:mod:`multiprocessing` --- Process-based "threading" interface ============================================================== .. module:: multiprocessing :synopsis: Process-based "threading" interface. Introduction ------------ :mod:`multiprocessing` is a package that supports spawning processes using an API similar to the :mod:`threading` module. The :mod:`multiprocessing` package offers both local and remote concurrency, effectively side-stepping the :term:`Global Interpreter Lock` by using subprocesses instead of threads. Due to this, the :mod:`multiprocessing` module allows the programmer to fully leverage multiple processors on a given machine. It runs on both Unix and Windows. .. warning:: Some of this package's functionality requires a functioning shared semaphore implementation on the host operating system. Without one, the :mod:`multiprocessing.synchronize` module will be disabled, and attempts to import it will result in an :exc:`ImportError`. See :issue:`3770` for additional information. .. note:: Functionality within this package requires that the ``__main__`` method be importable by the children. This is covered in :ref:`multiprocessing-programming` however it is worth pointing out here. This means that some examples, such as the :class:`multiprocessing.Pool` examples will not work in the interactive interpreter. For example:: >>> from multiprocessing import Pool >>> p = Pool(5) >>> def f(x): ... return x*x ... >>> p.map(f, [1,2,3]) Process PoolWorker-1: Process PoolWorker-2: Traceback (most recent call last): Traceback (most recent call last): AttributeError: 'module' object has no attribute 'f' AttributeError: 'module' object has no attribute 'f' AttributeError: 'module' object has no attribute 'f' The :class:`Process` class ~~~~~~~~~~~~~~~~~~~~~~~~~~ In :mod:`multiprocessing`, processes are spawned by creating a :class:`Process` object and then calling its :meth:`~Process.start` method. :class:`Process` follows the API of :class:`threading.Thread`. A trivial example of a multiprocess program is :: from multiprocessing import Process def f(name): print('hello', name) if __name__ == '__main__': p = Process(target=f, args=('bob',)) p.start() p.join() To show the individual process IDs involved, here is an expanded example:: from multiprocessing import Process import os def info(title): print title print 'module name:', __name__ print 'parent process:', os.getppid() print 'process id:', os.getpid() def f(name): info('function f') print 'hello', name if __name__ == '__main__': info('main line') p = Process(target=f, args=('bob',)) p.start() p.join() For an explanation of why (on Windows) the ``if __name__ == '__main__'`` part is necessary, see :ref:`multiprocessing-programming`. Exchanging objects between processes ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :mod:`multiprocessing` supports two types of communication channel between processes: **Queues** The :class:`Queue` class is a near clone of :class:`queue.Queue`. For example:: from multiprocessing import Process, Queue def f(q): q.put([42, None, 'hello']) if __name__ == '__main__': q = Queue() p = Process(target=f, args=(q,)) p.start() print(q.get()) # prints "[42, None, 'hello']" p.join() Queues are thread and process safe. **Pipes** The :func:`Pipe` function returns a pair of connection objects connected by a pipe which by default is duplex (two-way). For example:: from multiprocessing import Process, Pipe def f(conn): conn.send([42, None, 'hello']) conn.close() if __name__ == '__main__': parent_conn, child_conn = Pipe() p = Process(target=f, args=(child_conn,)) p.start() print(parent_conn.recv()) # prints "[42, None, 'hello']" p.join() The two connection objects returned by :func:`Pipe` represent the two ends of the pipe. Each connection object has :meth:`~Connection.send` and :meth:`~Connection.recv` methods (among others). Note that data in a pipe may become corrupted if two processes (or threads) try to read from or write to the *same* end of the pipe at the same time. Of course there is no risk of corruption from processes using different ends of the pipe at the same time. Synchronization between processes ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ :mod:`multiprocessing` contains equivalents of all the synchronization primitives from :mod:`threading`. For instance one can use a lock to ensure that only one process prints to standard output at a time:: from multiprocessing import Process, Lock def f(l, i): l.acquire() print('hello world', i) l.release() if __name__ == '__main__': lock = Lock() for num in range(10): Process(target=f, args=(lock, num)).start() Without using the lock output from the different processes is liable to get all mixed up. Sharing state between processes ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ As mentioned above, when doing concurrent programming it is usually best to avoid using shared state as far as possible. This is particularly true when using multiple processes. However, if you really do need to use some shared data then :mod:`multiprocessing` provides a couple of ways of doing so. **Shared memory** Data can be stored in a shared memory map using :class:`Value` or :class:`Array`. For example, the following code :: from multiprocessing import Process, Value, Array def f(n, a): n.value = 3.1415927 for i in range(len(a)): a[i] = -a[i] if __name__ == '__main__': num = Value('d', 0.0) arr = Array('i', range(10)) p = Process(target=f, args=(num, arr)) p.start() p.join() print(num.value) print(arr[:]) will print :: 3.1415927 [0, -1, -2, -3, -4, -5, -6, -7, -8, -9] The ``'d'`` and ``'i'`` arguments used when creating ``num`` and ``arr`` are typecodes of the kind used by the :mod:`array` module: ``'d'`` indicates a double precision float and ``'i'`` indicates a signed integer. These shared objects will be process and thread safe. For more flexibility in using shared memory one can use the :mod:`multiprocessing.sharedctypes` module which supports the creation of arbitrary ctypes objects allocated from shared memory. **Server process** A manager object returned by :func:`Manager` controls a server process which holds Python objects and allows other processes to manipulate them using proxies. A manager returned by :func:`Manager` will support types :class:`list`, :class:`dict`, :class:`Namespace`, :class:`Lock`, :class:`RLock`, :class:`Semaphore`, :class:`BoundedSemaphore`, :class:`Condition`, :class:`Event`, :class:`Queue`, :class:`Value` and :class:`Array`. For example, :: from multiprocessing import Process, Manager def f(d, l): d[1] = '1' d['2'] = 2 d[0.25] = None l.reverse() if __name__ == '__main__': manager = Manager() d = manager.dict() l = manager.list(range(10)) p = Process(target=f, args=(d, l)) p.start() p.join() print(d) print(l) will print :: {0.25: None, 1: '1', '2': 2} [9, 8, 7, 6, 5, 4, 3, 2, 1, 0] Server process managers are more flexible than using shared memory objects because they can be made to support arbitrary object types. Also, a single manager can be shared by processes on different computers over a network. They are, however, slower than using shared memory. Using a pool of workers ~~~~~~~~~~~~~~~~~~~~~~~ The :class:`~multiprocessing.pool.Pool` class represents a pool of worker processes. It has methods which allows tasks to be offloaded to the worker processes in a few different ways. For example:: from multiprocessing import Pool def f(x): return x*x if __name__ == '__main__': pool = Pool(processes=4) # start 4 worker processes result = pool.apply_async(f, [10]) # evaluate "f(10)" asynchronously print result.get(timeout=1) # prints "100" unless your computer is *very* slow print pool.map(f, range(10)) # prints "[0, 1, 4,..., 81]" Reference --------- The :mod:`multiprocessing` package mostly replicates the API of the :mod:`threading` module. :class:`Process` and exceptions ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .. class:: Process([group[, target[, name[, args[, kwargs]]]]]) Process objects represent activity that is run in a separate process. The :class:`Process` class has equivalents of all the methods of :class:`threading.Thread`. The constructor should always be called with keyword arguments. *group* should always be ``None``; it exists solely for compatibility with :class:`threading.Thread`. *target* is the callable object to be invoked by the :meth:`run()` method. It defaults to ``None``, meaning nothing is called. *name* is the process name. By default, a unique name is constructed of the form 'Process-N\ :sub:`1`:N\ :sub:`2`:...:N\ :sub:`k`' where N\ :sub:`1`,N\ :sub:`2`,...,N\ :sub:`k` is a sequence of integers whose length is determined by the *generation* of the process. *args* is the argument tuple for the target invocation. *kwargs* is a dictionary of keyword arguments for the target invocation. By default, no arguments are passed to *target*. If a subclass overrides the constructor, it must make sure it invokes the base class constructor (:meth:`Process.