def create_source_import(self, vcs, vcs_url, vcs_username=github.GithubObject.NotSet, vcs_password=github.GithubObject.NotSet):
        """
        :calls: `PUT /repos/:owner/:repo/import <https://developer.github.com/v3/migration/source_imports/#start-an-import>`_
        :param vcs: string
        :param vcs_url: string
        :param vcs_username: string
        :param vcs_password: string
        :rtype: :class:`github.SourceImport.SourceImport`
        """
        assert isinstance(vcs, (str, unicode)), vcs
        assert isinstance(vcs_url, (str, unicode)), vcs_url
        assert vcs_username is github.GithubObject.NotSet or isinstance(vcs_username, (str, unicode)), vcs_username
        assert vcs_password is github.GithubObject.NotSet or isinstance(vcs_password, (str, unicode)), vcs_password
        put_parameters = {
            "vcs": vcs,
            "vcs_url": vcs_url
        }

        if vcs_username is not github.GithubObject.NotSet:
            put_parameters["vcs_username"] = vcs_username

        if vcs_password is not github.GithubObject.NotSet:
            put_parameters["vcs_password"] = vcs_password

        import_header = {"Accept": Consts.mediaTypeImportPreview}

        headers, data = self._requester.requestJsonAndCheck(
            "PUT",
            self.url + "/import",
            headers=import_header,
            input=put_parameters
        )

        return github.SourceImport.SourceImport(self._requester, headers, data, completed=False)