Synopsis#

Methods#

def __init__()
def lockForRead()
def lockForWrite()
def tryLockForRead()
def tryLockForWrite()
def unlock()

Note

This documentation may contain snippets that were automatically translated from C++ to Python. We always welcome contributions to the snippet translation. If you see an issue with the translation, you can also let us know by creating a ticket on https:/bugreports.qt.io/projects/PYSIDE

Detailed Description#

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

A read-write lock is a synchronization tool for protecting resources that can be accessed for reading and writing. This type of lock is useful if you want to allow multiple threads to have simultaneous read-only access, but as soon as one thread wants to write to the resource, all other threads must be blocked until the writing is complete.

In many cases, QReadWriteLock is a direct competitor to QMutex . QReadWriteLock is a good choice if there are many concurrent reads and writing occurs infrequently.

Example:

lock = QReadWriteLock()
def run(self):

    ...
    lock.lockForRead()
    read_file()
    lock.unlock()
    ...

def run(self):

    ...
    lock.lockForWrite()
    write_file()
    lock.unlock()
    ...

To ensure that writers aren’t blocked forever by readers, readers attempting to obtain a lock will not succeed if there is a blocked writer waiting for access, even if the lock is currently only accessed by other readers. Also, if the lock is accessed by a writer and another writer comes in, that writer will have priority over any readers that might also be waiting.

Like QMutex , a QReadWriteLock can be recursively locked by the same thread when constructed with Recursive as RecursionMode . In such cases, unlock() must be called the same number of times lockForWrite() or lockForRead() was called. Note that the lock type cannot be changed when trying to lock recursively, i.e. it is not possible to lock for reading in a thread that already has locked for writing (and vice versa).

See also

QReadLocker QWriteLocker QMutex QSemaphore

class RecursionMode#

Constant	Description
QReadWriteLock.Recursive	In this mode, a thread can lock the same `QReadWriteLock` multiple times. The `QReadWriteLock` won’t be unlocked until a corresponding number of `unlock()` calls have been made.
QReadWriteLock.NonRecursive	In this mode, a thread may only lock a `QReadWriteLock` once.