sqlalchemy_dlock.baselock module#
- class sqlalchemy_dlock.baselock.BaseSadLock(connection_or_session, key, *args, **kwargs)#
Bases:
_local
Base class of database lock implementation
Note
It’s Thread-Local (
threading.local
)It’s an abstract class, do not manual instantiate
The
acquire()
andrelease()
methods can be used as context managers for awith
statement.acquire()
will be called when the block is entered, andrelease()
will be called when the block is exited. Hence, the following snippet:with some_lock: # do something... pass
is equivalent to:
some_lock.acquire() try: # do something... pass finally: some_lock.release()
- Parameters:
connection_or_session (Connection | Session | scoped_session) – Connection or Session object SQL locking functions will be invoked on it
key – ID or name of the SQL locking function
- property connection_or_session: Connection | Session | scoped_session#
Returns connection_or_session parameter of the constructor
- property key#
Returns key parameter of the constructor
- acquire(block=True, timeout=None, *args, **kwargs)#
Acquire a lock, blocking or non-blocking.
With the block argument set to
True
(the default), the method call will block until the lock is in an unlocked state, then set it to locked and returnTrue
.With the block argument set to
False
, the method call does not block. If the lock is currently in a locked state, returnFalse
; otherwise set the lock to a locked state and returnTrue
.When invoked with a positive, floating-point value for timeout, block for at most the number of seconds specified by timeout as long as the lock can not be acquired. Invocations with a negative value for timeout are equivalent to a timeout of zero. Invocations with a timeout value of
None
(the default) set the timeout period to infinite. The timeout argument has no practical implications if the block argument is set toFalse
and is thus ignored. ReturnsTrue
if the lock has been acquired orFalse
if the timeout period has elapsed.
- release(*args, **kwargs)#
Release a lock.
Since the class is thread-local, this cannot be called from other thread or process, and also can not be called from other connection. (Although PostgreSQL’s shared advisory lock supports so).
When the lock is locked, reset it to unlocked, and return. If any other threads are blocked waiting for the lock to become unlocked, allow exactly one of them to proceed.
When invoked on an unlocked lock, a
ValueError
is raised.There is no return value.
- close(*args, **kwargs)#
Same as
release()
Except that the
ValueError
is NOT raised when invoked on an unlocked lock.An invocation of this method is equivalent to:
if not some_lock.locked: some_lock.release()
This method maybe useful together with
contextlib.closing()
, when we need awith
statement, but don’t want it to acquire at the beginning of the block.eg:
# ... from contextlib import closing from sqlalchemy_dlock import create_sadlock # ... with closing(create_sadlock(some_connection, some_key)) as lock: # will not acquire at the begin of with-block assert not lock.locked # ... # lock when need lock.acquire() assert lock.locked # ... # `close` will be called at the end with-block assert not lock.locked