def _get_ncbi_db_url():
if "<STR_LIT>" in os.environ:<EOL><INDENT>return os.environ["<STR_LIT>"]<EOL><DEDENT>if "<STR_LIT>" in os.environ:<EOL><INDENT>url_key = os.environ["<STR_LIT>"]<EOL><DEDENT>else:<EOL><INDENT>sdlc = _stage_from_version(hgvs.__version__)<EOL>url_key = "<STR_LIT>".format(sdlc=sdlc)<EOL><DEDENT>return hgvs.global_config['<STR_LIT>'][url_key]<EOL>
returns NCBI DB URL based on environment variables and code version

    * if NCBI_DB_URL is set, use that
    * Otherwise, if _NCBI_URL_KEY is set, use that as the name of a
      config file entry and use the corresponding URL
    * Otherwise,