Using SQLite in a Python program

 

I have created a Python module that creates and populates several SQLite tables. Now, I want to use it in a program but I don't really know how to call it properly. All the tutorials I've found are essentially "inline", ie they walk through using SQLite in a linear fashion rather than how to actually use it in production.

What I'm trying to do is have a method check to see if the database is already created. If so, then I can use it. If not, an exception is raised and the program will create the database. (Or use if/else statements, whichever is better).

I created a test script to see if my logic is correct but it's not working. When I create the try statement, it just creates a new database rather than checking if one already exists. The next time I run the script, I get an error that the table already exists, even if I tried catching the exception. (I haven't used try/except before but figured this is a good time to learn).

Are there any good tutorials for using SQLite operationally or any suggestions on how to code this? I've looked through the pysqlite tutorial and others I found but they don't address this.

AFAIK an SQLITE database is just a file. To check if the database exists, check for file existence.

When you open a SQLITE database it will automatically create one if the file that backs it up is not in place.

If you try and open a file as a sqlite3 database that is NOT a database, you will get this:

"sqlite3.DatabaseError: file is encrypted or is not a database"

so check to see if the file exists and also make sure to try and catch the exception in case the file is not a sqlite3 database

Don't make this more complex than it needs to be. The big, independent databases have complex setup and configuration requirements. SQLite is just a file you access with SQL, it's much simpler.

Do the following.

  • Add a table to your database for "Components" or "Versions" or "Configuration" or "Release" or something administrative like that.

    CREATE TABLE REVISION( RELEASE_NUMBER CHAR(20) );

  • In your application, connect to your database normally.

  • Execute a simple query against the revision table. Here's what can happen.
  • The query fails to execute: your database doesn't exist, so execute a series of CREATE statements to build it.
  • The query succeeds but returns no rows or the release number is lower than expected: your database exists, but is out of date. You need to migrate from that release to the current release. Hopefully, you have a sequence of DROP, CREATE and ALTER statements to do this.
  • The query succeeds, and the release number is the expected value. Do nothing more, your database is configured correctly.

    • SQLite automatically creates the database file the first time you try to use it. The SQL statements for creating tables can use IF NOT EXISTS to make the commands only take effect if the table has not been created This way you don't need to check for the database's existence beforehand: SQLite can take care of that for you.

    The main thing I would still be worried about is that executing CREATE TABLE IF EXISTS for every web transaction (say) would be inefficient; you can avoid that by having the program keep an (in-memory) variable saying whether it has created the database today, so it runs the CREATE TABLE script once per run. This would still allow for you to delete the database and start over during debugging.

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