async with await psycopg.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)
An AsyncConnection can be used by several asyncio.Task at the same time.
However, as with threads, all the AsyncCursor on the same connection will
share the same session and will have their access to the connection
serialized.
Changed in version 3.1: AsyncConnection.connect() performs DNS name resolution in a non-blocking
Warning
Before version 3.1, AsyncConnection.connect() may still block on DNS
name resolution. To avoid that you should set the hostaddr connection
parameter, or use the resolve_hostaddr_async() to
do it automatically.
Warning
On Windows, Psycopg is not compatible with the default
ProactorEventLoop. Please use a different loop, for instance
the SelectorEventLoop.
For instance, you can use, early in your program:
asyncio.set_event_loop_policy(
asyncio.WindowsSelectorEventLoopPolicy()
with async connections
As seen in the basic usage, connections and cursors can act as
context managers, so you can run:
with psycopg.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 objects manually and only 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 psycopg.AsyncConnection.connect()
async with aconn:
async with aconn.cursor() as cur:
await cur.execute(...)
which can be condensed into async with await:
async with await psycopg.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.
Note that the AsyncConnection.cursor() function is not an async function
(it never performs I/O), so you don’t need an await on it; as a consequence
you can use the normal async with context manager.
Interrupting async operations
If a long running operation is interrupted by a Ctrl-C on a normal connection
running in the main thread, the operation will be cancelled and the connection
will be put in error state, from which can be recovered with a normal
rollback().
An async connection provides similar behavior in that if the async task is
cancelled, any operation on the connection will similarly be cancelled. This
can happen either indirectly via Ctrl-C or similar signal, or directly by
cancelling the Python Task via the normal way. Psycopg will ask the
PostgreSQL postmaster to cancel the operation when it encounters the standard
Python CancelledError.
Remember that cancelling the Python Task does not guarantee that the operation
will not complete, even if the task ultimately exits prematurely due to
CancelledError. If you need to know the ultimate outcome of the statement,
then consider calling Connection.cancel() as an alternative to cancelling
the task.
Previous versions of Psycopg recommended setting up signal handlers to
manually cancel connections. This should no longer be necessary.
Gevent support
Psycopg 3 supports gevent out of the box. If the
select module is found patched by functions such as
gevent.monkey.patch_select() or patch_all(), psycopg will behave in a
collaborative way.
Unlike with psycopg2, using the psycogreen module is not required.
Warning
gevent support was initially accidental, and was accidentally broken in
psycopg 3.1.4.
gevent is officially supported only starting from psycopg 3.1.14.
Server messages
PostgreSQL can send, together with the query results, informative messages
about the operation just performed, such as warnings or debug information.
Notices may be raised even if the operations are successful and don’t indicate
an error. You are probably familiar with some of them, because they are
reported by psql:
$ psql
=# ROLLBACK;
WARNING: there is no transaction in progress
ROLLBACK
Messages can be also sent by the PL/pgSQL ‘RAISE’ statement (at a level
lower than EXCEPTION, otherwise the appropriate DatabaseError will be
raised). The level of the messages received can be controlled using the
client_min_messages setting.
By default, the messages received are ignored. If you want to process them on
the client you can use the Connection.add_notice_handler() function to
register a function that will be invoked whenever a message is received. The
message is passed to the callback as a Diagnostic instance,
containing all the information passed by the server, such as the message text
and the severity. The object is the same found on the diag
attribute of the errors raised by the server:
>>> import psycopg
>>> def log_notice(diag):
... print(f"The server says: {diag.severity} - {diag.message_primary}")
>>> conn = psycopg.connect(autocommit=True)
>>> conn.add_notice_handler(log_notice)
>>> cur = conn.execute("ROLLBACK")
The server says: WARNING - there is no transaction in progress
>>> print(cur.statusmessage)
ROLLBACK
Warning
The Diagnostic object received by the callback should not be used after
the callback function terminates, because its data is deallocated after
the callbacks have been processed. If you need to use the information
later please extract the attributes requested and forward them instead of
forwarding the whole Diagnostic object.
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(). Starting from Psycopg 3.2, the method supports options to receive
notifications only for a certain time or up to a certain number.
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 psycopg
conn = psycopg.connect("", autocommit=True)
conn.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.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 1… Don’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 create
an additional connection and wait for a notification from the OS that this
connection has something to say: only then you can run some checks. You
can dedicate a thread (or an asyncio task) to wait on this 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 psycopg.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 psycopg.OperationalError:
# Guess what happened
