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 eagerly formatted as we want it to get logged as if someone
    #   typed this entire message out.
    sentences = [
        (reason, DEPRECATION_MSG_PREFIX + "{}"),
        (gone_in, "pip {} will remove support for this functionality."),
        (replacement, "A possible replacement is {}."),
        (issue, (
            "You can find discussion regarding this at "
            "https://github.com/pypa/pip/issues/{}."
        )),
    ]
    message = " ".join(
        template.format(val) for val, template in sentences if val is not None
    )

    # 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.
    
product/gradle-plugin/src/main/python/pip/_internal/utils/deprecation.py