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PEP: 319
Title: Python Synchronize/Asynchronize Block
Version: d3493191e8c8
Last-Modified:  2009-01-18 09:47:54 +0000 (Sun, 18 Jan 2009)
Author: Michel Pelletier <michel at users.sourceforge.net>
Status: Rejected
Type: Standards Track
Created: 24-Feb-2003
Python-Version: 2.4?
Post-History: 

Abstract

    This PEP proposes adding two new keywords to Python, `synchronize'
    and 'asynchronize'.  

Pronouncement

    This PEP is rejected in favor of PEP 343.

The `synchronize' Keyword

    The concept of code synchronization in Python is too low-level.
    To synchronize code a programmer must be aware of the details of
    the following pseudo-code pattern:

        initialize_lock()

        ...

        acquire_lock()
        try:
            change_shared_data()
        finally:
            release_lock()

    This synchronized block pattern is not the only pattern (more
    discussed below) but it is very common.  This PEP proposes
    replacing the above code with the following equivalent:

        synchronize:
            change_shared_data()

    The advantages of this scheme are simpler syntax and less room for
    user error.  Currently users are required to write code about
    acquiring and releasing thread locks in 'try/finally' blocks;
    errors in this code can cause notoriously difficult concurrent
    thread locking issues.


The `asynchronize' Keyword

    While executing a `synchronize' block of code a programmer may
    want to "drop back" to running asynchronously momentarily to run
    blocking input/output routines or something else that might take a
    indeterminate amount of time and does not require synchronization.
    This code usually follows the pattern:

        initialize_lock()

        ...

        acquire_lock()
        try:    
            change_shared_data()
            release_lock()             # become async
            do_blocking_io()
            acquire_lock()             # sync again
            change_shared_data2()

        finally:
            release_lock()

    The asynchronous section of the code is not very obvious visually,
    so it is marked up with comments.  Using the proposed
    'asynchronize' keyword this code becomes much cleaner, easier to
    understand, and less prone to error:

        synchronize:
            change_shared_data()

            asynchronize:
               do_blocking_io()

            change_shared_data2()

    Encountering an `asynchronize' keyword inside a non-synchronized
    block can raise either an error or issue a warning (as all code
    blocks are implicitly asynchronous anyway).  It is important to
    note that the above example is *not* the same as:

        synchronize:
            change_shared_data()

        do_blocking_io()

        synchronize:
            change_shared_data2()

    Because both synchronized blocks of code may be running inside the
    same iteration of a loop, Consider:

        while in_main_loop():
            synchronize:
                change_shared_data()

                asynchronize:
                   do_blocking_io()

                change_shared_data2()

    Many threads may be looping through this code.  Without the
    'asynchronize' keyword one thread cannot stay in the loop and
    release the lock at the same time while blocking IO is going on.
    This pattern of releasing locks inside a main loop to do blocking
    IO is used extensively inside the CPython interpreter itself.


Synchronization Targets

    As proposed the `synchronize' and `asynchronize' keywords
    synchronize a block of code.  However programmers may want to
    specify a target object that threads synchronize on.  Any object
    can be a synchronization target.

    Consider a two-way queue object: two different objects are used by
    the same `synchronize' code block to synchronize both queues
    separately in the 'get' method:

        class TwoWayQueue:
            def __init__(self):
                self.front = []
                self.rear = []

            def putFront(self, item):
                self.put(item, self.front)

            def getFront(self):
                item = self.get(self.front)
                return item

            def putRear(self, item):
                self.put(item, self.rear)

            def getRear(self):
                item = self.get(self.rear)
                return item

            def put(self, item, queue):
                synchronize queue:
                    queue.append(item)

            def get(self, queue):
                synchronize queue:
                    item = queue[0]
                    del queue[0]
                    return item

