Python-Flask-Postgresql SQL block blocking

I tried looking in the Flask and SQLAlchemy documentation, but I couldn't find an answer ...

Do the sessions used with SQLAlchemy do automatic locking on the DB? I am reading (say it takes a long time) and it seems to block any other reads I try to do on my Postgres database. Is this something I need to configure manually on Postgres using constraints or is there something I can change using Flask / SQLAlchemy?

thank