__init__`) before doing anything else to the process. .. method:: run() Method representing the process's activity. You may override this method in a subclass. The standard :meth:`run` method invokes the callable object passed to the object's constructor as the target argument, if any, with sequential and keyword arguments taken from the *args* and *kwargs* arguments, respectively. .. method:: start() Start the process's activity. This must be called at most once per process object. It arranges for the object's :meth:`run` method to be invoked in a separate process. .. method:: join([timeout]) Block the calling thread until the process whose :meth:`join` method is called terminates or until the optional timeout occurs. If *timeout* is ``None`` then there is no timeout. A process can be joined many times. A process cannot join itself because this would cause a deadlock. It is an error to attempt to join a process before it has been started. .. attribute:: name The process's name. The name is a string used for identification purposes only. It has no semantics. Multiple processes may be given the same name. The initial name is set by the constructor. .. method:: is_alive Return whether the process is alive. Roughly, a process object is alive from the moment the :meth:`start` method returns until the child process terminates. .. attribute:: daemon The process's daemon flag, a Boolean value. This must be set before :meth:`start` is called. The initial value is inherited from the creating process. When a process exits, it attempts to terminate all of its daemonic child processes. Note that a daemonic process is not allowed to create child processes. Otherwise a daemonic process would leave its children orphaned if it gets terminated when its parent process exits. In addition to the :class:`Threading.Thread` API, :class:`Process` objects also support the following attributes and methods: .. attribute:: pid Return the process ID. Before the process is spawned, this will be ``None``. .. attribute:: exitcode The child's exit code. This will be ``None`` if the process has not yet terminated. A negative value *-N* indicates that the child was terminated by signal *N*. .. attribute:: authkey The process's authentication key (a byte string). When :mod:`multiprocessing` is initialized the main process is assigned a random string using :func:`os.random`. When a :class:`Process` object is created, it will inherit the authentication key of its parent process, although this may be changed by setting :attr:`authkey` to another byte string. See :ref:`multiprocessing-auth-keys`. .. method:: terminate() Terminate the process. On Unix this is done using the ``SIGTERM`` signal; on Windows :cfunc:`TerminateProcess` is used. Note that exit handlers and finally clauses, etc., will not be executed. Note that descendant processes of the process will *not* be terminated -- they will simply become orphaned. .. warning:: If this method is used when the associated process is using a pipe or queue then the pipe or queue is liable to become corrupted and may become unusable by other process. Similarly, if the process has acquired a lock or semaphore etc. then terminating it is liable to cause other processes to deadlock. Note that the :meth:`start`, :meth:`join`, :meth:`is_alive` and :attr:`exit_code` methods should only be called by the process that created the process object. Example usage of some of the methods of :class:`Process`:: >>> import multiprocessing, time, signal >>> p = multiprocessing.Process(target=time.sleep, args=(1000,)) >>> print(p, p.is_alive()) False >>> p.start() >>> print(p, p.is_alive()) True >>> p.terminate() >>> print(p, p.is_alive()) False >>> p.exitcode == -signal.SIGTERM True .. exception:: BufferTooShort Exception raised by :meth:`Connection.recv_bytes_into()` when the supplied buffer object is too small for the message read. If ``e`` is an instance of :exc:`BufferTooShort` then ``e.args[0]`` will give the message as a byte string. Pipes and Queues ~~~~~~~~~~~~~~~~ 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 :func:`Pipe` (for a connection between two processes) or a queue (which allows multiple producers and consumers). The :class:`Queue` and :class:`JoinableQueue` types are multi-producer, multi-consumer FIFO queues modelled on the :class:`queue.Queue` class in the standard library. They differ in that :class:`Queue` lacks the :meth:`~queue.Queue.task_done` and :meth:`~queue.Queue.join` methods introduced into Python 2.5's :class:`queue.Queue` class. If you use :class:`JoinableQueue` then you **must** call :meth:`JoinableQueue.task_done` for each task removed from the queue or else the semaphore used to count the number of unfinished tasks may eventually overflow raising an exception. Note that one can also create a shared queue by using a manager object -- see :ref:`multiprocessing-managers`. .. note:: :mod:`multiprocessing` uses the usual :exc:`queue.Empty` and :exc:`queue.Full` exceptions to signal a timeout. They are not available in the :mod:`multiprocessing` namespace so you need to import them from :mod:`queue`. .. warning:: If a process is killed using :meth:`Process.terminate` or :func:`os.kill` while it is trying to use a :class:`Queue`, then the data in the queue is likely to become corrupted. This may cause any other processes to get an exception when it tries to use the queue later on. .. warning:: As mentioned above, if a child process has put items on a queue (and it has not used :meth:`JoinableQueue.cancel_join_thread`), then that process will not terminate until all buffered items have been flushed to the pipe. This means that if you try joining that process you may get a deadlock unless you are sure that all items which have been put on the queue have been consumed. Similarly, if the child process is non-daemonic then the parent process may hang on exit when it tries to join all its non-daemonic children. Note that a queue created using a manager does not have this issue. See :ref:`multiprocessing-programming`. For an example of the usage of queues for interprocess communication see :ref:`multiprocessing-examples`. .. function:: Pipe([duplex]) Returns a pair ``(conn1, conn2)`` of :class:`Connection` objects representing the ends of a pipe. If *duplex* is ``True`` (the default) then the pipe is bidirectional. If *duplex* is ``False`` then the pipe is unidirectional: ``conn1`` can only be used for receiving messages and ``conn2`` can only be used for sending messages. .. class:: Queue([maxsize]) Returns a process shared queue implemented using a pipe and a few locks/semaphores. When a process first puts an item on the queue a feeder thread is started which transfers objects from a buffer into the pipe. The usual :exc:`queue.Empty` and :exc:`queue.Full` exceptions from the standard library's :mod:`Queue` module are raised to signal timeouts. :class:`Queue` implements all the methods of :class:`queue.Queue` except for :meth:`~queue.Queue.task_done` and :meth:`~queue.Queue.join`. .. method:: qsize() Return the approximate size of the queue. Because of multithreading/multiprocessing semantics, this number is not reliable. Note that this may raise :exc:`NotImplementedError` on Unix platforms like Mac OS X where ``sem_getvalue()`` is not implemented. .. method:: empty() Return ``True`` if the queue is empty, ``False`` otherwise. Because of multithreading/multiprocessing semantics, this is not reliable. .. method:: full() Return ``True`` if the queue is full, ``False`` otherwise. Because of multithreading/multiprocessing semantics, this is not reliable. .. method:: put(item[, block[, timeout]]) Put item into the queue. If the optional argument *block* is ``True`` (the default) and *timeout* is ``None`` (the default), block if necessary until a free slot is available. If *timeout* is a positive number, it blocks at most *timeout* seconds and raises the :exc:`queue.Full` exception if