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I have a slightly unusual problem with transaction state and error handling in SQLAlchemy. The short version: is there any way of preserving a transaction when SQLAlchemy raises a ProgrammingError and aborts it?

I'm working on an integration test suite for a legacy codebase. Right now, I'm designing a set of fixtures that will allow us to run all tests inside transactions, inspired by the SQLAlchemy documentation. The general paradigm involves opening a connection, starting a transaction, binding a session to that connection, and then mocking out most database access methods so that they make use of that transaction. (To get a sense of what this looks like, see the code provided in the docs link above, including the note at the end.) The goal is to allow ourselves to run methods from the codebase that perform a lot of database updates in the context of a test, with the assurance that any side effects that happen to alter the test database will get rolled back after the test has completed.

My problem is that the code often relies on handling DBAPI errors to accomplish control flow when running queries, and those errors automatically abort transactions (per the psycopg2 docs). This poses a problem, since I need to preserve the work that has been done in that transaction up to the point that the error is raised, and I need to continue using the transaction after the error handling is done.

Here's a representative method that uses error handling for control flow: