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 BufferTooShort
Exception raised by the recvBytesInto() 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 ex_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.
(On Mac OSX this is indistiguishable from Semaphore() because sem_getvalue() is not implemented on that platform).
- 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 clone of threading.Lock.
- RLock()
- Returns a recursive lock object: a clone of threading.RLock.
- Semaphore(value=1)
- Returns a bounded semaphore object: a clone of threading.Semaphore.
Acquiring with a timeout
The acquire() method of BoundedSemaphore, Lock, RLock and Semaphore has a timeout parameter not supported by the equivalents in threading. The signature is acquire(block=True, timeout=None) with keyword parameters being acceptable. If block is true and timeout is not None then it specifies a timeout in seconds. If block is false then timeout is ignored.
Interrupting the main thread
If the SIGINT signal generated by Ctrl-C arrives while the main thread is blocked by a call to BoundedSemaphore.acquire(), Lock.acquire(), RLock.acquire(), Semaphore.acquire(), Condition.acquire() or Condition.wait() then the call will be immediately interrupted and KeyboardInterrupt will be raised.
This differs from the behaviour of threading where SIGINT will be ignored while the equivalent blocking calls are in progress.
It is possible to create shared objects using shared memory which can be inherited by child processes.
- Value(typecode_or_type, *args, **, lock=True)
Returns a ctypes object allocated from shared memory. By default the return value is actually a synchronized wrapper for the object.
typecode_or_type determines the type of the returned object: it is either a ctypes type or a one character typecode of the kind used by the array module. *args is passed on to the constructor for the type.
If lock is true (the default) then a new lock object is created to synchronize access to the value. If lock is a Lock or RLock object then that will be used to synchronize access to the value. If lock is false then access to the returned object will not be automatically protected by a lock, so it will not necessarily be "process-safe".
Note that lock is a keyword only argument.
- Array(typecode_or_type, size_or_initializer, **, lock=True)
Returns a ctypes array allocated from shared memory. By default the return value is actually a synchronized wrapper for the array.
typecode_or_type determines the type of the elements of the returned array: it is either a ctypes type or a one character typecode of the kind used by the array module. If size_or_initializer is an integer then it determines the length of the array, and the array will be initially zeroed. Otherwise size_or_initializer is a sequence which is used to initialize the array and whose length determines the length of the array.
If lock is true (the default) then a new lock object is created to synchronize access to the value. If lock is a Lock or RLock object then that will be used to synchronize access to the value. If lock is false then access to the returned object will not be automatically protected by a lock, so it will not necessarily be "process-safe".
Note that lock is a keyword only argument.
Note that an array of ctypes.c_char has value and rawvalue attributes which allow one to use it to store and retrieve strings -- see the documentation for ctypes in the standard library.
See also sharedctypes.
Managers provide a way to create data which can be shared between different processes.
- 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(), Value(), Array(), Lock(), RLock(), Semaphore(), BoundedSemaphore(), Condition(), Event(), Queue().See SyncManager.
It is possible to create managers which support other types -- see Customized managers.
One can create a pool of processes which will carry out tasks submitted to it.
- Pool(processes=None, initializer=None, initargs=())
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.
processes is the number of worker processes to use. If processes is None then the number returned by cpuCount() is used. If initializer is not None then each worker process will call initializer(*initargs) when it starts.
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, 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 module in the standard library.
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().warning('doomed')
[WARNING/MainProcess] doomed
>>> m = processing.Manager()
[INFO/SyncManager-1] child process calling self.run()
[INFO/SyncManager-1] manager bound to '\\\\.\\pipe\\pyc-2776-0-lj0tfa'
>>> del m
[INFO/MainProcess] sending shutdown message to manager
[INFO/SyncManager-1] manager received shutdown message
[INFO/SyncManager-1] manager exiting with exitcode 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() Process(target=f).start()If the freezeSupport() line is missed out then trying to run the frozen executable will raise RuntimeError.
If the module is being run normally by the python interpreter then freezeSupport() has no effect.
Note