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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def ensure_virtualenv(args, return_values):
 """Ensure we have a valid virtualenv."""

    def adjust_options(options, args):
        # TODO-TEST: proper error message with no arguments
        venv_path = return_values.venv_path = args[0]

        if venv_path == DEFAULT_VIRTUALENV_PATH or options.prompt == '<dirname>':
            from os.path import abspath, basename, dirname
            options.prompt = '(%s)' % basename(dirname(abspath(venv_path)))
        # end of option munging.

        # there are two python interpreters involved here:
        # 1) the interpreter we're instructing virtualenv to copy
        if options.python is None:
            source_python = None
        else:
            source_python = virtualenv.resolve_interpreter(options.python)
        # 2) the interpreter virtualenv will create
        destination_python = venv_python(venv_path)

        if exists(destination_python):
            reason = invalid_virtualenv_reason(venv_path, source_python, destination_python, options)
            if reason:
                info('Removing invalidated virtualenv. (%s)' % reason)
                run(('rm', '-rf', venv_path))
            else:
                info('Keeping valid virtualenv from previous run.')
                raise SystemExit(0)  # looks good! we're done here.

    # this is actually a documented extension point:
    #   http://virtualenv.readthedocs.org/en/latest/reference.html#adjust_options
    import virtualenv
    virtualenv.adjust_options = adjust_options

    from sys import argv
    argv[:] = ('virtualenv',) + args
    info(colorize(argv))
    raise_on_failure(virtualenv.main)
    # There might not be a venv_path if doing something like "venv= --version"
    # and not actually asking virtualenv to make a venv.
    if return_values.venv_path is not None:
        run(('rm', '-rf', join(return_values.venv_path, 'local')))