reSTify PEP 319 (#350)
This commit is contained in:
Huang Huang
Brett Cannon
parent
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commit
86a462a7b4
270
pep-0319.txt
270
pep-0319.txt
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@ -5,25 +5,27 @@ Last-Modified: $Date$
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Author: Michel Pelletier <michel@users.sourceforge.net>
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Status: Rejected
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 24-Feb-2003
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Python-Version: 2.4?
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Post-History:
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Abstract
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========
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This PEP proposes adding two new keywords to Python, `synchronize'
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and 'asynchronize'.
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This PEP proposes adding two new keywords to Python, 'synchronize'
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and 'asynchronize'.
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Pronouncement
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=============
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This PEP is rejected in favor of PEP 343.
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The `synchronize' Keyword
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This PEP is rejected in favor of PEP 343.
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The 'synchronize' Keyword
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The concept of code synchronization in Python is too low-level.
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To synchronize code a programmer must be aware of the details of
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the following pseudo-code pattern:
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the following pseudo-code pattern::
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initialize_lock()
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@ -37,7 +39,7 @@ The `synchronize' Keyword
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This synchronized block pattern is not the only pattern (more
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discussed below) but it is very common. This PEP proposes
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replacing the above code with the following equivalent:
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replacing the above code with the following equivalent::
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synchronize:
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change_shared_data()
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@ -49,13 +51,12 @@ The `synchronize' Keyword
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thread locking issues.
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The `asynchronize' Keyword
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While executing a `synchronize' block of code a programmer may
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The 'asynchronize' Keyword
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While executing a 'synchronize' block of code a programmer may
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want to "drop back" to running asynchronously momentarily to run
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blocking input/output routines or something else that might take a
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indeterminate amount of time and does not require synchronization.
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This code usually follows the pattern:
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This code usually follows the pattern::
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initialize_lock()
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@ -75,7 +76,7 @@ The `asynchronize' Keyword
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The asynchronous section of the code is not very obvious visually,
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so it is marked up with comments. Using the proposed
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'asynchronize' keyword this code becomes much cleaner, easier to
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understand, and less prone to error:
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understand, and less prone to error::
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synchronize:
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change_shared_data()
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@ -85,10 +86,10 @@ The `asynchronize' Keyword
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change_shared_data2()
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Encountering an `asynchronize' keyword inside a non-synchronized
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Encountering an 'asynchronize' keyword inside a non-synchronized
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block can raise either an error or issue a warning (as all code
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blocks are implicitly asynchronous anyway). It is important to
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note that the above example is *not* the same as:
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note that the above example is **not** the same as::
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synchronize:
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change_shared_data()
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@ -99,7 +100,7 @@ The `asynchronize' Keyword
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change_shared_data2()
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Because both synchronized blocks of code may be running inside the
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same iteration of a loop, Consider:
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same iteration of a loop, Consider::
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while in_main_loop():
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synchronize:
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@ -118,15 +119,16 @@ The `asynchronize' Keyword
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Synchronization Targets
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=======================
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As proposed the `synchronize' and `asynchronize' keywords
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synchronize a block of code. However programmers may want to
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specify a target object that threads synchronize on. Any object
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can be a synchronization target.
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As proposed the 'synchronize' and 'asynchronize' keywords
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synchronize a block of code. However programmers may want to
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specify a target object that threads synchronize on. Any object
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can be a synchronization target.
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Consider a two-way queue object: two different objects are used by
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the same `synchronize' code block to synchronize both queues
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separately in the 'get' method:
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Consider a two-way queue object: two different objects are used by
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the same 'synchronize' code block to synchronize both queues
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separately in the 'get' method::
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class TwoWayQueue:
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def __init__(self):
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@ -157,8 +159,8 @@ Synchronization Targets
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del queue[0]
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return item
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Here is the equivalent code in Python as it is now without a
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`synchronize' keyword:
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Here is the equivalent code in Python as it is now without a
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'synchronize' keyword::
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import thread
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@ -203,24 +205,25 @@ Synchronization Targets
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finally:
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queue.lock.release()
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The last example had to define an extra class to associate a lock
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with the queue where the first example the `synchronize' keyword
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does this association internally and transparently.
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The last example had to define an extra class to associate a lock
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with the queue where the first example the 'synchronize' keyword
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does this association internally and transparently.
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Other Patterns that Synchronize
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===============================
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There are some situations where the `synchronize' and
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`asynchronize' keywords cannot entirely replace the use of lock
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methods like `acquire' and `release'. Some examples are if the
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programmer wants to provide arguments for `acquire' or if a lock
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is acquired in one code block but released in another, as shown
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below.
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There are some situations where the 'synchronize' and
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'asynchronize' keywords cannot entirely replace the use of lock
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methods like ``acquire`` and ``release``. Some examples are if the
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programmer wants to provide arguments for ``acquire`` or if a lock
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is acquired in one code block but released in another, as shown
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below.
