[Python-checkins] r55816 - peps/trunk/pep-0000.txt peps/trunk/pep-0344.txt peps/trunk/pep-0367.txt peps/trunk/pep-3134.txt peps/trunk/pep-3135.txt
guido.van.rossum
python-checkins at python.org
Fri Jun 8 00:17:58 CEST 2007
Author: guido.van.rossum
Date: Fri Jun 8 00:17:56 2007
New Revision: 55816
Added:
peps/trunk/pep-3134.txt (contents, props changed)
peps/trunk/pep-3135.txt (contents, props changed)
Modified:
peps/trunk/pep-0000.txt
peps/trunk/pep-0344.txt
peps/trunk/pep-0367.txt
Log:
Renumber 344 -> 3134 and 367 -> 3135.
Modified: peps/trunk/pep-0000.txt
==============================================================================
--- peps/trunk/pep-0000.txt (original)
+++ peps/trunk/pep-0000.txt Fri Jun 8 00:17:56 2007
@@ -91,18 +91,18 @@
S 286 Enhanced Argument Tuples von Loewis
S 335 Overloadable Boolean Operators Ewing
S 337 Logging Usage in the Standard Library Dubner
- S 344 Exception Chaining and Embedded Tracebacks Yee
S 345 Metadata for Python Software Packages 1.2 Jones
S 362 Function Signature Object Cannon, Seo
S 364 Transitioning to the Py3K Standard Library Warsaw
S 365 Adding the pkg_resources module Eby
S 366 Main module explicit relative imports Coghlan
- S 367 New Super Spealman
S 3101 Advanced String Formatting Talin
S 3108 Standard Library Reorganization Cannon
S 3116 New I/O Stutzbach, Verdone, GvR
S 3118 Revising the buffer protocol Oliphant, Banks
S 3124 Overloading, Generic Functions, Interfaces Eby
+ S 3134 Exception Chaining and Embedded Tracebacks Yee
+ S 3135 New Super Spealman, Delaney
S 3141 A Type Hierarchy for Numbers Yasskin
Finished PEPs (done, implemented in Subversion)
@@ -251,6 +251,7 @@
SW 334 Simple Coroutines via SuspendIteration Evans
SR 336 Make None Callable McClelland
SR 340 Anonymous Block Statements GvR
+ SR 344 Exception Chaining and Embedded Tracebacks Yee
SW 346 User Defined ("with") Statements Coghlan
SR 348 Exception Reorganization for Python 3.0 Cannon
SD 349 Allow str() to return unicode strings Schemenauer
@@ -260,6 +261,7 @@
SR 355 Path - Object oriented filesystem paths Lindqvist
SW 359 The "make" Statement Bethard
SR 363 Syntax For Dynamic Attribute Access North
+ SR 367 New Super Spealman, Delaney
SR 666 Reject Foolish Indentation Creighton
SR 754 IEEE 754 Floating Point Special Values Warnes
SR 3103 A Switch/Case Statement GvR
@@ -440,7 +442,7 @@
SF 341 Unifying try-except and try-finally Brandl
SF 342 Coroutines via Enhanced Generators GvR, Eby
SF 343 Anonymous Block Redux and Generator Enhancements GvR
- S 344 Exception Chaining and Embedded Tracebacks Yee
+ SR 344 Exception Chaining and Embedded Tracebacks Yee
S 345 Metadata for Python Software Packages 1.2 Jones
SW 346 User Defined ("with") Statements Coghlan
PA 347 Migrating the Python CVS to Subversion von Löwis
@@ -463,7 +465,7 @@
S 364 Transitioning to the Py3K Standard Library Warsaw
S 365 Adding the pkg_resources module Eby
S 366 Main module explicit relative imports Coghlan
- S 367 New Super Spealman
+ SR 367 New Super Spealman, Delaney
SR 666 Reject Foolish Indentation Creighton
SR 754 IEEE 754 Floating Point Special Values Warnes
P 3000 Python 3000 GvR
@@ -504,6 +506,8 @@
SA 3131 Supporting Non-ASCII Identifiers von Löwis
SF 3132 Extended Iterable Unpacking Brandl
SR 3133 Introducing Roles Winter
+ S 3134 Exception Chaining and Embedded Tracebacks Yee
+ S 3135 New Super Spealman, Delaney
S 3141 A Type Hierarchy for Numbers Yasskin
@@ -544,6 +548,7 @@
Cole, Dave djc at object-craft.com.au
Craig, Christopher python-pep at ccraig.org
Creighton, Laura lac at strakt.com
+ Delaney, Tim timothy.c.delaney at gmail.com
Dörwald, Walter
Drake, Fred fdrake at acm.org
Dubner, Michael P. dubnerm at mindless.com
Modified: peps/trunk/pep-0344.txt
==============================================================================
--- peps/trunk/pep-0344.txt (original)
+++ peps/trunk/pep-0344.txt Fri Jun 8 00:17:56 2007
@@ -3,13 +3,19 @@
Version: $Revision$
Last-Modified: $Date$
Author: Ka-Ping Yee
-Status: Active
+Status: Replaced
Type: Standards Track
Content-Type: text/plain
Created: 12-May-2005
Python-Version: 2.5
+Numbering Note
+
+ This PEP has been renumbered to PEP 3134. The text below is the
+ last version submitted under the old number.
