Use python 2 shelf in python 3

Question

I have data stored in a shelf file created with python 2.7

When I try to access the file from python 3.4, I get an error:

>>> import shelve
>         


        

Answer 1

I don't think it's possible to use a Python 2 shelf with Python 3's shelve module. The underlying files are completely different, at least in my tests.

In Python 2*, a shelf is represented as a single file with the filename you originally gave it.

In Python 3*, a shelf consists of three files: filename.bak, filename.dat, and filename.dir. Without any of these files present, the shelf cannot be opened by the Python 3 library (though it appears that just the .dat file is sufficient for opening, if not actual reading).

@Ricardo Cárdenes has given an overview of why this may be--it's likely an issue with the underlying database modules used in storing the shelved data. It's possible that the databases are backwards compatible, but I don't know and a quick search hasn't turned up any obvious answers.

I think it's likely that some of the possible databases implemented by dbm are backwards-compatible, whereas others are not: this could be the cause of the discrepancy between answers here, where some people, but not all, are able to open older databases directly by specifying a protocol.

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*On every machine I've tested, using Python 2.7.6 vs Pythons 3.2.5, 3.3.4, and 3.4.1