no free slot was available within that time. Otherwise (*block* is ``False``), put an item on the queue if a free slot is immediately available, else raise the :exc:`queue.Full` exception (*timeout* is ignored in that case). .. method:: put_nowait(item) Equivalent to ``put(item, False)``. .. method:: get([block[, timeout]]) Remove and return an item from the queue. If optional args *block* is ``True`` (the default) and *timeout* is ``None`` (the default), block if necessary until an item is available. If *timeout* is a positive number, it blocks at most *timeout* seconds and raises the :exc:`queue.Empty` exception if no item was available within that time. Otherwise (block is ``False``), return an item if one is immediately available, else raise the :exc:`queue.Empty` exception (*timeout* is ignored in that case). .. method:: get_nowait() get_no_wait() Equivalent to ``get(False)``. :class:`multiprocessing.Queue` has a few additional methods not found in :class:`queue.Queue`. These methods are usually unnecessary for most code: .. method:: close() Indicate that no more data will be put on this queue by the current process. The background thread will quit once it has flushed all buffered data to the pipe. This is called automatically when the queue is garbage collected. .. method:: join_thread() Join the background thread. This can only be used after :meth:`close` has been called. It blocks until the background thread exits, ensuring that all data in the buffer has been flushed to the pipe. By default if a process is not the creator of the queue then on exit it will attempt to join the queue's background thread. The process can call :meth:`cancel_join_thread` to make :meth:`join_thread` do nothing. .. method:: cancel_join_thread() Prevent :meth:`join_thread` from blocking. In particular, this prevents the background thread from being joined automatically when the process exits -- see :meth:`join_thread`. .. class:: JoinableQueue([maxsize]) :class:`JoinableQueue`, a :class:`Queue` subclass, is a queue which additionally has :meth:`task_done` and :meth:`join` methods. .. method:: task_done() Indicate that a formerly enqueued task is complete. Used by queue consumer threads. For each :meth:`~Queue.get` used to fetch a task, a subsequent call to :meth:`task_done` tells the queue that the processing on the task is complete. If a :meth:`~Queue.join` is currently blocking, it will resume when all items have been processed (meaning that a :meth:`task_done` call was received for every item that had been :meth:`~Queue.put` into the queue). Raises a :exc:`ValueError` if called more times than there were items placed in the queue. .. method:: join() Block until all items in the queue have been gotten and processed. The count of unfinished tasks goes up whenever an item is added to the queue. The count goes down whenever a consumer thread calls :meth:`task_done` to indicate that the item was retrieved and all work on it is complete. When the count of unfinished tasks drops to zero, :meth:`~Queue.join` unblocks. Miscellaneous ~~~~~~~~~~~~~ .. function:: active_children() Return list of all live children of the current process. Calling this has the side affect of "joining" any processes which have already finished. .. function:: cpu_count() Return the number of CPUs in the system. May raise :exc:`NotImplementedError`. .. function:: current_process() Return the :class:`Process` object corresponding to the current process. An analogue of :func:`threading.current_thread`. .. function:: freeze_support() Add support for when a program which uses :mod:`multiprocessing` 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 multiprocessing import Process, freeze_support def f(): print('hello world!') if __name__ == '__main__': freeze_support() Process(target=f).start() If the ``freeze_support()`` line is missed out then trying to run the frozen executable will raise :exc:`RuntimeError`. If the module is being run normally by the Python interpreter then :func:`freeze_support` has no effect. .. function:: set_executable() Sets the path of the python interpreter to use when starting a child process. (By default :data:`sys.executable` is used). Embedders will probably need to do some thing like :: setExecutable(os.path.join(sys.exec_prefix, 'pythonw.exe')) before they can create child processes. (Windows only) .. note:: :mod:`multiprocessing` contains no analogues of :func:`threading.active_count`, :func:`threading.enumerate`, :func:`threading.settrace`, :func:`threading.setprofile`, :class:`threading.Timer`, or :class:`threading.local`. Connection Objects ~~~~~~~~~~~~~~~~~~ Connection objects allow the sending and receiving of picklable objects or strings. They can be thought of as message oriented connected sockets. Connection objects usually created using :func:`Pipe` -- see also :ref:`multiprocessing-listeners-clients`. .. class:: Connection .. method:: send(obj) Send an object to the other end of the connection which should be read using :meth:`recv`. The object must be picklable. Very large pickles (approximately 32 MB+, though it depends on the OS) may raise a ValueError exception. .. method:: recv() Return an object sent from the other end of the connection using :meth:`send`. Raises :exc:`EOFError` if there is nothing left to receive and the other end was closed. .. method:: fileno() Returns the file descriptor or handle used by the connection. .. method:: close() Close the connection. This is called automatically when the connection is garbage collected. .. method:: poll([timeout]) Return whether there is any data available to be read. If *timeout* is not specified then it will return immediately. If *timeout* is a number then this specifies the maximum time in seconds to block. If *timeout* is ``None`` then an infinite timeout is used. .. method:: send_bytes(buffer[, offset[, size]]) Send byte data from an object supporting the buffer interface as a complete message. If *offset* is given then data is read from that position in *buffer*. If *size* is given then that many bytes will be read from buffer. Very large buffers (approximately 32 MB+, though it depends on the OS) may raise a ValueError exception .. method:: recv_bytes([maxlength]) Return a complete message of byte data sent from the other end of the connection as a string. Raises :exc:`EOFError` if there is nothing left to receive and the other end has closed. If *maxlength* is specified and the message is longer than *maxlength* then :exc:`IOError` is raised and the connection will no longer be readable. .. method:: recv_bytes_into(buffer[, offset]) Read into *buffer* a complete message of byte data sent from the other end of the connection and return the number of bytes in the message. Raises :exc:`EOFError` if there is nothing left to receive and the other end was closed. *buffer* must be an object satisfying the writable buffer interface. If *offset* is given then the message will be written into the buffer from *that position. Offset must be a non-negative integer less than the *length of *buffer* (in bytes). If the buffer is too short then a :exc:`BufferTooShort` exception is raised and the complete message is available as ``e.args[0]`` where ``e`` is the exception instance. For example: >>> from multiprocessing import Pipe >>> a, b = Pipe() >>> a.send([1, 'hello', None]) >>> b.recv() [1, 'hello', None] >>> b.send_bytes('thank you') >>> a.recv_bytes() 'thank you' >>> import array >>> arr1 = array.array('i', range(5)) >>> arr2 = array.array('i', [0] * 10) >>> a.send_bytes(arr1) >>> count = b.recv_bytes_into(arr2) >>> assert count == len(arr1) * arr1.itemsize >>> arr2 array('i', [0, 1, 2, 3, 4, 0, 0, 0, 0, 0]) .. warning:: The :meth:`Connection.recv` method automatically unpickles the data it receives, which can be a security risk unless you can trust the process which sent the message. Therefore, unless the connection object was produced using :func:`Pipe` you should only use the :meth:`~Connection.recv` and :meth:`~Connection.send` methods after performing some sort of authentication. See :ref:`multiprocessing-auth-keys`. .. warning:: If a process is killed while it is trying to read or write to a pipe then the data in the pipe is likely to become corrupted, because it may become impossible to be sure where the message boundaries lie. Synchronization primitives ~~~~~~~~~~~~~~~~~~~~~~~~~~ Generally synchronization primitives are not as necessary in a multiprocess program as they are in a multithreaded program. See the documentation for :mod:`threading` module. Note that one can also create synchronization primitives by using a manager object -- see :ref:`multiprocessing-managers`. .. class:: BoundedSemaphore([value]) A bounded semaphore object: a clone of :class:`threading.BoundedSemaphore`. (On Mac OS X this is indistinguishable from :class:`Semaphore` because ``sem_getvalue()`` is not implemented on that platform). .. class:: Condition([lock]) A condition variable: a clone of :class:`threading.Condition`. If *lock* is specified then it should be a :class:`Lock` or :class:`RLock` object from :mod:`multiprocessing`. .. class:: Event() A clone of :class:`threading.Event`. This method returns the state of the internal semaphore on exit, so it will always return ``True`` except if a timeout is given and the operation times out. .. versionchanged:: 2.7 Previously, the method always returned ``None``. .. class:: Lock() A non-recursive lock object: a clone of :class:`threading.Lock`. .. class:: RLock() A recursive lock object: a clone of :class:`threading.RLock`. .. class:: Semaphore([value]) A bounded semaphore object: a clone of :class:`threading.Semaphore`. .. note:: The :meth:`acquire` method of :class:`BoundedSemaphore`, :class:`Lock`, :class:`RLock` and :class:`Semaphore` has a timeout parameter not supported by the equivalents in :mod:`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. Note that on OS/X ``sem_timedwait`` is unsupported, so timeout arguments for these will be ignored. .. note:: If the SIGINT signal generated by Ctrl-C arrives while the main thread is blocked by a call to :meth:`BoundedSemaphore.acquire`, :meth:`Lock.acquire`, :meth:`RLock.acquire`, :meth:`Semaphore.acquire`, :meth:`Condition.acquire` or :meth:`Condition.wait` then the call will be immediately interrupted and :exc:`KeyboardInterrupt` will be raised. This differs from the behaviour of :mod:`threading` where SIGINT will be ignored while the equivalent blocking calls are in progress. Shared :mod:`ctypes` Objects ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It is possible to create shared objects using shared memory which can be inherited by child processes. .. function:: Value(typecode_or_type, *args[, lock]) Return a :mod:`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 :mod:`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 :class:`Lock` or :class:`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. .. function:: Array(typecode_or_type, size_or_initializer, *, lock=True) Return 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 :mod:`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 :class:`Lock` or :class:`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 :data:`ctypes.c_char` has *value* and *raw* attributes which allow one to use it to store and retrieve strings. The :mod:`multiprocessing.sharedctypes` module >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> .. module:: multiprocessing.sharedctypes :synopsis: Allocate ctypes objects from shared memory. The :mod:`multiprocessing.sharedctypes` module provides functions for allocating :mod:`ctypes` objects from shared memory which can be inherited by child processes. .. note:: Although it is possible to store a pointer in shared memory remember that this will refer to a location in the address space of a specific process. However, the pointer is quite likely to be invalid in the context of a second process and trying to dereference the pointer from the second process may cause a crash. .. function:: RawArray(typecode_or_type, size_or_initializer) Return a ctypes array allocated from shared memory. *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 :mod:`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. Note that setting and getting an element is potentially non-atomic -- use :func:`Array` instead to make sure that access is automatically synchronized using a lock. .. function:: RawValue(typecode_or_type, *args) Return a ctypes object allocated from shared memory. *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 :mod:`array` module. *\*args* is passed on to the constructor for the type. Note that setting and getting the value is potentially non-atomic -- use :func:`Value` instead to make sure that access is automatically synchronized using a lock. Note that an array of :data:`ctypes.c_char` has ``value`` and ``raw`` attributes which allow one to use it to store and retrieve strings -- see documentation for :mod:`ctypes`. .. function:: Array(typecode_or_type, size_or_initializer, *args[, lock]) The same as :func:`RawArray` except that depending on the value of *lock* a process-safe synchronization wrapper may be returned instead of a raw ctypes array. If *lock* is ``True`` (the default) then a new lock object is created to synchronize access to the value. If *lock* is a :class:`Lock` or :class:`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. .. function:: Value(typecode_or_type, *args[, lock]) The same as :func:`RawValue` except that depending on the value of *lock* a process-safe synchronization wrapper may be returned instead of a raw ctypes object. If *lock* is ``True`` (the default) then a new lock object is created to synchronize access to the value. If *lock* is a :class:`Lock` or :class:`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. .. function:: copy(obj) Return a ctypes object allocated from shared memory which is a copy of the ctypes object *obj*. .. function:: synchronized(obj[, lock]) Return a process-safe wrapper object for a ctypes object which uses *lock* to synchronize access. If *lock* is ``None`` (the default) then a :class:`multiprocessing.RLock` object is created automatically. A synchronized wrapper will have two methods in addition to those of the object it wraps: :meth:`get_obj` returns the wrapped object and :meth:`get_lock` returns the lock object used for synchronization. Note that accessing the ctypes object through the wrapper can be a lot slower than accessing the raw ctypes object. The table below compares the syntax for creating shared ctypes objects from shared memory with the normal ctypes syntax. (In the table ``MyStruct`` is some subclass of :class:`ctypes.Structure`.) ==================== ========================== =========================== ctypes sharedctypes using type sharedctypes using typecode ==================== ========================== =========================== c_double(2.4) RawValue(c_double, 2.4) RawValue('d', 2.4) MyStruct(4, 6) RawValue(MyStruct, 4, 6) (c_short * 7)() RawArray(c_short, 7) RawArray('h', 7) (c_int * 3)(9, 2, 8) RawArray(c_int, (9, 2, 8)) RawArray('i', (9, 2, 8)) ==================== ========================== =========================== Below is an example where a number of ctypes objects are modified by a child process:: from multiprocessing import Process, Lock from multiprocessing.sharedctypes import Value, Array from ctypes import Structure, c_double class Point(Structure): _fields_ = [('x', c_double), ('y', c_double)] def modify(n, x, s, A): n.value **= 2 x.value **= 2 s.value = s.value.upper() for a in A: a.x **= 2 a.y **= 2 if __name__ == '__main__': lock = Lock() n = Value('i', 7) x = Value(ctypes.c_double, 1.0/3.0, lock=False) s = Array('c', 'hello world', lock=lock) A = Array(Point, [(1.875,-6.25), (-5.75,2.0), (2.375,9.5)], lock=lock) p = Process(target=modify, args=(n, x, s, A)) p.start() p.join() print(n.value) print(x.value) print(s.value) print([(a.x, a.y) for a in A]) .. highlight:: none The results printed are :: 49 0.1111111111111111 HELLO WORLD [(3.515625, 39.0625), (33.0625, 4.0), (5.640625, 90.25)] .. highlight:: python .. _multiprocessing-managers: Managers ~~~~~~~~ Managers provide a way to create data which can be shared between different processes. A manager object controls a server process which manages *shared objects*. Other processes can access the shared objects by using proxies. .. function:: multiprocessing.Manager() Returns a started :class:`~multiprocessing.managers.