def preprocess(content, options):
    # type: (Text, Optional[optparse.Values]) -> ReqFileLines
    """Split, filter, and join lines, and return a line iterator

    :param content: the content of the requirements file
    :param options: cli options
    """
    lines_enum = enumerate(content.splitlines(), start=1)  # type: ReqFileLines
    lines_enum = join_lines(lines_enum)
    lines_enum = ignore_comments(lines_enum)
    lines_enum = skip_regex(lines_enum, options)
    lines_enum = expand_env_variables(lines_enum)
    return lines_enum
Split, filter, and join lines, and return a line iterator

    :param content: the content of the requirements file
    :param options: cli options
    
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_internal/req/req_file.py