Shelve module uses "import dbm" instruction while opening database. Dbm module has global dictionary "_modules". This dictionary modifies during database opening operation. When different threads simultaneously try to open different databases, unexpected behavior occurred. In my case I've got such message: 

    with shelve.open('some_file') as f:
  File "/usr/local/lib/python3.6/shelve.py", line 243, in open
    return DbfilenameShelf(filename, flag, protocol, writeback)
  File "/usr/local/lib/python3.6/shelve.py", line 227, in init
    Shelf.init(self, dbm.open(filename, flag), protocol, writeback)
  File "/usr/local/lib/python3.6/dbm/__init__.py", line 94, in open
    return mod.open(file, flag, mode)
AttributeError: module 'dbm.ndbm' has no attribute 'open'

Behavior is the same on python 3.6 and 3.7. Error is spontaneous.

I added import experts because the issue looks relates to race condition in import.

Willy, can the issue be reproduced on more modern Python releases (3.8 or 3.9)? It could help if you provide minimal script which reproduces it.

I suggest removing the global state from dbm and just recomputing the underlying database for every call to open().

Loading modules every time without using a cache (sys.modules) can add significant overhead.