+
+
Abstract
This PEP proposes three standard attributes on exception instances:
Modified: peps/trunk/pep-0367.txt
==============================================================================
--- peps/trunk/pep-0367.txt (original)
+++ peps/trunk/pep-0367.txt Fri Jun 8 00:17:56 2007
@@ -4,13 +4,19 @@
Last-Modified: $Date$
Author: Calvin Spealman <ironfroggy at gmail.com>,
Tim Delaney <timothy.c.delaney at gmail.com>
-Status: Draft
+Status: Replaced
Type: Standards Track
Content-Type: text/x-rst
Created: 28-Apr-2007
Python-Version: 2.6
Post-History: 28-Apr-2007, 29-Apr-2007 (1), 29-Apr-2007 (2), 14-May-2007
+Numbering Note
+==============
+
+This PEP has been renumbered to PEP 3135. The text below is the last
+version submitted under the old number.
+
Abstract
========
Added: peps/trunk/pep-3134.txt
==============================================================================
--- (empty file)
+++ peps/trunk/pep-3134.txt Fri Jun 8 00:17:56 2007
@@ -0,0 +1,544 @@
+PEP: 3134
+Title: Exception Chaining and Embedded Tracebacks
+Version: $Revision$
+Last-Modified: $Date$
+Author: Ka-Ping Yee
+Status: Active
+Type: Standards Track
+Content-Type: text/plain
+Created: 12-May-2005
+Python-Version: 3.0
+
+
+Numbering Note
+
+ This PEP started its life as PEP 344. Since it is now targeted
+ for Python 3000, it has been moved into the 3xxx space.
+
+
+Abstract
+
+ This PEP proposes three standard attributes on exception instances:
+ the '__context__' attribute for implicitly chained exceptions, the
+ '__cause__' attribute for explicitly chained exceptions, and the
+ '__traceback__' attribute for the traceback. A new "raise ... from"
+ statement sets the '__cause__' attribute.
+
+
+Motivation
+
+ During the handling of one exception (exception A), it is possible
+ that another exception (exception B) may occur. In today's Python
+ (version 2.4), if this happens, exception B is propagated outward
+ and exception A is lost. In order to debug the problem, it is
+ useful to know about both exceptions. The '__context__' attribute
+ retains this information automatically.
+
+ Sometimes it can be useful for an exception handler to intentionally
+ re-raise an exception, either to provide extra information or to
+ translate an exception to another type. The '__cause__' attribute
+ provides an explicit way to record the direct cause of an exception.
+
+ In today's Python implementation, exceptions are composed of three
+ parts: the type, the value, and the traceback. The 'sys' module,
+ exposes the current exception in three parallel variables, exc_type,
+ exc_value, and exc_traceback, the sys.exc_info() function returns a
+ tuple of these three parts, and the 'raise' statement has a
+ three-argument form accepting these three parts. Manipulating
+ exceptions often requires passing these three things in parallel,
+ which can be tedious and error-prone. Additionally, the 'except'
+ statement can only provide access to the value, not the traceback.
+ Adding the '__traceback__' attribute to exception values makes all
+ the exception information accessible from a single place.