    Here is the equivalent code in Python as it is now without a
    `synchronize' keyword:

        import thread

        class LockableQueue:

            def __init__(self):
                self.queue = []
                self.lock = thread.allocate_lock()

        class TwoWayQueue:
            def __init__(self):
                self.front = LockableQueue()
                self.rear = LockableQueue()

            def putFront(self, item):
                self.put(item, self.front)

            def getFront(self):
                item = self.get(self.front)
                return item

            def putRear(self, item):
                self.put(item, self.rear)

            def getRear(self):
                item = self.get(self.rear)
                return item

            def put(self, item, queue):
                queue.lock.acquire()
                try:
                    queue.append(item)
                finally:
                    queue.lock.release()

            def get(self, queue):
                queue.lock.acquire()
                try:
                    item = queue[0]
                    del queue[0]
                    return item
                finally:
                    queue.lock.release()

    The last example had to define an extra class to associate a lock
    with the queue where the first example the `synchronize' keyword
    does this association internally and transparently.


Other Patterns that Synchronize

    There are some situations where the `synchronize' and
    `asynchronize' keywords cannot entirely replace the use of lock
    methods like `acquire' and `release'.  Some examples are if the
    programmer wants to provide arguments for `acquire' or if a lock
    is acquired in one code block but released in another, as shown
    below.

    Here is a class from Zope modified to use both the `synchronize'
    and `asynchronize' keywords and also uses a pool of explicit locks
    that are acquired and released in different code blocks and thus
    don't use `synchronize':

        import thread
        from ZServerPublisher import ZServerPublisher

        class ZRendevous:

            def __init__(self, n=1):
                pool=[]
                self._lists=pool, [], []

                synchronize:
                    while n > 0:
                        l=thread.allocate_lock()
                        l.acquire()
                        pool.append(l)
                        thread.start_new_thread(ZServerPublisher,
                                                (self.accept,))
                        n=n-1

            def accept(self):
                synchronize:
                    pool, requests, ready = self._lists
                    while not requests:
                        l=pool[-1]
                        del pool[-1]
                        ready.append(l)

                        asynchronize:
                            l.acquire()

                        pool.append(l)

                    r=requests[0]
                    del requests[0]
                    return r

            def handle(self, name, request, response):
                synchronize:
                    pool, requests, ready = self._lists
                    requests.append((name, request, response))
                    if ready:
                        l=ready[-1]
                        del ready[-1]
                        l.release()

    Here is the original class as found in the
    'Zope/ZServer/PubCore/ZRendevous.py' module.  The "convenience" of
    the '_a' and '_r' shortcut names obscure the code:

        import thread
        from ZServerPublisher import ZServerPublisher

        class ZRendevous:

            def __init__(self, n=1):
                sync=thread.allocate_lock()
                self._a=sync.acquire
                self._r=sync.release
                pool=[]
                self._lists=pool, [], []
                self._a()
                try:
                    while n > 0:
                        l=thread.allocate_lock()
                        l.acquire()
                        pool.append(l)
                        thread.start_new_thread(ZServerPublisher,
                                                (self.accept,))
                        n=n-1
                finally: self._r()

            def accept(self):
                self._a()
                try:
                    pool, requests, ready = self._lists
                    while not requests:
                        l=pool[-1]
                        del pool[-1]
                        ready.append(l)
                        self._r()
                        l.acquire()
                        self._a()
                        pool.append(l)

                    r=requests[0]
                    del requests[0]
                    return r
                finally: self._r()

            def handle(self, name, request, response):
                self._a()
                try:
                    pool, requests, ready = self._lists
                    requests.append((name, request, response))
                    if ready:
                        l=ready[-1]
                        del ready[-1]
                        l.release()
                finally: self._r()

    In particular the asynchronize section of the `accept' method is
    not very obvious.  To beginner programmers, `synchronize' and
    `asynchronize' remove many of the problems encountered when
    juggling multiple `acquire' and `release' methods on different
    locks in different `try/finally' blocks.


Formal Syntax

    Python syntax is defined in a modified BNF grammar notation
    described in the Python Language Reference [1].  This section
    describes the proposed synchronization syntax using this grammar:

        synchronize_stmt: 'synchronize' [test] ':' suite
        asynchronize_stmt: 'asynchronize' [test] ':' suite
        compound_stmt: ... | synchronized_stmt | asynchronize_stmt
        
    (The '...' indicates other compound statements elided).


