[sqlalchemy] How to determine if a session has any uncommitted changes

[Tom Dalton]

Using SQLAlchemy 0.9.

I am writing a server, that's running in gevent. As part of the library 
code, I am trying to write a session context manager that checks if the 
session has uncommitted changes when it's exited (this situation represents 
a logic/programming error, as I expect every server 'request' to finish 
with a committed or rolled back transaction. Each request has its own 
session but there may be multiple transactions within that request/session.

Based on the docs for Session and this question at stackoverflow 
(http://stackoverflow.com/questions/13910576/find-out-how-many-uncommitted-items-are-in-the-session)
 
I wrote a context manager like so:

@contextmanager
def checked_scoped_session():
    session = get_session()
    try:
        yield session
    except:
        session.rollback()
        raise
    finally:
        if session.new or session.dirty or session.deleted:
            new = len(session.new)
            dirty = len(session.dirty)
            deleted = len(session.deleted)
            msg = "Session left with ({} new, {} dirty, {} deleted) 
instances"\
                " uncommitted".format(new, dirty, deleted)
            raise SessionLeftUnclean(msg)
        session.close()

While testing, I discovered that session.new, .dirty and .deleted appear to 
actually be UNFLUSHED instances, and not uncommitted. This is a problem 
since I am using (and want to keep using) autoflush. I have been reading up 
on session.transaction, however, it's still not clear to me how I can tell 
if the transaction has (uncommitted) changes, and this seems to be further 
complicated by the possibility of sub-transactions.

Is there an easy way to do what I want? E.g. Ideally I want to do:

    finally:
        if session.has_uncommitted_changes():
            raise SessionLeftUnclean()

Any and all help gratefully appreciated!

Regards,

Tom

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