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. .. module:: multiprocessing.managers :synopsis: Share data between process with shared objects. Manager processes will be shutdown as soon as they are garbage collected or their parent process exits. The manager classes are defined in the :mod:`multiprocessing.managers` module: .. class:: BaseManager([address[, authkey]]) Create a BaseManager object. Once created one should call :meth:`start` or :meth:`serve_forever` to ensure that the manager object refers to a started manager process. *address* is the address on which the manager process listens for new connections. If *address* is ``None`` then an arbitrary one is chosen. *authkey* is the authentication key which will be used to check the validity of incoming connections to the server process. If *authkey* is ``None`` then ``current_process().authkey``. Otherwise *authkey* is used and it must be a string. .. method:: start() Start a subprocess to start the manager. .. method:: serve_forever() Run the server in the current process. .. method:: from_address(address, authkey) A class method which creates a manager object referring to a pre-existing server process which is using the given address and authentication key. .. method:: get_server() Returns a :class:`Server` object which represents the actual server under the control of the Manager. The :class:`Server` object supports the :meth:`serve_forever` method: >>> from multiprocessing.managers import BaseManager >>> m = BaseManager(address=('', 50000), authkey='abc')) >>> server = m.get_server() >>> s.serve_forever() :class:`Server` additionally have an :attr:`address` attribute. .. method:: connect() Connect a local manager object to a remote manager process: >>> from multiprocessing.managers import BaseManager >>> m = BaseManager(address='127.0.0.1', authkey='abc)) >>> m.connect() .. method:: shutdown() Stop the process used by the manager. This is only available if :meth:`start` has been used to start the server process. This can be called multiple times. .. method:: register(typeid[, callable[, proxytype[, exposed[, method_to_typeid[, create_method]]]]]) A classmethod which can be used for registering a type or callable with the manager class. *typeid* is a "type identifier" which is used to identify a particular type of shared object. This must be a string. *callable* is a callable used for creating objects for this type identifier. If a manager instance will be created using the :meth:`from_address` classmethod or if the *create_method* argument is ``False`` then this can be left as ``None``. *proxytype* is a subclass of :class:`BaseProxy` which is used to create proxies for shared objects with this *typeid*. If ``None`` then a proxy class is created automatically. *exposed* is used to specify a sequence of method names which proxies for this typeid should be allowed to access using :meth:`BaseProxy._callMethod`. (If *exposed* is ``None`` then :attr:`proxytype._exposed_` is used instead if it exists.) In the case where no exposed list is specified, all "public methods" of the shared object will be accessible. (Here a "public method" means any attribute which has a :meth:`__call__` method and whose name does not begin with ``'_'``.) *method_to_typeid* is a mapping used to specify the return type of those exposed methods which should return a proxy. It maps method names to typeid strings. (If *method_to_typeid* is ``None`` then :attr:`proxytype._method_to_typeid_` is used instead if it exists.) If a method's name is not a key of this mapping or if the mapping is ``None`` then the object returned by the method will be copied by value. *create_method* determines whether a method should be created with name *typeid* which can be used to tell the server process to create a new shared object and return a proxy for it. By default it is ``True``. :class:`BaseManager` instances also have one read-only property: .. attribute:: address The address used by the manager. .. class:: SyncManager A subclass of :class:`BaseManager` which can be used for the synchronization of processes. Objects of this type are returned by :func:`multiprocessing.Manager`. It also supports creation of shared lists and dictionaries. .. method:: BoundedSemaphore([value]) Create a shared :class:`threading.BoundedSemaphore` object and return a proxy for it. .. method:: Condition([lock]) Create a shared :class:`threading.Condition` object and return a proxy for it. If *lock* is supplied then it should be a proxy for a :class:`threading.Lock` or :class:`threading.RLock` object. .. method:: Event() Create a shared :class:`threading.Event` object and return a proxy for it. .. method:: Lock() Create a shared :class:`threading.Lock` object and return a proxy for it. .. method:: Namespace() Create a shared :class:`Namespace` object and return a proxy for it. .. method:: Queue([maxsize]) Create a shared :class:`queue.Queue` object and return a proxy for it. .. method:: RLock() Create a shared :class:`threading.RLock` object and return a proxy for it. .. method:: Semaphore([value]) Create a shared :class:`threading.Semaphore` object and return a proxy for it. .. method:: Array(typecode, sequence) Create an array and return a proxy for it. .. method:: Value(typecode, value) Create an object with a writable ``value`` attribute and return a proxy for it. .. method:: dict() dict(mapping) dict(sequence) Create a shared ``dict`` object and return a proxy for it. .. method:: list() list(sequence) Create a shared ``list`` object and return a proxy for it. Namespace objects >>>>>>>>>>>>>>>>> A namespace object has no public methods, but does have writable attributes. Its representation shows the values of its attributes. However, when using a proxy for a namespace object, an attribute beginning with ``'_'`` will be an attribute of the proxy and not an attribute of the referent:: >>> manager = multiprocessing.Manager() >>> Global = manager.Namespace() >>> Global.x = 10 >>> Global.y = 'hello' >>> Global._z = 12.3 # this is an attribute of the proxy >>> print(Global) Namespace(x=10, y='hello') Customized managers >>>>>>>>>>>>>>>>>>> To create one's own manager, one creates a subclass of :class:`BaseManager` and use the :meth:`~BaseManager.register` classmethod to register new types or callables with the manager class. For example:: from multiprocessing.managers import BaseManager class MathsClass(object): def add(self, x, y): return x + y def mul(self, x, y): return x * y class MyManager(BaseManager): pass MyManager.register('Maths', MathsClass) if __name__ == '__main__': manager = MyManager() manager.start() maths = manager.Maths() print(maths.add(4, 3)) # prints 7 print(maths.mul(7, 8)) # prints 56 Using a remote manager >>>>>>>>>>>>>>>>>>>>>> It is possible to run a manager server on one machine and have clients use it from other machines (assuming that the firewalls involved allow it). Running the following commands creates a server for a single shared queue which remote clients can access:: >>> from multiprocessing.managers import BaseManager >>> import queue >>> queue = queue.Queue() >>> class QueueManager(BaseManager): pass ... >>> QueueManager.register('get_queue', callable=lambda:queue) >>> m = QueueManager(address=('', 50000), authkey='abracadabra') >>> s = m.get_server() >>> s.serveForever() One client can access the server as follows:: >>> from multiprocessing.managers import BaseManager >>> class QueueManager(BaseManager): pass ... >>> QueueManager.register('get_queue') >>> m = QueueManager(address=('foo.bar.org', 50000), authkey='abracadabra') >>> m.connect() >>> queue = m.get_queue() >>> queue.put('hello') Another client can also use it:: >>> from multiprocessing.managers import BaseManager >>> class QueueManager(BaseManager): pass ... >>> QueueManager.register('getQueue') >>> m = QueueManager.from_address(address=('foo.bar.org', 50000), authkey='abracadabra') >>> queue = m.getQueue() >>> queue.get() 'hello' Local processes can also access that queue, using the code from above on the client to access it remotely:: >>> from multiprocessing import Process, Queue >>> from multiprocessing.managers import BaseManager >>> class Worker(Process): ... def __init__(self, q): ... self.q = q ... super(Worker, self).