def deprecated(reason, replacement, gone_in, issue=None):
    # type: (str, Optional[str], Optional[str], Optional[int]) -> None
    """Helper to deprecate existing functionality.

    reason:
        Textual reason shown to the user about why this functionality has
        been deprecated.
    replacement:
        Textual suggestion shown to the user about what alternative
        functionality they can use.
    gone_in:
        The version of pip does this functionality should get removed in.
        Raises errors if pip's current version is greater than or equal to
        this.
    issue:
        Issue number on the tracker that would serve as a useful place for
        users to find related discussion and provide feedback.

    Always pass replacement, gone_in and issue as keyword arguments for clarity
    at the call site.
    """

    # Construct a nice message.
    # This is purposely eagerly formatted as we want it to appear as if someone
    # typed this entire message out.
    message = "DEPRECATION: " + reason
    if replacement is not None:
        message += " A possible replacement is {}.".format(replacement)
    if issue is not None:
        url = "https://github.com/pypa/pip/issues/" + str(issue)
        message += " You can find discussion regarding this at {}.".format(url)

    # Raise as an error if it has to be removed.
    if gone_in is not None and parse(current_version) >= parse(gone_in):
        raise PipDeprecationWarning(message)
    warnings.warn(message, category=PipDeprecationWarning, stacklevel=2)
Helper to deprecate existing functionality.

    reason:
        Textual reason shown to the user about why this functionality has
        been deprecated.
    replacement:
        Textual suggestion shown to the user about what alternative
        functionality they can use.
    gone_in:
        The version of pip does this functionality should get removed in.
        Raises errors if pip's current version is greater than or equal to
        this.
    issue:
        Issue number on the tracker that would serve as a useful place for
        users to find related discussion and provide feedback.

    Always pass replacement, gone_in and issue as keyword arguments for clarity
    at the call site.
    
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_internal/utils/deprecation.py