+
+
+History
+
+ Raymond Hettinger [1] raised the issue of masked exceptions on
+ Python-Dev in January 2003 and proposed a PyErr_FormatAppend()
+ function that C modules could use to augment the currently active
+ exception with more information. Brett Cannon [2] brought up
+ chained exceptions again in June 2003, prompting a long discussion.
+
+ Greg Ewing [3] identified the case of an exception occuring in a
+ 'finally' block during unwinding triggered by an original exception,
+ as distinct from the case of an exception occuring in an 'except'
+ block that is handling the original exception.
+
+ Greg Ewing [4] and Guido van Rossum [5], and probably others, have
+ previously mentioned adding a traceback attribute to Exception
+ instances. This is noted in PEP 3000.
+
+ This PEP was motivated by yet another recent Python-Dev reposting
+ of the same ideas [6] [7].
+
+
+Rationale
+
+ The Python-Dev discussions revealed interest in exception chaining
+ for two quite different purposes. To handle the unexpected raising
+ of a secondary exception, the exception must be retained implicitly.
+ To support intentional translation of an exception, there must be a
+ way to chain exceptions explicitly. This PEP addresses both.
+
+ Several attribute names for chained exceptions have been suggested
+ on Python-Dev [2], including 'cause', 'antecedent', 'reason',
+ 'original', 'chain', 'chainedexc', 'exc_chain', 'excprev',
+ 'previous', and 'precursor'. For an explicitly chained exception,
+ this PEP suggests '__cause__' because of its specific meaning. For
+ an implicitly chained exception, this PEP proposes the name
+ '__context__' because the intended meaning is more specific than
+ temporal precedence but less specific than causation: an exception
+ occurs in the context of handling another exception.
+
+ This PEP suggests names with leading and trailing double-underscores
+ for these three attributes because they are set by the Python VM.
+ Only in very special cases should they be set by normal assignment.
+
+ This PEP handles exceptions that occur during 'except' blocks and
+ 'finally' blocks in the same way. Reading the traceback makes it
+ clear where the exceptions occurred, so additional mechanisms for
+ distinguishing the two cases would only add unnecessary complexity.
+
+ This PEP proposes that the outermost exception object (the one
+ exposed for matching by 'except' clauses) be the most recently
+ raised exception for compatibility with current behaviour.
+
+ This PEP proposes that tracebacks display the outermost exception
+ last, because this would be consistent with the chronological order
+ of tracebacks (from oldest to most recent frame) and because the
+ actual thrown exception is easier to find on the last line.
+
+ To keep things simpler, the C API calls for setting an exception
+ will not automatically set the exception's '__context__'. Guido
+ van Rossum has has expressed concerns with making such changes [8].
+
+ As for other languages, Java and Ruby both discard the original
+ exception when another exception occurs in a 'catch'/'rescue' or
+ 'finally'/'ensure' clause. Perl 5 lacks built-in structured
+ exception handling. For Perl 6, RFC 88 [9] proposes an exception
+ mechanism that implicitly retains chained exceptions in an array
+ named @@. In that RFC, the most recently raised exception is
+ exposed for matching, as in this PEP; also, arbitrary expressions
+ (possibly involving @@) can be evaluated for exception matching.
+
+ Exceptions in C# contain a read-only 'InnerException' property that
+ may point to another exception. Its documentation [10] says that
+ "When an exception X is thrown as a direct result of a previous
+ exception Y, the InnerException property of X should contain a
+ reference to Y." This property is not set by the VM automatically;
+ rather, all exception constructors take an optional 'innerException'
+ argument to set it explicitly. The '__cause__' attribute fulfills
+ the same purpose as InnerException, but this PEP proposes a new form
+ of 'raise' rather than extending the constructors of all exceptions.
+ C# also provides a GetBaseException method that jumps directly to
+ the end of the InnerException chain; this PEP proposes no analog.
+
+ The reason all three of these attributes are presented together in
+ one proposal is that the '__traceback__' attribute provides
+ convenient access to the traceback on chained exceptions.
+
+
+Implicit Exception Chaining
+
+ Here is an example to illustrate the '__context__' attribute.
