Update database.py

[KeksKeksKeks]

Don't write db if _default is an empty Table.

If there are no documents in the _default table, TinyDB will write (and thus, re-serialize) the entire database.
This is because bool({}) == False, although the table does exist.

I have a JSON file under version control, which doesn't use the default table. Recently, I noticed that the file changes although I didn't change any document within the database.
Apparently, TinyDB.init reads the default table, which is empty in my case. An empty document evaluates to False in a boolean context, and TinyDB will write a new, empty table, effectively re-serializing the database on read.

[KeksKeksKeks]

Classical case of RTFM :(

I should have read the contribution instructions first. I have added a test to demonstrate the original behaviour, introduced changes to make it go away, and added the neccessary changes to make the coverage check happy.

Kudos for such a thorough code infrastructure! I enjoyed using (and now, contributing to) TinyDB.

[msiemens]

That makes sense, thank you! Just out of curiosity: I find it interesting that the VCS tracks the file modification time. Which one are you using?

[KeksKeksKeks]

You're welcome!

I use git, which doesn't track file modification time. Instead, because python dicts are unordered, two JSON serialisations of the same database will look different. And a different file content is, of course, then detected by the VCS.

[msiemens]

Ah, I see! Unexpected reordering of Python's dicts have bitten me more than once 😄

[eugene-eeo]

Turns out there is a sort_keys option in the json.dumpsfunction. However I think that'd degrade performance slightly, but anyways here it is: http://stackoverflow.com/questions/10844064/items-in-json-object-are-out-of-order-using-json-dumps