Stepping through the code, it seems under ndbm it is creating a file with a '.db' extension:

```
(Pdb) list
 47  	             'g': b'intended',
 48  	             }
 49  	
 50  	    def init_db(self):
 51  	        import pdb; pdb.set_trace()
 52  ->	        with dbm.open(_fname, 'n') as f:
 53  	            for k in self._dict:
 54  	                f[k.encode("ascii")] = self._dict[k]
 55  	
 56  	    def keys_helper(self, f):
 57  	        keys = sorted(k.decode("ascii") for k in f.keys())
(Pdb) n
> /home/asottile/workspace/cpython/Lib/test/test_dbm.py(53)init_db()
-> for k in self._dict:
(Pdb) os.listdir('.')
['@test_8371_tmp.db']
(Pdb) _fname
'@test_8371_tmp'
```