+
+ def compute(a, b):
+ try:
+ a/b
+ except Exception, exc:
+ log(exc)
+
+ def log(exc):
+ file = open('logfile.txt') # oops, forgot the 'w'
+ print >>file, exc
+ file.close()
+
+ Calling compute(0, 0) causes a ZeroDivisionError. The compute()
+ function catches this exception and calls log(exc), but the log()
+ function also raises an exception when it tries to write to a
+ file that wasn't opened for writing.
+
+ In today's Python, the caller of compute() gets thrown an IOError.
+ The ZeroDivisionError is lost. With the proposed change, the
+ instance of IOError has an additional '__context__' attribute that
+ retains the ZeroDivisionError.
+
+ The following more elaborate example demonstrates the handling of a
+ mixture of 'finally' and 'except' clauses:
+
+ def main(filename):
+ file = open(filename) # oops, forgot the 'w'
+ try:
+ try:
+ compute()
+ except Exception, exc:
+ log(file, exc)
+ finally:
+ file.clos() # oops, misspelled 'close'
+
+ def compute():
+ 1/0
+
+ def log(file, exc):
+ try:
+ print >>file, exc # oops, file is not writable
+ except:
+ display(exc)
+
+ def display(exc):
+ print ex # oops, misspelled 'exc'
+
+ Calling main() with the name of an existing file will trigger four
+ exceptions. The ultimate result will be an AttributeError due to
+ the misspelling of 'clos', whose __context__ points to a NameError
+ due to the misspelling of 'ex', whose __context__ points to an
+ IOError due to the file being read-only, whose __context__ points to
+ a ZeroDivisionError, whose __context__ attribute is None.
+
+ The proposed semantics are as follows:
+
+ 1. Each thread has an exception context initially set to None.
+
+ 2. Whenever an exception is raised, if the exception instance does
+ not already have a '__context__' attribute, the interpreter sets
+ it equal to the thread's exception context.
+
+ 3. Immediately after an exception is raised, the thread's exception
+ context is set to the exception.
+
+ 4. Whenever the interpreter exits an 'except' block by reaching the
+ end or executing a 'return', 'yield', 'continue', or 'break'
+ statement, the thread's exception context is set to None.
+
+
+Explicit Exception Chaining
+
+ The '__cause__' attribute on exception objects is always initialized
+ to None. It is set by a new form of the 'raise' statement:
+
+ raise EXCEPTION from CAUSE
+
+ which is equivalent to:
+
+ exc = EXCEPTION
+ exc.__cause__ = CAUSE
+ raise exc
+
+ In the following example, a database provides implementations for a
+ few different kinds of storage, with file storage as one kind. The
+ database designer wants errors to propagate as DatabaseError objects
+ so that the client doesn't have to be aware of the storage-specific
+ details, but doesn't want to lose the underlying error information.
+
+ class DatabaseError(StandardError):
+ pass
+
+ class FileDatabase(Database):
+ def __init__(self, filename):
+ try:
+ self.file = open(filename)
+ except IOError, exc:
+ raise DatabaseError('failed to open') from exc
+
+ If the call to open() raises an exception, the problem will be
+ reported as a DatabaseError, with a __cause__ attribute that reveals
+ the IOError as the original cause.
+
+
+Traceback Attribute
+
+ The following example illustrates the '__traceback__' attribute.
+
+ def do_logged(file, work):
+ try:
+ work()
+ except Exception, exc:
+ write_exception(file, exc)
+ raise exc
+
+ from traceback import format_tb
+
+ def write_exception(file, exc):
+ ...
+ type = exc.__class__
+ message = str(exc)
+ lines = format_tb(exc.__traceback__)
+ file.write(... type ... message ... lines ...)
+ ...
+
+ In today's Python, the do_logged() function would have to extract
+ the traceback from sys.exc_traceback or sys.exc_info()[2] and pass
+ both the value and the traceback to write_exception(). With the
+ proposed change, write_exception() simply gets one argument and
+ obtains the exception using the '__traceback__' attribute.
+
+ The proposed semantics are as follows:
+
+ 1. Whenever an exception is caught, if the exception instance does
+ not already have a '__traceback__' attribute, the interpreter
+ sets it to the newly caught traceback.
