Any recommended approach for creating a new dialect?

[Ralph Heinkel]

Hello dialect experts,

what would be the best approach for creating a SqlAlchemy dialect for a new database system?
Are there any recipes available for this area, or is the way to go to read code of existing dialects and derive my own dialect from those?

I had a first glance at some built-in dialects, and also some in external packages ... it is not always obvious to me why certain classes and methods have been implemented.
The obvious thing is to create a subclass of sqlalchemy.enginedefault.DefaultDialect, but how would I know which methods and class attributes to override/implement, except for going through the trial and error approach?
And then there are other classes which are implemented in some dialects, like compiler.DDLCompiler, compiler.GenericTypeCompiler, and so on ... where and how would I start best?

Any help would be very much appreciated.

Ciao ciao

Ralph

[Mike Bayer]

Start with the README for new dialects:

https://bitbucket.org/zzzeek/sqlalchemy/src/44a9820b4e02f65b3884fa2c016efce9663e4910/README.dialects.rst?at=master

that will show the guidelines for writing new dialects.

Then to see some examples of that layout, take a look at some of the 3rd party dialects listed at:

http://docs.sqlalchemy.org/en/latest/dialects/index.html#production-ready

Also the "sqlalchemy-access" dialect is basically something of a "demo" for the layout, which I basically put there after extracting it from SQLAlchemy main where it had been for many years.   It might not be 100% up to date, but mostly follows that guideline and even passed tests at one point, that's at https://bitbucket.org/zzzeek/sqlalchemy-access.

The key thing you'll be doing is running the suite tests, which will be part of the test suite within your own dialect.  So yes, you start with a fairly plain subclass of DefaultDialect, then you probably want to get a "hello world" kind of program going where you just see if "create_engine" and then "engine.execute("select * from table")" work at all, and then the suite tests should test a lot more.

Ciao ciao

Ralph

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[Ralph Heinkel]

Hi Michael,

thanks for the prompt response and your directions. This was really helpful. I will start as you've described in your reply.

Ralph

[Richard Gerd Kuesters | Pollux Automation]

Mike, I remember an article of yours where you described much of the process of creating a new dialect for SA, for a Java database if I'm not mistaken. I wasn't able to find it, though.

:)