error: db type could not be determined

[John Machin]

neutrinman at myrealbox.com wrote:
> why does the following error occur?

I don't know; I've never used the shelve module. Let's see what as two
utter n00bz we can find out. Let's check out where it clagged:
lib\anydbm.py, line 80, in open ...

Hmm, reading backwards a little, looks like it called whichdb.whichdb
to nut out what sort of database it was, and whichdb reported back that
it was an existing file, of unknown type. Flicking through whichdb.py
confirms this.

Let's see if we can reproduce that:

=== step 1: file doesn't exist ===
C:\junk>c:\python23\python
Python 2.3.4 (#53, May 25 2004, 21:17:02) [MSC v.1200 32 bit (Intel)]
on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import shelve
>>> df = shelve.open('mydata.dat', 'c')
>>> ^Z

C:\junk>dir mydata.dat
[snip]
26/02/2005  08:40p              24,576 mydata.dat

=== looks like file created OK ==

C:\junk>del mydata.dat
C:\junk>copy con mydata.dat
any old codswallop
^Z
        1 file(s) copied.

=== OK, now mydata.dat is trash.

C:\junk>c:\python23\python
Python 2.3.4 (#53, May 25 2004, 21:17:02) [MSC v.1200 32 bit (Intel)]
on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import shelve
>>> df = shelve.open('mydata.dat', 'c')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "c:\python23\lib\shelve.py", line 231, in open
    return DbfilenameShelf(filename, flag, protocol, writeback, binary)
  File "c:\python23\lib\shelve.py", line 212, in __init__
    Shelf.__init__(self, anydbm.open(filename, flag), protocol,
writeback, binary)
  File "c:\python23\lib\anydbm.py", line 80, in open
    raise error, "db type could not be determined"
anydbm.error: db type could not be determined
>>>

Uh-huh.

By the way, it's probably not a good idea to use a ".dat" extension;
evidently (read whichdb.py for the gory details) some of the dbms add
an extension to the supplied name. One of them uses ".dat". Could
become a source of confusion.

Take a hint: they say "Google is your friend", but better still is the
source in lib\*.py -- it's quite legible, you don't need an Internet
connection, and there sure ain't no ads in the margin. And don't just
open it in emergencies: pick a module that covers a topic that
interests you and just read it. You'll see good coding style, good ways
of doing things, wise utterances by the timbot, ...

HTH,
John