+
+
+Enhanced Reporting
+
+ The default exception handler will be modified to report chained
+ exceptions. The chain of exceptions is traversed by following the
+ '__cause__' and '__context__' attributes, with '__cause__' taking
+ priority. In keeping with the chronological order of tracebacks,
+ the most recently raised exception is displayed last; that is, the
+ display begins with the description of the innermost exception and
+ backs up the chain to the outermost exception. The tracebacks are
+ formatted as usual, with one of the lines:
+
+ The above exception was the direct cause of the following exception:
+
+ or
+
+ During handling of the above exception, another exception occurred:
+
+ between tracebacks, depending whether they are linked by __cause__
+ or __context__ respectively. Here is a sketch of the procedure:
+
+ def print_chain(exc):
+ if exc.__cause__:
+ print_chain(exc.__cause__)
+ print '\nThe above exception was the direct cause...'
+ elif exc.__context__:
+ print_chain(exc.__context__)
+ print '\nDuring handling of the above exception, ...'
+ print_exc(exc)
+
+ In the 'traceback' module, the format_exception, print_exception,
+ print_exc, and print_last functions will be updated to accept an
+ optional 'chain' argument, True by default. When this argument is
+ True, these functions will format or display the entire chain of
+ exceptions as just described. When it is False, these functions
+ will format or display only the outermost exception.
+
+ The 'cgitb' module should also be updated to display the entire
+ chain of exceptions.
+
+
+C API
+
+ The PyErr_Set* calls for setting exceptions will not set the
+ '__context__' attribute on exceptions. PyErr_NormalizeException
+ will always set the 'traceback' attribute to its 'tb' argument and
+ the '__context__' and '__cause__' attributes to None.
+
+ A new API function, PyErr_SetContext(context), will help C
+ programmers provide chained exception information. This function
+ will first normalize the current exception so it is an instance,
+ then set its '__context__' attribute. A similar API function,
+ PyErr_SetCause(cause), will set the '__cause__' attribute.
+
+
+Compatibility
+
+ Chained exceptions expose the type of the most recent exception, so
+ they will still match the same 'except' clauses as they do now.
+
+ The proposed changes should not break any code unless it sets or
+ uses attributes named '__context__', '__cause__', or '__traceback__'
+ on exception instances. As of 2005-05-12, the Python standard
+ library contains no mention of such attributes.
+
+
+Open Issue: Extra Information
+
+ Walter Dörwald [11] expressed a desire to attach extra information
+ to an exception during its upward propagation without changing its
+ type. This could be a useful feature, but it is not addressed by
+ this PEP. It could conceivably be addressed by a separate PEP
+ establishing conventions for other informational attributes on
+ exceptions.
+
+
+Open Issue: Suppressing Context
+
+ As written, this PEP makes it impossible to suppress '__context__',
+ since setting exc.__context__ to None in an 'except' or 'finally'
+ clause will only result in it being set again when exc is raised.
+
+
+Open Issue: Limiting Exception Types
+
+ To improve encapsulation, library implementors may want to wrap all
+ implementation-level exceptions with an application-level exception.
+ One could try to wrap exceptions by writing this:
+
+ try:
+ ... implementation may raise an exception ...
+ except:
+ import sys
+ raise ApplicationError from sys.exc_value
+
+ or this:
+
+ try:
+ ... implementation may raise an exception ...
+ except Exception, exc:
+ raise ApplicationError from exc
+
+ but both are somewhat flawed. It would be nice to be able to name
+ the current exception in a catch-all 'except' clause, but that isn't
+ addressed here. Such a feature would allow something like this:
+
+ try:
+ ... implementation may raise an exception ...
+ except *, exc:
+ raise ApplicationError from exc
+
+
+Open Issue: yield
+
+ The exception context is lost when a 'yield' statement is executed;
+ resuming the frame after the 'yield' does not restore the context.
+ Addressing this problem is out of the scope of this PEP; it is not a
+ new problem, as demonstrated by the following example:
+
+ >>> def gen():
+ ... try:
+ ... 1/0
+ ... except:
+ ... yield 3
+ ... raise
+ ...
+ >>> g = gen()
+ >>> g.next()
+ 3
+ >>> g.next()
+ TypeError: exceptions must be classes, instances, or strings
+ (deprecated), not NoneType
+
+
+Open Issue: Garbage Collection
+
+ The strongest objection to this proposal has been that it creates
+ cycles between exceptions and stack frames [12]. Collection of
+ cyclic garbage (and therefore resource release) can be greatly
+ delayed.