__init__() ... def run(self): ... self.q.put('local hello') ... >>> queue = Queue() >>> w = Worker(queue) >>> w.start() >>> class QueueManager(BaseManager): pass ... >>> QueueManager.register('get_queue', callable=lambda: queue) >>> m = QueueManager(address=('', 50000), authkey='abracadabra') >>> s = m.get_server() >>> s.serve_forever() Proxy Objects ~~~~~~~~~~~~~ A proxy is an object which *refers* to a shared object which lives (presumably) in a different process. The shared object is said to be the *referent* of the proxy. Multiple proxy objects may have the same referent. A proxy object has methods which invoke corresponding methods of its referent (although not every method of the referent will necessarily be available through the proxy). A proxy can usually be used in most of the same ways that its referent can:: >>> from multiprocessing import Manager >>> manager = Manager() >>> l = manager.list([i*i for i in range(10)]) >>> print(l) [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] >>> print(repr(l)) >>> l[4] 16 >>> l[2:5] [4, 9, 16] Notice that applying :func:`str` to a proxy will return the representation of the referent, whereas applying :func:`repr` will return the representation of the proxy. An important feature of proxy objects is that they are picklable so they can be passed between processes. Note, however, that if a proxy is sent to the corresponding manager's process then unpickling it will produce the referent itself. This means, for example, that one shared object can contain a second:: >>> a = manager.list() >>> b = manager.list() >>> a.append(b) # referent of a now contains referent of b >>> print(a, b) [[]] [] >>> b.append('hello') >>> print(a, b) [['hello']] ['hello'] .. note:: The proxy types in :mod:`multiprocessing` do nothing to support comparisons by value. So, for instance, :: manager.list([1,2,3]) == [1,2,3] will return ``False``. One should just use a copy of the referent instead when making comparisons. .. class:: BaseProxy Proxy objects are instances of subclasses of :class:`BaseProxy`. .. method:: _callmethod(methodname[, args[, kwds]]) Call and return the result of a method of the proxy's referent. If ``proxy`` is a proxy whose referent is ``obj`` then the expression :: proxy._callmethod(methodname, args, kwds) will evaluate the expression :: getattr(obj, methodname)(*args, **kwds) in the manager's process. The returned value will be a copy of the result of the call or a proxy to a new shared object -- see documentation for the *method_to_typeid* argument of :meth:`BaseManager.register`. If an exception is raised by the call, then then is re-raised by :meth:`_callmethod`. If some other exception is raised in the manager's process then this is converted into a :exc:`RemoteError` exception and is raised by :meth:`_callmethod`. Note in particular that an exception will be raised if *methodname* has not been *exposed* An example of the usage of :meth:`_callmethod`:: >>> l = manager.list(range(10)) >>> l._callmethod('__len__') 10 >>> l._callmethod('__getslice__', (2, 7)) # equiv to `l[2:7]` [2, 3, 4, 5, 6] >>> l._callmethod('__getitem__', (20,)) # equiv to `l[20]` Traceback (most recent call last): ... IndexError: list index out of range .. method:: _getvalue() Return a copy of the referent. If the referent is unpicklable then this will raise an exception. .. method:: __repr__ Return a representation of the proxy object. .. method:: __str__ Return the representation of the referent. Cleanup >>>>>>> A proxy object uses a weakref callback so that when it gets garbage collected it deregisters itself from the manager which owns its referent. A shared object gets deleted from the manager process when there are no longer any proxies referring to it. Process Pools ~~~~~~~~~~~~~ .. module:: multiprocessing.pool :synopsis: Create pools of processes. One can create a pool of processes which will carry out tasks submitted to it with the :class:`Pool` class. .. class:: multiprocessing.Pool([processes[, initializer[, initargs]]]) 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 :func:`cpu_count` is used. If *initializer* is not ``None`` then each worker process will call ``initializer(*initargs)`` when it starts. .. method:: apply(func[, args[, kwds]]) Call *func* with arguments *args* and keyword arguments *kwds*. It blocks till the result is ready. Given this blocks - :meth:`apply_async` is better suited for performing work in parallel. Additionally, the passed in function is only executed in one of the workers of the pool. .. method:: apply_async(func[, args[, kwds[, callback]]]) A variant of the :meth:`apply` method which returns a result object. If *callback* is specified then it should be a callable which accepts a single argument. When the result becomes ready *callback* is applied to it (unless the call failed). *callback* should complete immediately since otherwise the thread which handles the results will get blocked. .. method:: map(func, iterable[, chunksize]) A parallel equivalent of the :func:`map` builtin function (it supports only one *iterable* argument though). It blocks till the result is ready. This method chops the iterable into a number of chunks which it submits to the process pool as separate tasks. The (approximate) size of these chunks can be specified by setting *chunksize* to a positive integer. .. method:: map_async(func, iterable[, chunksize[, callback]]) A variant of the :meth:`map` method which returns a result object. If *callback* is specified then it should be a callable which accepts a single argument. When the result becomes ready *callback* is applied to it (unless the call failed). *callback* should complete immediately since otherwise the thread which handles the results will get blocked. .. method:: imap(func, iterable[, chunksize]) A lazier version of :meth:`map`. The *chunksize* argument is the same as the one used by the :meth:`.map` method. For very long iterables using a large value for *chunksize* can make make the job complete **much** faster than using the default value of ``1``. Also if *chunksize* is ``1`` then the :meth:`next` method of the iterator returned by the :meth:`imap` method has an optional *timeout* parameter: ``next(timeout)`` will raise :exc:`multiprocessing.TimeoutError` if the result cannot be returned within *timeout* seconds. .. method:: imap_unordered(func, iterable[, chunksize]) The same as :meth:`imap` except that the ordering of the results from the returned iterator should be considered arbitrary. (Only when there is only one worker process is the order guaranteed to be "correct".) .. method:: close() Prevents any more tasks from being submitted to the pool. Once all the tasks have been completed the worker processes will exit. .. method:: terminate() Stops the worker processes immediately without completing outstanding work. When the pool object is garbage collected :meth:`terminate` will be called immediately. .. method:: join() Wait for the worker processes to exit. One must call :meth:`close` or :meth:`terminate` before using :meth:`join`. .. class:: AsyncResult The class of the result returned by :meth:`Pool.apply_async` and :meth:`Pool.map_async`. .. method:: get([timeout]) Return the result when it arrives. If *timeout* is not ``None`` and the result does not arrive within *timeout* seconds then :exc:`multiprocessing.TimeoutError` is raised. If the remote call raised an exception then that exception will be reraised by :meth:`get`. .. method:: wait([timeout]) Wait until the result is available or until *timeout* seconds pass. .. method:: ready() Return whether the call has completed. .. method:: successful() Return whether the call completed without raising an exception. Will raise :exc:`AssertionError` if the result is not ready. The following example demonstrates the use of a pool:: from multiprocessing import Pool def f(x): return x*x if __name__ == '__main__': pool = Pool(processes=4) # start 4 worker processes result = pool.apply_async(f, (10,)) # evaluate "f(10)" asynchronously print(result.get(timeout=1)) # prints "100" unless your computer is *very* slow print(pool.map(f, range(10))) # prints "[0, 1, 4,..., 81]" it = pool.imap(f, range(10)) print(next(it)) # prints "0" print(next(it)) # prints "1" print(it.next(timeout=1)) # prints "4" unless your computer is *very* slow import time result = pool.apply_async(time.sleep, (10,)) print(result.get(timeout=1)) # raises TimeoutError .. _multiprocessing-listeners-clients: Listeners and Clients ~~~~~~~~~~~~~~~~~~~~~ .. module:: multiprocessing.connection :synopsis: API for dealing with sockets. Usually message passing between processes is done using queues or by using :class:`Connection` objects returned by :func:`Pipe`. However, the :mod:`multiprocessing.connection` module allows some extra flexibility. It basically gives a high level message oriented API for dealing with sockets or Windows named pipes, and also has support for *digest authentication* using the :mod:`hmac` module. .. function:: deliver_challenge(connection, authkey) Send a randomly generated