --tox-pyenv-no-fallback.
  
      If this option is set, do not allow fallback to tox's built-in
      strategy for looking up python executables if the call to `pyenv which`
      by this plugin fails. This will allow the error to raise instead
      of falling back to tox's default behavior.