def __init__(self, url, comes_from=None, requires_python=None):
        # type: (str, Optional[Union[str, HTMLPage]], Optional[str]) -> None
        """
        url:
            url of the resource pointed to (href of the link)
        comes_from:
            instance of HTMLPage where the link was found, or string.
        requires_python:
            String containing the `Requires-Python` metadata field, specified
            in PEP 345. This may be specified by a data-requires-python
            attribute in the HTML link tag, as described in PEP 503.
        """

        # url can be a UNC windows share
        if url.startswith('\\\\'):
            url = path_to_url(url)

        self.url = url
        self.comes_from = comes_from
        self.requires_python = requires_python if requires_python else None

        super(Link, self).__init__(
            key=(self.url),
            defining_class=Link
        )

        url:
            url of the resource pointed to (href of the link)
        comes_from:
            instance of HTMLPage where the link was found, or string.
        requires_python:
            String containing the `Requires-Python` metadata field, specified
            in PEP 345. This may be specified by a data-requires-python
            attribute in the HTML link tag, as described in PEP 503.
        
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_internal/models/link.py