message to the other end of the connection and wait for a reply. If the reply matches the digest of the message using *authkey* as the key then a welcome message is sent to the other end of the connection. Otherwise :exc:`AuthenticationError` is raised. .. function:: answerChallenge(connection, authkey) Receive a message, calculate the digest of the message using *authkey* as the key, and then send the digest back. If a welcome message is not received, then :exc:`AuthenticationError` is raised. .. function:: Client(address[, family[, authenticate[, authkey]]]) Attempt to set up a connection to the listener which is using address *address*, returning a :class:`~multiprocessing.Connection`. The type of the connection is determined by *family* argument, but this can generally be omitted since it can usually be inferred from the format of *address*. (See :ref:`multiprocessing-address-formats`) If *authentication* is ``True`` or *authkey* is a string then digest authentication is used. The key used for authentication will be either *authkey* or ``current_process().authkey)`` if *authkey* is ``None``. If authentication fails then :exc:`AuthenticationError` is raised. See :ref:`multiprocessing-auth-keys`. .. class:: Listener([address[, family[, backlog[, authenticate[, authkey]]]]]) A wrapper for a bound socket or Windows named pipe which is 'listening' for connections. *address* is the address to be used by the bound socket or named pipe of the listener object. .. note:: If an address of '0.0.0.0' is used, the address will not be a connectable end point on Windows. If you require a connectable end-point, you should use '127.0.0.1'. *family* is the type of socket (or named pipe) to use. This can be one of the strings ``'AF_INET'`` (for a TCP socket), ``'AF_UNIX'`` (for a Unix domain socket) or ``'AF_PIPE'`` (for a Windows named pipe). Of these only the first is guaranteed to be available. If *family* is ``None`` then the family is inferred from the format of *address*. If *address* is also ``None`` then a default is chosen. This default is the family which is assumed to be the fastest available. See :ref:`multiprocessing-address-formats`. Note that if *family* is ``'AF_UNIX'`` and address is ``None`` then the socket will be created in a private temporary directory created using :func:`tempfile.mkstemp`. If the listener object uses a socket then *backlog* (1 by default) is passed to the :meth:`listen` method of the socket once it has been bound. If *authenticate* is ``True`` (``False`` by default) or *authkey* is not ``None`` then digest authentication is used. If *authkey* is a string then it will be used as the authentication key; otherwise it must be *None*. If *authkey* is ``None`` and *authenticate* is ``True`` then ``current_process().authkey`` is used as the authentication key. If *authkey* is ``None`` and *authentication* is ``False`` then no authentication is done. If authentication fails then :exc:`AuthenticationError` is raised. See :ref:`multiprocessing-auth-keys`. .. method:: accept() Accept a connection on the bound socket or named pipe of the listener object and return a :class:`Connection` object. If authentication is attempted and fails, then :exc:`AuthenticationError` is raised. .. method:: close() Close the bound socket or named pipe of the listener object. This is called automatically when the listener is garbage collected. However it is advisable to call it explicitly. Listener objects have the following read-only properties: .. attribute:: address The address which is being used by the Listener object. .. attribute:: last_accepted The address from which the last accepted connection came. If this is unavailable then it is ``None``. The module defines two exceptions: .. exception:: AuthenticationError Exception raised when there is an authentication error. **Examples** The following server code creates a listener which uses ``'secret password'`` as an authentication key. It then waits for a connection and sends some data to the client:: from multiprocessing.connection import Listener from array import array address = ('localhost', 6000) # family is deduced to be 'AF_INET' listener = Listener(address, authkey='secret password') conn = listener.accept() print('connection accepted from', listener.last_accepted) conn.send([2.25, None, 'junk', float]) conn.send_bytes('hello') conn.send_bytes(array('i', [42, 1729])) conn.close() listener.close() The following code connects to the server and receives some data from the server:: from multiprocessing.connection import Client from array import array address = ('localhost', 6000) conn = Client(address, authkey='secret password') print(conn.recv()) # => [2.25, None, 'junk', float] print(conn.recv_bytes()) # => 'hello' arr = array('i', [0, 0, 0, 0, 0]) print(conn.recv_bytes_into(arr)) # => 8 print(arr) # => array('i', [42, 1729, 0, 0, 0]) conn.close() .. _multiprocessing-address-formats: Address Formats >>>>>>>>>>>>>>> * An ``'AF_INET'`` address is a tuple of the form ``(hostname, port)`` where *hostname* is a string and *port* is an integer. * An ``'AF_UNIX'`` address is a string representing a filename on the filesystem. * An ``'AF_PIPE'`` address is a string of the form :samp:`r'\\\\.\\pipe\\{PipeName}'`. To use :func:`Client` to connect to a named pipe on a remote computer called *ServerName* one should use an address of the form :samp:`r'\\\\{ServerName}\\pipe\\{PipeName}'` instead. Note that any string beginning with two backslashes is assumed by default to be an ``'AF_PIPE'`` address rather than an ``'AF_UNIX'`` address. .. _multiprocessing-auth-keys: Authentication keys ~~~~~~~~~~~~~~~~~~~ When one uses :meth:`Connection.recv`, the data received is automatically unpickled. Unfortunately unpickling data from an untrusted source is a security risk. Therefore :class:`Listener` and :func:`Client` use the :mod:`hmac` module to provide digest authentication. An authentication key is a string which can be thought of as a password: once a connection is established both ends will demand proof that the other knows the authentication key. (Demonstrating that both ends are using the same key does **not** involve sending the key over the connection.) If authentication is requested but do authentication key is specified then the return value of ``current_process().authkey`` is used (see :class:`~multiprocessing.Process`). This value will automatically inherited by any :class:`~multiprocessing.Process` object that the current process creates. This means that (by default) all processes of a multi-process program will share a single authentication key which can be used when setting up connections between themselves. Suitable authentication keys can also be generated by using :func:`os.urandom`. Logging ~~~~~~~ Some support for logging is available. Note, however, that the :mod:`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. .. currentmodule:: multiprocessing .. function:: get_logger() Returns the logger used by :mod:`multiprocessing`. If necessary, a new one will be created. When first created the logger has level :data:`logging.NOTSET` and no default handler. Messages sent to this logger will not by default propagate to the root logger. Note that on Windows child processes will only inherit the level of the parent process's logger -- any other customization of the logger will not be inherited. .. currentmodule:: multiprocessing .. function:: log_to_stderr() This function performs a call to :func:`get_logger` but in addition to returning the logger created by get_logger, it adds a handler which sends output to :data:`sys.stderr` using format ``'[%(levelname)s/%(processName)s] %(message)s'``. Below is an example session with logging turned on:: >>> import multiprocessing, logging >>> logger = multiprocessing.log_to_stderr() >>> logger.setLevel(logging.INFO) >>> logger.warning('doomed') [WARNING/MainProcess] doomed >>> m = multiprocessing.Manager() [INFO/SyncManager-1] child process calling self.run() [INFO/SyncManager-1] created temp directory /.../pymp-Wh47O_ [INFO/SyncManager-1] manager serving at '/.../listener-lWsERs' >>> del m [INFO/MainProcess] sending shutdown message to manager [INFO/SyncManager-1] manager exiting with exitcode 0 In addition to having these two logging functions, the multiprocessing also exposes two additional logging level attributes. These are :const:`SUBWARNING` and :const:`SUBDEBUG`. The table below illustrates where theses fit in the