SQLAlchemy or psycopg2?

SQLAlchemy is a ORM, psycopg2 is a database driver. These are completely different things: SQLAlchemy generates SQL statements and psycopg2 sends SQL statements to the database. SQLAlchemy depends on psycopg2 or other database drivers to communicate with the database!

As a rather complex software layer SQLAlchemy does add some overhead but it also is a huge boost to development speed, at least once you learned the library. SQLAlchemy is an excellent library and will teach you the whole ORM concept, but if you don't want to generate SQL statements to begin with then you don't want SQLAlchemy.

To talk with database any one need driver for that. If you are using client like SQL Plus for oracle, MysqlCLI for Mysql then it will direct run the query and that client come with DBServer pack.

To communicate from outside with any language like java, c, python, C#... We need driver to for that database. psycopg2 is driver to run query for PostgreSQL from python.

SQLAlchemy is the ORM which is not same as database driver. It will give you flexibility so you can write your code without any database specific standard. ORM provide database independence for programmer. If you write object.save in ORM then it will check, which database is associated with that object and it will generate insert query according to the backend database.