The processing package mostly replicates the API of the threading module.
- class Process(group=None, target=None, name=None, args=(), kwargs={})
An analogue of threading.Thread.
See Process objects.
- exception ProcessExit
- Exception raised in a target process when the Process.stop() method is used. This is a subclass of SystemExit.
- exception BufferTooShort
Exception raised by the recvbytes_into() method of a connection object when the supplied buffer object is too small for the message read.
If e is an instance of BufferTooShort then e.args[0] will give the message as a byte string.
When using multiple processes one generally uses message passing for communication between processes and avoids having to use any synchronization primitives like locks.
For passing messages one can use a pipe (for a connection between two processes) or a queue (which allows multiple producers and consumers).
Note that one can also create a shared queue by using a manager object -- see Managers.
For an example of the usage of queues for interprocess communication see test_workers.py.
- Pipe(duplex=True)
Returns a pair (conn1, conn2) of connection objects representing the ends of a pipe.
If duplex is true then the pipe is two way; otherwise conn1 can only be used for receiving messages and conn2 can only be used for sending messages.
See Connection objects.
- Queue(maxsize=0)
Returns a process shared queue object. The usual Empty and Full exceptions from the standard library's Queue module are raised to signal timeouts.
See Queue objects.
Generally synchronization primitives are not as necessary in a multiprocess program as they are in a mulithreaded program. See the documentation for the standard library's threading module.
Note that one can also create synchronization primitves by using a manager object -- see Managers.
- BoundedSemaphore(value=1)
- Returns a bounded semaphore object: a clone of threading.BoundedSemaphore.
- Condition(lock=None)
Returns a condition variable: a clone of threading.Condition.
If lock is specified then it should be a Lock or RLock object from processing.
- Event()
- Returns an event object: a clone of threading.Event.
- Lock()
Returns a non-recursive lock object: a near clone of threading.Lock.
There are two differences from threading.Lock: trying to acquire a lock already owned by the current thread raises an exception instead of deadlocking; and trying to release a lock held by a different thread/process will raise and exception.
- RLock()
- Returns a recursive lock object: a clone of threading.RLock.
- Semaphore(value=1)
- Returns a bounded semaphore object: a clone of threading.Semaphore.
Managers provide a way to create data which can be shared between different processes.
- LocalManager()
Returns a manager object which uses shared memory instead of a server process. It has instance methods
SharedValue, SharedStruct, SharedArrayfor creating objects stored in shared memory map.
See LocalManager.
- Manager()
Returns a started SyncManager object which can be used for sharing objects between processes. The returned manager object corresponds to a spawned child process and has methods which will create shared objects and return corresponding proxies.
The methods for creating shared objects are
list(), dict(), Namespace(), SharedValue(), SharedStruct(), SharedArray(), Lock(), RLock(), Semaphore(), BoundedSemaphore(), Condition(), Event(), Queue().See SyncManager.
One can create a pool of processes which will carry out tasks submitted to it.
- Pool(processes=None)
Returns a process pool object which controls a pool of worker processes to which jobs can be submitted.
It supports asynchronous results with timeouts and callbacks and has a parallel map implementation.
If processes is None then the number returned by cpuCount() is used.
See Pool objects.
Some support for logging is available. Note, however, that the logging package does not use process shared locks so it is possible (depending on the handler type) for messages from different processes to get mixed up.
- enableLogging(level=10, HandlerType=None, handlerArgs=(), format=None)
Enables logging and sets the debug level used by the package's logger to level -- see documentation for the logging package in the standard library. The default logging level is 10 which is equivalent to logging.DEBUG.
If HandlerType is specified then a handler is created using HandlerType(*handlerArgs) and this will be used by the logger -- any previous handlers will be discarded. If format is specified then this will be used for the handler; otherwise format defaults to '[%(levelname)s/%(processName)s] %(message)s'. (The logger used by processing allows use of the non-standard '%(processName)s' format.)
If HandlerType is not specified and the logger has no handlers then a default one is created which prints to sys.stderr.
Note: on Windows a child process does not directly inherit its parent's logger; instead it will automatically call enableLogging() with the same arguments which were used when its parent process last called enableLogging() (if it ever did).
- getLogger()
- Returns the logger used by processing. If enableLogging() has not yet been called then None is returned.
Below is an example session with logging turned on:
>>> import processing, logging >>> processing.enableLogging(level=logging.INFO) >>> processing.getLogger().warn('doomed') [WARNING/MainProcess] doomed >>> m = processing.Manager() [INFO/SyncManager-1] process starting up [INFO/SyncManager-1] manager bound to '\\\\.\\pipe\\pyc-1352-0-r97d0b' >>> del m [INFO/MainProcess] sending shutdown message to manager [INFO/SyncManager-1] manager received shutdown message [INFO/SyncManager-1] running all "atexit" finalizers [INFO/SyncManager-1] process exiting with `os.exit(0)`
- activeChildren()
Return list of all live children of the current process.
Calling this has the side affect of "joining" any processes which have already finished.
- cpuCount()
- Returns the number of CPUs in the system. May raise NotImplementedError.
- currentProcess()
An analogue of threading.currentThread
Returns the object corresponding to the current process.
- freezeSupport()
Adds support for when a program which uses the processing package has been frozen to produce a Windows executable. (Has been tested with py2exe, PyInstaller and cx_Freeze.)
One needs to call this function straight after the if __name__ == '__main__' line of the main module. For example
from processing import Process, freezeSupport def f(): print "hello world!" if __name__ == '__main__': freezeSupport() p = Process(target=f) p.start()If the freezeSupport() line is missed out then the frozen executable produced from this module would (on Windows) recursively create new processes.
If the module is being run normally by the python interpreter then freezeSupport() has no effect.
Note