Python Database Pooling with psycopg2

Question

Below is a database pooling example. I don't understand the following.

1. Why the getcursor function use "yield"?

2. What is the context manager?

    from psycopg2.pool import SimpleConnectionPool from contextlib import contextmanager dbConnection = "dbname='dbname' user='postgres' host='localhost' password='postgres'" # pool define with 10 live connections connectionpool = SimpleConnectionPool(1,10,dsn=dbConnection) @contextmanager def getcursor(): con = connectionpool.getconn() try: yield con.cursor() finally: connectionpool.putconn(con) def main_work(): try: # with here will take care of put connection when its done with getcursor() as cur: cur.execute("select * from \\"TableName\\"") result_set = cur.fetchall() except Exception as e: print "error in executing with exception: ", e**

Answer 1

Both of your questions are related. In python context managers are what we're using whenever you see a with statement. Classically, they're written like this.

class getcursor(object):
    def __enter__(self):
        con = connectionpool.getconn()
        return con

    def __exit__(self, *args):
        connectionpool.putconn(con)

Now when you use a context manager, it calls the __enter__ method on the with statement and the __exit__ method when it exits the context. Think of it like this.

cursor = getcursor()
with cursor as cur: # calls cursor.__enter__()
   cur.execute("select * from \"TableName\"")
   result_set = cur.fetchall()
# We're now exiting the context so it calls `cursor.__exit__()` 
# with some exception info if relevant
x = 1

The @contextmanager decorator is some sugar to make creating a context manager easier. Basically, it uses the yield statement to give the execution back to the caller. Everything up to and including the yield statement is the __enter__ method and everything after that is effectively the __exit__ statement.