I highly suspect you don't have gdbm installed in your environment and `import dbm.gnu` will fail. When simply using `dbm.open`, it searches through [dbm.gnu, dbm.ndbm, dbm.dumb]. In my environment, macOS 10.13.3, dbm.gnu works correctly(so dbm works correctly) and dbm.ndbm fails with same error. Currently I cannot see any code in Python _dbm module could lead to this error. And POSIX only requires dbm library to support at least 1023 bytes long key/value pairs.