def preprocess(content):
    # type: (Text) -> ReqFileLines
    """Split, filter, and join lines, and return a line iterator

    :param content: the content of the requirements file
    """
    lines_enum = enumerate(content.splitlines(), start=1)  # type: ReqFileLines
    lines_enum = join_lines(lines_enum)
    lines_enum = ignore_comments(lines_enum)
    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
    
product/gradle-plugin/src/main/python/pip/_internal/req/req_file.py