def __init__(
        self,
        find_links,  # type: List[str]
        index_urls,  # type: List[str]
        allow_all_prereleases=False,  # type: bool
        trusted_hosts=None,  # type: Optional[Iterable[str]]
        session=None,  # type: Optional[PipSession]
        format_control=None,  # type: Optional[FormatControl]
        platform=None,  # type: Optional[str]
        versions=None,  # type: Optional[List[str]]
        abi=None,  # type: Optional[str]
        implementation=None,  # type: Optional[str]
        prefer_binary=False  # type: bool
    ):
        # type: (...) -> None
        """Create a PackageFinder.

        :param format_control: A FormatControl object or None. Used to control
            the selection of source packages / binary packages when consulting
            the index and links.
        :param platform: A string or None. If None, searches for packages
            that are supported by the current system. Otherwise, will find
            packages that can be built on the platform passed in. These
            packages will only be downloaded for distribution: they will
            not be built locally.
        :param versions: A list of strings or None. This is passed directly
            to pep425tags.py in the get_supported() method.
        :param abi: A string or None. This is passed directly
            to pep425tags.py in the get_supported() method.
        :param implementation: A string or None. This is passed directly
            to pep425tags.py in the get_supported() method.
        """
        if session is None:
            raise TypeError(
                "PackageFinder() missing 1 required keyword argument: "
                "'session'"
            )

        # Build find_links. If an argument starts with ~, it may be
        # a local file relative to a home directory. So try normalizing
        # it and if it exists, use the normalized version.
        # This is deliberately conservative - it might be fine just to
        # blindly normalize anything starting with a ~...
        self.find_links = []  # type: List[str]
        for link in find_links:
            if link.startswith('~'):
                new_link = normalize_path(link)
                if os.path.exists(new_link):
                    link = new_link
            self.find_links.append(link)

        self.index_urls = index_urls

        # These are boring links that have already been logged somehow:
        self.logged_links = set()  # type: Set[Link]

        self.format_control = format_control or FormatControl(set(), set())

        # Domains that we won't emit warnings for when not using HTTPS
        self.secure_origins = [
            ("*", host, "*")
            for host in (trusted_hosts if trusted_hosts else [])
        ]  # type: List[SecureOrigin]

        # Do we want to allow _all_ pre-releases?
        self.allow_all_prereleases = allow_all_prereleases

        # The Session we'll use to make requests
        self.session = session

        # The valid tags to check potential found wheel candidates against
        self.valid_tags = get_supported(
            versions=versions,
            platform=platform,
            abi=abi,
            impl=implementation,
        )

        # Do we prefer old, but valid, binary dist over new source dist
        self.prefer_binary = prefer_binary

        # If we don't have TLS enabled, then WARN if anyplace we're looking
        # relies on TLS.
        if not HAS_TLS:
            for link in itertools.chain(self.index_urls, self.find_links):
                parsed = urllib_parse.urlparse(link)
                if parsed.scheme == "https":
                    logger.warning(
                        "pip is configured with locations that require "
                        "TLS/SSL, however the ssl module in Python is not "
                        "available."
                    )
                    break
Create a PackageFinder.

        :param format_control: A FormatControl object or None. Used to control
            the selection of source packages / binary packages when consulting
            the index and links.
        :param platform: A string or None. If None, searches for packages
            that are supported by the current system. Otherwise, will find
            packages that can be built on the platform passed in. These
            packages will only be downloaded for distribution: they will
            not be built locally.
        :param versions: A list of strings or None. This is passed directly
            to pep425tags.py in the get_supported() method.
        :param abi: A string or None. This is passed directly
            to pep425tags.py in the get_supported() method.
        :param implementation: A string or None. This is passed directly
            to pep425tags.py in the get_supported() method.
        
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_internal/index.py