Proposed Implementation

    The author of this PEP has not explored an implementation yet.
    There are several implementation issues that must be resolved.
    The main implementation issue is what exactly gets locked and
    unlocked during a synchronized block.

    During an unqualified synchronized block (the use of the
    `synchronize' keyword without an target argument) a lock could be
    created and associated with the synchronized code block object.
    Any threads that are to execute the block must first acquire the
    code block lock.

    When an `asynchronize' keyword is encountered in a `synchronize'
    block the code block lock is unlocked before the inner block is
    executed and re-locked when the inner block terminates.

    When a synchronized block target is specified the object is
    associated with a lock.  How this is implemented cleanly is
    probably the highest risk of this proposal.  Java Virtual Machines
    typically associate a special hidden lock object with target
    object and use it to synchronized the block around the target
    only.


Backward Compatibility

    Backward compatibility is solved with the new `from __future__'
    Python syntax [2], and the new warning framework [3] to evolve the
    Python language into phasing out any conflicting names that use
    the new keywords `synchronize' and `asynchronize'.  To use the
    syntax now, a developer could use the statement:

        from __future__ import threadsync  # or whatever

    In addition, any code that uses the keyword `synchronize' or
    `asynchronize' as an identifier will be issued a warning from
    Python.  After the appropriate period of time, the syntax would
    become standard, the above import statement would do nothing, and
    any identifiers named `synchronize' or `asynchronize' would raise
    an exception.


PEP 310 Reliable Acquisition/Release Pairs

    PEP 310 [4] proposes the 'with' keyword that can serve the same
    function as 'synchronize' (but no facility for 'asynchronize').
    The pattern:

        initialize_lock()

        with the_lock:
            change_shared_data()

    is equivalent to the proposed:

        synchronize the_lock:
            change_shared_data()

    PEP 310 must synchronize on an exsiting lock, while this PEP
    proposes that unqualified 'synchronize' statements synchronize on
    a global, internal, transparent lock in addition to qualifiled
    'synchronize' statements.  The 'with' statement also requires lock
    initialization, while the 'synchronize' statment can synchronize
    on any target object *including* locks.

    While limited in this fashion, the 'with' statment is more
    abstract and serves more purposes than synchronization.  For
    example, transactions could be used with the 'with' keyword:

        initialize_transaction()

        with my_transaction:
            do_in_transaction()

        # when the block terminates, the transaction is committed.

    The 'synchronize' and 'asynchronize' keywords cannot serve this or
    any other general acquire/release pattern other than thread
    synchronization.


How Java Does It

    Java defines a 'synchronized' keyword (note the grammatical tense
    different between the Java keyword and this PEP's 'synchronize')
    which must be qualified on any object.  The syntax is:

        synchronized (Expression) Block 

    Expression must yeild a valid object (null raises an error and
    exceptions during 'Expression' terminate the 'synchronized' block
    for the same reason) upon which 'Block' is synchronized.


How Jython Does It

    Jython uses a 'synchronize' class with the static method
    'make_synchronized' that accepts one callable argument and returns
    a newly created, synchronized, callable "wrapper" around the
    argument.


Summary of Proposed Changes to Python

    Adding new `synchronize' and `asynchronize' keywords to the
    language.


Risks

    This PEP proposes adding two keywords to the Python language. This
    may break code.

    There is no implementation to test.

    It's not the most important problem facing Python programmers
    today (although it is a fairly notorious one).

    The equivalent Java keyword is the past participle 'synchronized'.
    This PEP proposes the present tense, 'synchronize' as being more
    in spirit with Python (there being less distinction between
    compile-time and run-time in Python than Java).


Dissenting Opinion

    This PEP has not been discussed on python-dev.
        

References

    [1] The Python Language Reference
        http://docs.python.org/reference/

    [2] PEP 236, Back to the __future__, Peters
        http://www.python.org/dev/peps/pep-0236/

    [3] PEP 230, Warning Framework, van Rossum
        http://www.python.org/dev/peps/pep-0230/

    [4] PEP 310, Reliable Acquisition/Release Pairs, Hudson, Moore
        http://www.python.org/dev/peps/pep-0310/


Copyright

    This document has been placed in the public domain.