Question

Being new to SQLAlchemy, I was wondering at what time would Session() should be called, in say, a view. Should it be defined as a global variable, or should a new session be created for each request.

Answer 1

I strongly recommend you follow the Pyramid SQLAlchemy tutorial. It teaches you how to use SQLAlchemy with Pyramid in a simple Wiki application.

You’ll note that tutorial a models.py module is created which defines a DBSession item. This gives you access to the SQLAlchemy session, scoped to a Pyramid thread and tied to the Pyramid transaction model, and which is imported whenever you need the session:

from pyramid.view import view_config

from .models import (
    DBSession,
    MyModel,
    )

@view_config(route_name='home', renderer='templates/mytemplate.pt')
def my_view(request):
    one = DBSession.query(MyModel).filter(MyModel.name=='one').first()
    return {'one':one, 'project':'tutorial'}

In models.py the DBSession variable is defined as follows:

from sqlalchemy.orm import (
    scoped_session,
    sessionmaker,
    )

from zope.sqlalchemy import ZopeTransactionExtension

DBSession = scoped_session(sessionmaker(extension=ZopeTransactionExtension()))

Note the ZopeTransactionExtension; Pyramid automatically starts a new transaction with each request, committing it on successful responses, aborting it when an exception occurs. This relieves you of most transaction handling duties. Just remember to .flush your session when you need to see updates to the database (such as auto-incrementing primary keys).

Again, the tutorial expands on all this and more.