|Title:||Python Synchronize/Asynchronize Block|
|Author:||Michel Pelletier <michel at users.sourceforge.net>|
- Synchronization Targets
- Other Patterns that Synchronize
- Formal Syntax
- Proposed Implementation
- Backward Compatibility
- PEP 310 Reliable Acquisition/Release Pairs
- How Java Does It
- How Jython Does It
- Summary of Proposed Changes to Python
- Dissenting Opinion
This PEP proposes adding two new keywords to Python, 'synchronize' and 'asynchronize'.
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:
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 an 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.
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 del queue 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 del queue 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.
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 del requests 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 del requests 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.
Python syntax is defined in a modified BNF grammar notation described in the Python Language Reference . 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).
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 a 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 is solved with the new from __future__ Python syntax , and the new warning framework  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
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 existing lock, while this PEP proposes that unqualified 'synchronize' statements synchronize on a global, internal, transparent lock in addition to qualified 'synchronize' statements. The 'with' statement also requires lock initialization, while the 'synchronize' statement can synchronize on any target object including locks.
While limited in this fashion, the 'with' statement 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.
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 yield a valid object (null raises an error and exceptions during 'Expression' terminate the 'synchronized' block for the same reason) upon which 'Block' is synchronized.
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.
Adding new 'synchronize' and 'asynchronize' keywords to the language.
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).
This PEP has not been discussed on python-dev.
|||The Python Language Reference http://docs.python.org/reference/|
|||PEP 236, Back to the __future__, Peters http://www.python.org/dev/peps/pep-0236/|
|||PEP 230, Warning Framework, van Rossum http://www.python.org/dev/peps/pep-0230/|
|||PEP 310, Reliable Acquisition/Release Pairs, Hudson, Moore http://www.python.org/dev/peps/pep-0310/|
This document has been placed in the public domain.