def __init__(self, vcs, rev=None, extra_args=None):
        # type: (VersionControl, Optional[str], Optional[List[str]]) -> None
        """
        Args:
          vcs: a VersionControl object.
          rev: the name of the revision to install.
          extra_args: a list of extra options.
        """
        if extra_args is None:
            extra_args = []

        self.extra_args = extra_args
        self.rev = rev
        self.vcs = vcs

        Args:
          vcs: a VersionControl object.
          rev: the name of the revision to install.
          extra_args: a list of extra options.
        
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_internal/vcs/__init__.py