Async operations

psycopg3 Connection and Cursor have counterparts AsyncConnection and AsyncCursor supporting an asyncio interface.

The design of the asynchronous objects is pretty much the same of the sync ones: in order to use them you will only have to scatter the await keyword here and there.

async with await psycopg3.AsyncConnection.connect(
        "dbname=test user=postgres") as aconn:
    async with aconn.cursor() as acur:
        await acur.execute(
            "INSERT INTO test (num, data) VALUES (%s, %s)",
            (100, "abc'def"))
        await acur.execute("SELECT * FROM test")
        await acur.fetchone()
        # will return (1, 100, "abc'def")
        async for record in acur:
            print(record)

with async connections

As seen in the basic usage, connections and cursors can act as context managers, so you can run:

with psycopg3.connect("dbname=test user=postgres") as conn:
    with conn.cursor() as cur:
        cur.execute(...)
    # the cursor is closed upon leaving the context
# the transaction is committed, the connection closed

For asynchronous connections it’s almost what you’d expect, but not quite. Please note that connect() and cursor() don’t return a context: they are both factory methods which return an object which can be used as a context. That’s because there are several use cases where it’s useful to handle the object manually and close() them when required.

As a consequence you cannot use async with connect(): you have to do it in two steps instead, as in

aconn = await psycopg3.AsyncConnection.connect():
async with aconn:
    async with aconn.cursor() as cur:
        await cur.execute(...)

which can be condensed as:

async with await psycopg3.AsyncConnection.connect() as aconn:
    async with aconn.cursor() as cur:
        await cur.execute(...)

…but no less than that: you still need to do the double async thing.

The AsyncConnection.cursor() function is not marked as async (it never performs I/O), so you don’t need an await on it and you can use the normal async with context manager.

Asynchronous notifications

Psycopg allows asynchronous interaction with other database sessions using the facilities offered by PostgreSQL commands LISTEN and NOTIFY. Please refer to the PostgreSQL documentation for examples about how to use this form of communication.

Because of the way sessions interact with notifications (see NOTIFY documentation), you should keep the connection in autocommit mode if you wish to receive or send notifications in a timely manner.

Notifications are received as instances of Notify. If you are reserving a connection only to receive notifications, the simplest way is to consume the Connection.notifies generator. The generator can be stopped using close().

Note

You don’t need an AsyncConnection to handle notifications: a normal blocking Connection is perfectly valid.

The following example will print notifications and stop when one containing the stop message is received.

import psycopg3
conn = psycopg3.connect("", autocommit=True)
conn.cursor().execute("LISTEN mychan")
gen = conn.notifies()
for notify in gen:
    print(notify)
    if notify.payload == "stop":
        gen.close()
print("there, I stopped")

If you run some NOTIFY in a psql session:

=# notify mychan, 'hello';
NOTIFY
=# notify mychan, 'hey';
NOTIFY
=# notify mychan, 'stop';
NOTIFY

You may get output from the Python process such as:

Notify(channel='mychan', payload='hello', pid=961823)
Notify(channel='mychan', payload='hey', pid=961823)
Notify(channel='mychan', payload='stop', pid=961823)
there, I stopped

Alternatively, you can use add_notify_handler() to register a callback function, which will be invoked whenever a notification is received, during the normal query processing; you will be then able to use the connection normally. Please note that in this case notifications will not be received immediately, but only during a connection operation, such as a query.

conn.add_notify_handler(lambda n: print(f"got this: {n}"))

# meanwhile in psql...
# =# notify mychan, 'hey';
# NOTIFY

print(conn.cursor().execute("select 1").fetchone())
# got this: Notify(channel='mychan', payload='hey', pid=961823)
# (1,)

Detecting disconnections

Sometimes it is useful to detect immediately when the connection with the database is lost. One brutal way to do so is to poll a connection in a loop running an endless stream of SELECT 1Don’t do so: polling is so out of fashion. Besides, it is inefficient (unless what you really want is a client-server generator of ones), it generates useless traffic and will only detect a disconnection with an average delay of half the polling time.

A more efficient and timely way to detect a server disconnection is to get a notification from the OS that the connection has something to say: only then you can test the connection. You can dedicate a thread (or an asyncio task) to wait on a connection: such thread will perform no activity until awaken by the OS.

In a normal (non asyncio) program you can use the selectors module. Because the Connection implements a fileno() method you can just register it as a file-like object. You can run such code in a dedicated thread (and using a dedicated connection) if the rest of the program happens to have something else to do too.

import selectors

sel = selectors.DefaultSelector()
sel.register(conn, selectors.EVENT_READ)
while True:
    if not sel.select(timeout=60.0):
        continue  # No FD activity detected in one minute

    # Activity detected. Is the connection still ok?
    try:
        conn.execute("select 1")
    except psycopg3.OperationalError:
        # You were disconnected: do something useful such as panicking
        logger.error("we lost our database!")
        sys.exit(1)

In an asyncio program you can dedicate a Task instead and do something similar using add_reader:

import asyncio

ev = asyncio.Event()
loop = asyncio.get_event_loop()
loop.add_reader(conn.fileno(), ev.set)

while True:
    try:
        await asyncio.wait_for(ev.wait(), 60.0)
    except asyncio.TimeoutError:
        continue  # No FD activity detected in one minute

    # Activity detected. Is the connection still ok?
    try:
        await conn.execute("select 1")
    except psycopg3.OperationalError:
        # Guess what happened
        ...