+
+ >>> try:
+ >>> 1/0
+ >>> except Exception, err:
+ >>> pass
+
+ will introduce a cycle from err -> traceback -> stack frame -> err,
+ keeping all locals in the same scope alive until the next GC happens.
+
+ Today, these locals would go out of scope. There is lots of code
+ which assumes that "local" resources -- particularly open files -- will
+ be closed quickly. If closure has to wait for the next GC, a program
+ (which runs fine today) may run out of file handles.
+
+ Making the __traceback__ attribute a weak reference would avoid the
+ problems with cyclic garbage. Unfortunately, it would make saving
+ the Exception for later (as unittest does) more awkward, and it would
+ not allow as much cleanup of the sys module.
+
+ A possible alternate solution, suggested by Adam Olsen, would be to
+ instead turn the reference from the stack frame to the 'err' variable
+ into a weak reference when the variable goes out of scope [13].
+
+
+Possible Future Compatible Changes
+
+ These changes are consistent with the appearance of exceptions as
+ a single object rather than a triple at the interpreter level.
+
+ - If PEP 340 or PEP 343 is accepted, replace the three (type, value,
+ traceback) arguments to __exit__ with a single exception argument.
+
+ - Deprecate sys.exc_type, sys.exc_value, sys.exc_traceback, and
+ sys.exc_info() in favour of a single member, sys.exception.
+
+ - Deprecate sys.last_type, sys.last_value, and sys.last_traceback
+ in favour of a single member, sys.last_exception.
+
+ - Deprecate the three-argument form of the 'raise' statement in
+ favour of the one-argument form.
+
+ - Upgrade cgitb.html() to accept a single value as its first
+ argument as an alternative to a (type, value, traceback) tuple.
+
+
+Possible Future Incompatible Changes
+
+ These changes might be worth considering for Python 3000.
+
+ - Remove sys.exc_type, sys.exc_value, sys.exc_traceback, and
+ sys.exc_info().
+
+ - Remove sys.last_type, sys.last_value, and sys.last_traceback.
+
+ - Replace the three-argument sys.excepthook with a one-argument
+ API, and changing the 'cgitb' module to match.
+
+ - Remove the three-argument form of the 'raise' statement.
+
+ - Upgrade traceback.print_exception to accept an 'exception'
+ argument instead of the type, value, and traceback arguments.
+
+
+Acknowledgements
+
+ Brett Cannon, Greg Ewing, Guido van Rossum, Jeremy Hylton, Phillip
+ J. Eby, Raymond Hettinger, Walter Dörwald, and others.
+
+
+References
+
+ [1] Raymond Hettinger, "Idea for avoiding exception masking"
+ http://mail.python.org/pipermail/python-dev/2003-January/032492.html
+
+ [2] Brett Cannon explains chained exceptions
+ http://mail.python.org/pipermail/python-dev/2003-June/036063.html
+
+ [3] Greg Ewing points out masking caused by exceptions during finally
+ http://mail.python.org/pipermail/python-dev/2003-June/036290.html
+
+ [4] Greg Ewing suggests storing the traceback in the exception object
+ http://mail.python.org/pipermail/python-dev/2003-June/036092.html
+
+ [5] Guido van Rossum mentions exceptions having a traceback attribute
+ http://mail.python.org/pipermail/python-dev/2005-April/053060.html
+
+ [6] Ka-Ping Yee, "Tidier Exceptions"
+ http://mail.python.org/pipermail/python-dev/2005-May/053671.html
+
+ [7] Ka-Ping Yee, "Chained Exceptions"
+ http://mail.python.org/pipermail/python-dev/2005-May/053672.html
+
+ [8] Guido van Rossum discusses automatic chaining in PyErr_Set*
+ http://mail.python.org/pipermail/python-dev/2003-June/036180.html
+
+ [9] Tony Olensky, "Omnibus Structured Exception/Error Handling Mechanism"
+ http://dev.perl.org/perl6/rfc/88.html
+
+ [10] MSDN .NET Framework Library, "Exception.InnerException Property"
+ http://msdn.microsoft.com/library/en-us/cpref/html/frlrfsystemexceptionclassinnerexceptiontopic.asp
+
+ [11] Walter Dörwald suggests wrapping exceptions to add details
+ http://mail.python.org/pipermail/python-dev/2003-June/036148.html
+
+ [12] Guido van Rossum restates the objection to cyclic trash
+ http://mail.python.org/pipermail/python-3000/2007-January/005322.html
+
+ [13] Adam Olsen suggests using a weakref from stack frame to exception
+ http://mail.python.org/pipermail/python-3000/2007-January/005363.html
+
+
+Copyright
+
+ This document has been placed in the public domain.
