def downloaded_reqs_from_path(path, argv):
    """Return a list of DownloadedReqs representing the requirements parsed
    out of a given requirements file.

    :arg path: The path to the requirements file
    :arg argv: The commandline args, starting after the subcommand

    """
    finder = package_finder(argv)

    def downloaded_reqs(parsed_reqs):
        """Just avoid repeating this list comp."""
        return [DownloadedReq(req, argv, finder) for req in parsed_reqs]

    try:
        return downloaded_reqs(parse_requirements(
            path, options=EmptyOptions(), finder=finder))
    except TypeError:
        # session is a required kwarg as of pip 6.0 and will raise
        # a TypeError if missing. It needs to be a PipSession instance,
        # but in older versions we can't import it from pip.download
        # (nor do we need it at all) so we only import it in this except block
        from pip.download import PipSession
        return downloaded_reqs(parse_requirements(
            path, options=EmptyOptions(), session=PipSession(), finder=finder)) 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     Global options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines = content.splitlines()
    lines = ignore_comments(lines)
    lines = join_lines(lines)
    lines = skip_regex(lines, options)

    for line_number, line in enumerate(lines, 1):
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def head(cls):
        return ("These packages were already installed, so we didn't need to download or build\n"
                "them again. If you installed them with peep in the first place, you should be\n"
                "safe. If not, uninstall them, then re-attempt your install with peep.\n") 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     Global options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines = content.splitlines()
    lines = ignore_comments(lines)
    lines = join_lines(lines)
    lines = skip_regex(lines, options)

    for line_number, line in enumerate(lines, 1):
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     Global options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines = content.splitlines()
    lines = ignore_comments(lines)
    lines = join_lines(lines)
    lines = skip_regex(lines, options)

    for line_number, line in enumerate(lines, 1):
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, wheel_cache=None):
    """
    Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     Global options.
    :param session:     Instance of pip.download.PipSession.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines = content.splitlines()
    lines = ignore_comments(lines)
    lines = join_lines(lines)
    lines = skip_regex(lines, options)

    for line_number, line in enumerate(lines, 1):
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     Global options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines = content.splitlines()
    lines = ignore_comments(lines)
    lines = join_lines(lines)
    lines = skip_regex(lines, options)

    for line_number, line in enumerate(lines, 1):
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def parse_requirements(filename, finder=None, comes_from=None, options=None,
                       session=None, constraint=False, wheel_cache=None):
    """Parse a requirements file and yield InstallRequirement instances.

    :param filename:    Path or url of requirements file.
    :param finder:      Instance of pip.index.PackageFinder.
    :param comes_from:  Origin description of requirements.
    :param options:     cli options.
    :param session:     Instance of pip.download.PipSession.
    :param constraint:  If true, parsing a constraint file rather than
        requirements file.
    :param wheel_cache: Instance of pip.wheel.WheelCache
    """
    if session is None:
        raise TypeError(
            "parse_requirements() missing 1 required keyword argument: "
            "'session'"
        )

    _, content = get_file_content(
        filename, comes_from=comes_from, session=session
    )

    lines_enum = preprocess(content, options)

    for line_number, line in lines_enum:
        req_iter = process_line(line, filename, line_number, finder,
                                comes_from, options, session, wheel_cache,
                                constraint=constraint)
        for req in req_iter:
            yield req 

def _downloaded_filename(self):
        """Download the package's archive if necessary, and return its
        filename.

        --no-deps is implied, as we have reimplemented the bits that would
        ordinarily do dependency resolution.

        """
        # Peep doesn't support requirements that don't come down as a single
        # file, because it can't hash them. Thus, it doesn't support editable
        # requirements, because pip itself doesn't support editable
        # requirements except for "local projects or a VCS url". Nor does it
        # support VCS requirements yet, because we haven't yet come up with a
        # portable, deterministic way to hash them. In summary, all we support
        # is == requirements and tarballs/zips/etc.

        # TODO: Stop on reqs that are editable or aren't ==.

        # If the requirement isn't already specified as a URL, get a URL
        # from an index:
        link = self._link() or self._finder.find_requirement(self._req, upgrade=False)

        if link:
            lower_scheme = link.scheme.lower()  # pip lower()s it for some reason.
            if lower_scheme == 'http' or lower_scheme == 'https':
                file_path = self._download(link)
                return basename(file_path)
            elif lower_scheme == 'file':
                # The following is inspired by pip's unpack_file_url():
                link_path = url_to_path(link.url_without_fragment)
                if isdir(link_path):
                    raise UnsupportedRequirementError(
                        "%s: %s is a directory. So that it can compute "
                        "a hash, peep supports only filesystem paths which "
                        "point to files" %
                        (self._req, link.url_without_fragment))
                else:
                    copy(link_path, self._temp_path)
                    return basename(link_path)
            else:
                raise UnsupportedRequirementError(
                    "%s: The download link, %s, would not result in a file "
                    "that can be hashed. Peep supports only == requirements, "
                    "file:// URLs pointing to files (not folders), and "
                    "http:// and https:// URLs pointing to tarballs, zips, "
                    "etc." % (self._req, link.url))
        else:
            raise UnsupportedRequirementError(
                "%s: couldn't determine where to download this requirement from."
                % (self._req,))