SQLAlchemy prevent “nesting” pattern - postgresql

Question

I am now using SQLAlchemy(async/core), PostgreSQL and FastAPI.
I made a #contextlib.contextmanager.
#contextlib.contextmanager
def transaction_begin(self) -> typing.Generator:
if not self.engine_connection.in_transaction():
with self.engine_connection.begin():
yield self.engine_connection
else:
yield self.engine_connection
It worked fine, but today I encountered an AttributeError: __enter__. It works somewhere in FastAPI router functions, it doesn't somewhere:
Traceback (most recent call last):
File "/var/www/myproj/app/auth/routers.py", line 161, in get_refresh_token
await CRUDModel(conn).insert(stmt)
File "/var/www/myproj/app/core/crud.py", line 61, in insert
with self.transaction_begin():
File "/home/me/.pyenv/versions/3.10.5/lib/python3.10/contextlib.py", line 135, in __enter__
return next(self.gen)
File "/var/www/myproj/app/core/crud.py", line 26, in transaction_begin
with self.engine_connection.begin():
AttributeError: __enter__
To solve the issue, I might rewrite the contextlib like this because PostgreSQL always implicit begins:
#contextlib.contextmanager
def transaction_begin(self) -> typing.Generator:
# BEGIN (implicit) by PostgreSQL
yield self.engine_connection
Actually, my contextmanager code is from here :
https://docs.sqlalchemy.org/en/14/core/connections.html#migrating-from-the-nesting-pattern
I would like to know that it is okay to get engine_connection without BEGIN if I use PostgreSQL. Otherwise, do I have to explicit begin transaction if not in transaction?
Thank you.