Python shelve: db type could not be determined

I hit a funny bug with the Python shelve module. I’d write a shelf database, then try to open it again and get this error:

db ='foo.db')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/", line 243, in open
    return DbfilenameShelf(filename, flag, protocol, writeback)
  File "/usr/lib/python3.6/", line 227, in __init__
    Shelf.__init__(self,, flag), protocol, writeback)
  File "/usr/lib/python3.6/dbm/", line 88, in open
    raise error[0]("db type could not be determined")
dbm.error: db type could not be determined

Turns out this is my mistake. I created the shelf with code like this:

db ='foo')

This creates a file named foo.db, a Berkeley DB file. And then I tried to open it:

db ='foo.db')

See the error? The shelve module is appending the .db for me; I shouldn’t have added it myself to the filename. Simple enough mistake I made, confusing error results. It’d be nicer if the code threw an error or opened a new database file named foo.db.db, but instead you get this confusing error.

Python 3.6.3 on Ubuntu 16.04, which I think is libdb 5.3.

Note this error is different from an often-referenced bug in older Pythons not recognizing GDBM databases consistently (issue 13007).