normal level hierarchy. +----------------+----------------+ | Level | Numeric value | +================+================+ | ``SUBWARNING`` | 25 | +----------------+----------------+ | ``SUBDEBUG`` | 5 | +----------------+----------------+ For a full table of logging levels, see the :mod:`logging` module. These additional logging levels are used primarily for certain debug messages within the multiprocessing module. Below is the same example as above, except with :const:`SUBDEBUG` enabled:: >>> import multiprocessing, logging >>> logger = multiprocessing.log_to_stderr() >>> logger.setLevel(multiprocessing.SUBDEBUG) >>> logger.warning('doomed') [WARNING/MainProcess] doomed >>> m = multiprocessing.Manager() [INFO/SyncManager-1] child process calling self.run() [INFO/SyncManager-1] created temp directory /.../pymp-djGBXN [INFO/SyncManager-1] manager serving at '/.../pymp-djGBXN/listener-knBYGe' >>> del m [SUBDEBUG/MainProcess] finalizer calling ... [INFO/MainProcess] sending shutdown message to manager [DEBUG/SyncManager-1] manager received shutdown message [SUBDEBUG/SyncManager-1] calling ... [SUBDEBUG/SyncManager-1] calling [SUBDEBUG/SyncManager-1] finalizer calling ... [INFO/SyncManager-1] manager exiting with exitcode 0 The :mod:`multiprocessing.dummy` module ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .. module:: multiprocessing.dummy :synopsis: Dumb wrapper around threading. :mod:`multiprocessing.dummy` replicates the API of :mod:`multiprocessing` but is no more than a wrapper around the :mod:`threading` module. .. _multiprocessing-programming: Programming guidelines ---------------------- There are certain guidelines and idioms which should be adhered to when using :mod:`multiprocessing`. All platforms ~~~~~~~~~~~~~ Avoid shared state As far as possible one should try to avoid shifting large amounts of data between processes. It is probably best to stick to using queues or pipes for communication between processes rather than using the lower level synchronization primitives from the :mod:`threading` module. Picklability Ensure that the arguments to the methods of proxies are picklable. Thread safety of proxies Do not use a proxy object from more than one thread unless you protect it with a lock. (There is never a problem with different processes using the *same* proxy.) Joining zombie processes On Unix when a process finishes but has not been joined it becomes a zombie. There should never be very many because each time a new process starts (or :func:`active_children` is called) all completed processes which have not yet been joined will be joined. Also calling a finished process's :meth:`Process.is_alive` will join the process. Even so it is probably good practice to explicitly join all the processes that you start. Better to inherit than pickle/unpickle On Windows many types from :mod:`multiprocessing` need to be picklable so that child processes can use them. However, one should generally avoid sending shared objects to other processes using pipes or queues. Instead you should arrange the program so that a process which need access to a shared resource created elsewhere can inherit it from an ancestor process. Avoid terminating processes Using the :meth:`Process.terminate` method to stop a process is liable to cause any shared resources (such as locks, semaphores, pipes and queues) currently being used by the process to become broken or unavailable to other processes. Therefore it is probably best to only consider using :meth:`Process.terminate` on processes which never use any shared resources. Joining processes that use queues Bear in mind that a process that has put items in a queue will wait before terminating until all the buffered items are fed by the "feeder" thread to the underlying pipe. (The child process can call the :meth:`Queue.cancel_join_thread` method of the queue to avoid this behaviour.) This means that whenever you use a queue you need to make sure that all items which have been put on the queue will eventually be removed before the process is joined. Otherwise you cannot be sure that processes which have put items on the queue will terminate. Remember also that non-daemonic processes will be automatically be joined. An example which will deadlock is the following:: from multiprocessing import Process, Queue def f(q): q.put('X' * 1000000) if __name__ == '__main__': queue = Queue() p = Process(target=f, args=(queue,)) p.start() p.join() # this deadlocks obj = queue.get() A fix here would be to swap the last two lines round (or simply remove the ``p.join()`` line). Explicitly pass resources to child processes On Unix a child process can make use of a shared resource created in a parent process using a global resource. However, it is better to pass the object as an argument to the constructor for the child process. Apart from making the code (potentially) compatible with Windows this also ensures that as long as the child process is still alive the object will not be garbage collected in the parent process. This might be important if some resource is freed when the object is garbage collected in the parent process. So for instance :: from multiprocessing import Process, Lock def f(): ... do something using "lock" ... if __name__ == '__main__': lock = Lock() for i in range(10): Process(target=f).start() should be rewritten as :: from multiprocessing import Process, Lock def f(l): ... do something using "l" ... if __name__ == '__main__': lock = Lock() for i in range(10): Process(target=f, args=(lock,)).start() Windows ~~~~~~~ Since Windows lacks :func:`os.fork` it has a few extra restrictions: More picklability Ensure that all arguments to :meth:`Process.__init__` are picklable. This means, in particular, that bound or unbound methods cannot be used directly as the ``target`` argument on Windows --- just define a function and use that instead. Also, if you subclass :class:`Process` then make sure that instances will be picklable when the :meth:`Process.start` method is called. Global variables Bear in mind that if code run in a child process tries to access a global variable, then the value it sees (if any) may not be the same as the value in the parent process at the time that :meth:`Process.start` was called. However, global variables which are just module level constants cause no problems. Safe importing of main module Make sure that the main module can be safely imported by a new Python interpreter without causing unintended side effects (such a starting a new process). For example, under Windows running the following module would fail with a :exc:`RuntimeError`:: from multiprocessing import Process def foo(): print('hello') p = Process(target=foo) p.start() Instead one should protect the "entry point" of the program by using ``if __name__ == '__main__':`` as follows:: from multiprocessing import Process, freeze_support def foo(): print('hello') if __name__ == '__main__': freeze_support() p = Process(target=foo) p.start() (The ``freeze_support()`` line can be omitted if the program will be run normally instead of frozen.) This allows the newly spawned Python interpreter to safely import the module and then run the module's ``foo()`` function. Similar restrictions apply if a pool or manager is created in the main module. .. _multiprocessing-examples: Examples -------- Demonstration of how to create and use customized managers and proxies: .. literalinclude:: ../includes/mp_newtype.py Using :class:`Pool`: .. literalinclude:: ../includes/mp_pool.py Synchronization types like locks, conditions and queues: .. literalinclude:: ../includes/mp_synchronize.py An showing how to use queues to feed tasks to a collection of worker process and collect the results: .. literalinclude:: ../includes/mp_workers.py An example of how a pool of worker processes can each run a :class:`SimpleHTTPServer.HttpServer` instance while sharing a single listening socket. .. literalinclude:: ../includes/mp_webserver.py Some simple benchmarks comparing :mod:`multiprocessing` with :mod:`threading`: .. literalinclude:: ../includes/mp_benchmarks.py An example/demo of how to use the :class:`managers.SyncManager`, :class:`Process` and others to build a system which can distribute processes and work via a distributed queue to a "cluster" of machines on a network, accessible via SSH. You will need to have private key authentication for all hosts configured for this to work. .. literalinclude:: ../includes/mp_distributing.py