Avoiding boilerplate session handling code in sqlalchemy functions

Question

I have a python application which has lots of small database access functions, using sqlalchemy. I\'m trying to avoid having lots of boilerplate session handling code around

Answer 1

morphyn's suggestion to use a context manager is good. You could make such a context manager by applying the contextlib.contextmanager decorator to a function very much like your first get_ticket_history, replacing the code between try and except with a yield statement and renaming it, say, transaction. PEP 343 has a near-identical example of that name.

Then, use that context manager with the with statement to reimplement get_ticket_history. It looks like SQLAlchemy already provides that function, though, as method begin:

http://docs.sqlalchemy.org/en/rel_0_8/orm/session.html#autocommit-mode

Answer 2

transaction handling (begin, commit/rolllback) using with clause

with engine.begin() as connection:
    r1 = connection.execute(table1.select())
    connection.execute(table1.insert(), {"col1": 7, "col2": "this is some data"})

Old questions, but I still stumbled upon it so here is the relevant link from the docu: https://docs.sqlalchemy.org/en/13/core/connections.html

Answer 3

The SQLAlchemy docs present a possible way of doing this with context managers.

http://docs.sqlalchemy.org/en/latest/orm/session_basics.html#when-do-i-construct-a-session-when-do-i-commit-it-and-when-do-i-close-it

Copying the code snippet here for completeness:

from contextlib import contextmanager

@contextmanager
def session_scope():
    """Provide a transactional scope around a series of operations."""
    session = Session()
    try:
        yield session
        session.commit()
    except:
        session.rollback()
        raise
    finally:
        session.close()

This session_scope can be used cleanly without repeating the boiler plate now.

class ThingOne(object):
    def go(self, session):
        session.query(FooBar).update({"x": 5})

class ThingTwo(object):
    def go(self, session):
        session.query(Widget).update({"q": 18})

def run_my_program():
    with session_scope() as session:
        ThingOne().go(session)
        ThingTwo().go(session)