Using python shelve cross-platform

Question

I am hoping for a little advice on shelves/databases in Python.

Problem: I have a database created on the mac, that I want to use on windows 7. I use Python 3.2, MacOS 10.7, and win 7.

When I open and save my shelve on the mac all is good and well. I get a file with a ".db" extension. On my windows-python it is not recognized. I can however create a new db on the pc and get files with ".bak, dat, .dir" extensions.

I am guessing that the python on the pc does not have the same underlying database that my mac-python uses?

I am not sure which is the correct approach here, but maybe I could:

Change the default-db that my systems uses? Find out which db my mac-python uses and add that on the pc? Change the way I store my data all together?

Speed is not an issue, the datasize is a few megabytes, and it's not accessed very often.

Hope to find a helping hand out there. Thanks in advance - any help is much appreciated.

/Esben

What I am doing:

Import shelve
db = shelve.open('mydb')
entries = db['list']
db.close

It's pretty straight forward, I have a working db-file called "mydb.db" on the mac but when I try to open it on the pc-python i get:

Traceback (most recent call last): File "/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/dbm/init.py", line 107, in whichdb f = io.open(filename + ".pag", "rb") IOError: [Errno 2] No such file or directory: 'mydb.pag'

Answer 1

I ran into the same issue and implemented a dict-based class that supports loading and writing the contents of the dict from and to disk.

from pathlib import Path
import pickle


class DiskDict(dict):
    def __init__(self, sync_path: Path):
        self.path = sync_path

        if self.path.exists():
            with open(self.path, "rb") as file:
                tmp_dct = pickle.load(file)
                super().update(tmp_dct)
                print(f"loaded DiskDict with {len(tmp_dct)} items from {self.path}")

    def sync_to_disk(self):
        with open(self.path, "wb") as file:
            tmp_dct = super().copy()
            pickle.dump(tmp_dct, file)
            print(f"saved DiskDict with {len(tmp_dct)} items to {self.path}")

Answer 2

Thank you for your reply!

I seems that shelves in python are not easily forced to use a specific db, however pickles works like a charm. At least from mac os -> windows 7.

So short answer: If you want portability, don't use shelves, use pickles directly.

/Esben