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Here is a class from Zope modified to use both the `synchronize'
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and `asynchronize' keywords and also uses a pool of explicit locks
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that are acquired and released in different code blocks and thus
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don't use `synchronize':
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Here is a class from Zope modified to use both the 'synchronize'
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and 'asynchronize' keywords and also uses a pool of explicit locks
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that are acquired and released in different code blocks and thus
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don't use 'synchronize'::
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import thread
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from ZServerPublisher import ZServerPublisher
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@ -266,9 +269,9 @@ Other Patterns that Synchronize
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del ready[-1]
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l.release()
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Here is the original class as found in the
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'Zope/ZServer/PubCore/ZRendevous.py' module. The "convenience" of
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the '_a' and '_r' shortcut names obscure the code:
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Here is the original class as found in the
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'Zope/ZServer/PubCore/ZRendevous.py' module. The "convenience" of
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the '_a' and '_r' shortcut names obscure the code::
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import thread
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from ZServerPublisher import ZServerPublisher
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@ -321,95 +324,99 @@ Other Patterns that Synchronize
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l.release()
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finally: self._r()
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In particular the asynchronize section of the `accept' method is
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not very obvious. To beginner programmers, `synchronize' and
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`asynchronize' remove many of the problems encountered when
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juggling multiple `acquire' and `release' methods on different
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locks in different `try/finally' blocks.
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In particular the asynchronize section of the ``accept`` method is
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not very obvious. To beginner programmers, 'synchronize' and
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'asynchronize' remove many of the problems encountered when
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juggling multiple ``acquire`` and ``release`` methods on different
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locks in different ``try/finally`` blocks.
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Formal Syntax
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=============
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Python syntax is defined in a modified BNF grammar notation
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described in the Python Language Reference [1]. This section
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describes the proposed synchronization syntax using this grammar:
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Python syntax is defined in a modified BNF grammar notation
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described in the Python Language Reference [1]_. This section
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describes the proposed synchronization syntax using this grammar::
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synchronize_stmt: 'synchronize' [test] ':' suite
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asynchronize_stmt: 'asynchronize' [test] ':' suite
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compound_stmt: ... | synchronized_stmt | asynchronize_stmt
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(The '...' indicates other compound statements elided).
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(The '...' indicates other compound statements elided).
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Proposed Implementation
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=======================
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The author of this PEP has not explored an implementation yet.
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There are several implementation issues that must be resolved.
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The main implementation issue is what exactly gets locked and
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unlocked during a synchronized block.
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The author of this PEP has not explored an implementation yet.
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There are several implementation issues that must be resolved.
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The main implementation issue is what exactly gets locked and
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unlocked during a synchronized block.
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During an unqualified synchronized block (the use of the
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`synchronize' keyword without a target argument) a lock could be
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created and associated with the synchronized code block object.
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Any threads that are to execute the block must first acquire the
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code block lock.
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During an unqualified synchronized block (the use of the
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'synchronize' keyword without a target argument) a lock could be
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created and associated with the synchronized code block object.
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Any threads that are to execute the block must first acquire the
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code block lock.
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When an `asynchronize' keyword is encountered in a `synchronize'
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block the code block lock is unlocked before the inner block is
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executed and re-locked when the inner block terminates.
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When an 'asynchronize' keyword is encountered in a 'synchronize'
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block the code block lock is unlocked before the inner block is
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executed and re-locked when the inner block terminates.
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When a synchronized block target is specified the object is
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associated with a lock. How this is implemented cleanly is
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probably the highest risk of this proposal. Java Virtual Machines
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typically associate a special hidden lock object with target
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object and use it to synchronized the block around the target
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only.
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When a synchronized block target is specified the object is
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associated with a lock. How this is implemented cleanly is
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probably the highest risk of this proposal. Java Virtual Machines
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typically associate a special hidden lock object with target
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object and use it to synchronized the block around the target
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only.
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Backward Compatibility
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======================
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Backward compatibility is solved with the new `from __future__'
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Python syntax [2], and the new warning framework [3] to evolve the
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Python language into phasing out any conflicting names that use
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the new keywords `synchronize' and `asynchronize'. To use the
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syntax now, a developer could use the statement:
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Backward compatibility is solved with the new ``from __future__``
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Python syntax [2]_, and the new warning framework [3]_ to evolve the
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Python language into phasing out any conflicting names that use
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the new keywords 'synchronize' and 'asynchronize'. To use the
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syntax now, a developer could use the statement::
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from __future__ import threadsync # or whatever
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In addition, any code that uses the keyword `synchronize' or
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`asynchronize' as an identifier will be issued a warning from
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Python. After the appropriate period of time, the syntax would
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become standard, the above import statement would do nothing, and
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any identifiers named `synchronize' or `asynchronize' would raise
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an exception.
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In addition, any code that uses the keyword 'synchronize' or
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'asynchronize' as an identifier will be issued a warning from
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Python. After the appropriate period of time, the syntax would
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become standard, the above import statement would do nothing, and
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any identifiers named 'synchronize' or 'asynchronize' would raise
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an exception.
