register_entities | InvalidRequestError: A transaction is already begun for this connection

Hello,

I set up an extremely basic test view in my env.py file to try out this package but when I ran alembic revision --autogenerate, alembic_utils raised the following exception:

  File "/usr/local/lib/python3.7/site-packages/alembic_utils/replaceable_entity.py", line 251, in compare_registered_entities
    sess.rollback()
UnboundLocalError: local variable 'sess' referenced before assignment

I dug into the replaceable_entity file and found that the culprit for this issue is this block of code:

try:
    transaction = connection.begin()
    sess = Session(bind=connection)
    ordered_entities: List[T] = solve_resolution_order(sess, entities)
finally:
    sess.rollback()

The core of the problem appears to be the connection.begin() line, which raises the following silent exception, causing sess to be undeclared:

  File "/usr/local/lib/python3.7/site-packages/alembic_utils/replaceable_entity.py", line 247, in compare_registered_entities
    transaction = connection.begin()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/future/engine.py", line 172, in begin
    return super(Connection, self).begin()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 741, in begin
    "a transaction is already begun for this connection"
sqlalchemy.exc.InvalidRequestError: a transaction is already begun for this connection

I found that I was able to avoid this issue if I simply commented out the transaction definition as it also appears to not be used as part of the proceeding code. Any ideas? I am confused as to why this is not an issue that has been raised before. After disabling the line, I was able to run my revision script without issue and correctly generate my test view.

pip reference:

alembic                            1.6.4
alembic-utils                      0.7.2

[quoimec]

Seems to be working for me too, thanks for the quick fix on this @olirice!

[ddemidov]

I can confirm the error with undefined sess is gone. There is another problem with async engine, I’ll open another issue.