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PEP 310 Reliable Acquisition/Release Pairs
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==========================================
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PEP 310 [4] proposes the 'with' keyword that can serve the same
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function as 'synchronize' (but no facility for 'asynchronize').
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The pattern:
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PEP 310 [4]_ proposes the 'with' keyword that can serve the same
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function as 'synchronize' (but no facility for 'asynchronize').
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The pattern::
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initialize_lock()
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with the_lock:
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change_shared_data()
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is equivalent to the proposed:
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is equivalent to the proposed::
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synchronize the_lock:
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change_shared_data()
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PEP 310 must synchronize on an exsiting lock, while this PEP
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proposes that unqualified 'synchronize' statements synchronize on
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a global, internal, transparent lock in addition to qualifiled
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'synchronize' statements. The 'with' statement also requires lock
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initialization, while the 'synchronize' statement can synchronize
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on any target object *including* locks.
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PEP 310 must synchronize on an exsiting lock, while this PEP
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proposes that unqualified 'synchronize' statements synchronize on
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a global, internal, transparent lock in addition to qualifiled
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'synchronize' statements. The 'with' statement also requires lock
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initialization, while the 'synchronize' statement can synchronize
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on any target object **including** locks.
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While limited in this fashion, the 'with' statement is more
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abstract and serves more purposes than synchronization. For
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example, transactions could be used with the 'with' keyword:
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While limited in this fashion, the 'with' statement is more
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abstract and serves more purposes than synchronization. For
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example, transactions could be used with the 'with' keyword::
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initialize_transaction()
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@ -418,83 +425,90 @@ PEP 310 Reliable Acquisition/Release Pairs
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# when the block terminates, the transaction is committed.
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The 'synchronize' and 'asynchronize' keywords cannot serve this or
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any other general acquire/release pattern other than thread
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synchronization.
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The 'synchronize' and 'asynchronize' keywords cannot serve this or
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any other general acquire/release pattern other than thread
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synchronization.
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How Java Does It
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================
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Java defines a 'synchronized' keyword (note the grammatical tense
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different between the Java keyword and this PEP's 'synchronize')
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which must be qualified on any object. The syntax is:
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Java defines a 'synchronized' keyword (note the grammatical tense
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different between the Java keyword and this PEP's 'synchronize')
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which must be qualified on any object. The syntax is::
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synchronized (Expression) Block
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Expression must yield a valid object (null raises an error and
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exceptions during 'Expression' terminate the 'synchronized' block
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for the same reason) upon which 'Block' is synchronized.
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Expression must yield a valid object (null raises an error and
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exceptions during 'Expression' terminate the 'synchronized' block
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for the same reason) upon which 'Block' is synchronized.
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How Jython Does It
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==================
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Jython uses a 'synchronize' class with the static method
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'make_synchronized' that accepts one callable argument and returns
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a newly created, synchronized, callable "wrapper" around the
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argument.
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Jython uses a 'synchronize' class with the static method
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'make_synchronized' that accepts one callable argument and returns
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a newly created, synchronized, callable "wrapper" around the
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argument.
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Summary of Proposed Changes to Python
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=====================================
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Adding new `synchronize' and `asynchronize' keywords to the
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language.
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Adding new 'synchronize' and 'asynchronize' keywords to the
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language.
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Risks
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=====
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This PEP proposes adding two keywords to the Python language. This
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may break code.
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This PEP proposes adding two keywords to the Python language. This
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may break code.
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There is no implementation to test.
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There is no implementation to test.
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It's not the most important problem facing Python programmers
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today (although it is a fairly notorious one).
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It's not the most important problem facing Python programmers
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today (although it is a fairly notorious one).
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The equivalent Java keyword is the past participle 'synchronized'.
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This PEP proposes the present tense, 'synchronize' as being more
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in spirit with Python (there being less distinction between
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compile-time and run-time in Python than Java).
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The equivalent Java keyword is the past participle 'synchronized'.
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This PEP proposes the present tense, 'synchronize' as being more
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in spirit with Python (there being less distinction between
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compile-time and run-time in Python than Java).
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Dissenting Opinion
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==================
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This PEP has not been discussed on python-dev.
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This PEP has not been discussed on python-dev.
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References
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==========
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[1] The Python Language Reference
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.. [1] The Python Language Reference
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http://docs.python.org/reference/
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[2] PEP 236, Back to the __future__, Peters
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.. [2] PEP 236, Back to the __future__, Peters
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http://www.python.org/dev/peps/pep-0236/
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[3] PEP 230, Warning Framework, van Rossum
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.. [3] PEP 230, Warning Framework, van Rossum
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http://www.python.org/dev/peps/pep-0230/
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[4] PEP 310, Reliable Acquisition/Release Pairs, Hudson, Moore
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.. [4] PEP 310, Reliable Acquisition/Release Pairs, Hudson, Moore
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http://www.python.org/dev/peps/pep-0310/
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Copyright
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=========
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This document has been placed in the public domain.
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This document has been placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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End:
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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End:
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