+
+
+Local Variables:
+mode: indented-text
+indent-tabs-mode: nil
+sentence-end-double-space: t
+fill-column: 70
+coding: utf-8
+End:
Added: peps/trunk/pep-3135.txt
==============================================================================
--- (empty file)
+++ peps/trunk/pep-3135.txt Fri Jun 8 00:17:56 2007
@@ -0,0 +1,591 @@
+PEP: 3135
+Title: New Super
+Version: $Revision$
+Last-Modified: $Date$
+Author: Calvin Spealman <ironfroggy at gmail.com>,
+ Tim Delaney <timothy.c.delaney at gmail.com>
+Status: Draft
+Type: Standards Track
+Content-Type: text/x-rst
+Created: 28-Apr-2007
+Python-Version: 3.0
+Post-History: 28-Apr-2007, 29-Apr-2007 (1), 29-Apr-2007 (2), 14-May-2007
+
+Numbering Note
+==============
+
+This PEP started its life as PEP 367. Since it is now targeted
+for Python 3000, it has been moved into the 3xxx space.
+
+Abstract
+========
+
+This PEP proposes syntactic sugar for use of the ``super`` type to automatically
+construct instances of the super type binding to the class that a method was
+defined in, and the instance (or class object for classmethods) that the method
+is currently acting upon.
+
+The premise of the new super usage suggested is as follows::
+
+ super.foo(1, 2)
+
+to replace the old::
+
+ super(Foo, self).foo(1, 2)
+
+and the current ``__builtin__.super`` be aliased to ``__builtin__.__super__``
+(with ``__builtin__.super`` to be removed in Python 3.0).
+
+It is further proposed that assignment to ``super`` become a ``SyntaxError``,
+similar to the behaviour of ``None``.
+
+
+Rationale
+=========
+
+The current usage of super requires an explicit passing of both the class and
+instance it must operate from, requiring a breaking of the DRY (Don't Repeat
+Yourself) rule. This hinders any change in class name, and is often considered
+a wart by many.
+
+
+Specification
+=============
+
+Within the specification section, some special terminology will be used to
+distinguish similar and closely related concepts. "super type" will refer to
+the actual builtin type named "super". A "super instance" is simply an instance
+of the super type, which is associated with a class and possibly with an
+instance of that class.
+
+Because the new ``super`` semantics are not backwards compatible with Python
+2.5, the new semantics will require a ``__future__`` import::
+
+ from __future__ import new_super
+
+The current ``__builtin__.super`` will be aliased to ``__builtin__.__super__``.
+This will occur regardless of whether the new ``super`` semantics are active.
+It is not possible to simply rename ``__builtin__.super``, as that would affect
+modules that do not use the new ``super`` semantics. In Python 3.0 it is
+proposed that the name ``__builtin__.super`` will be removed.
+
+Replacing the old usage of super, calls to the next class in the MRO (method
+resolution order) can be made without explicitly creating a ``super``
+instance (although doing so will still be supported via ``__super__``). Every
+function will have an implicit local named ``super``. This name behaves
+identically to a normal local, including use by inner functions via a cell,
+with the following exceptions:
+
+1. Assigning to the name ``super`` will raise a ``SyntaxError`` at compile time;
+
+2. Calling a static method or normal function that accesses the name ``super``
+ will raise a ``TypeError`` at runtime.
+
+Every function that uses the name ``super``, or has an inner function that
+uses the name ``super``, will include a preamble that performs the equivalent
+of::
+
+ super = __builtin__.__super__(<class>, <instance>)
+
+where ``<class>`` is the class that the method was defined in, and
+``<instance>`` is the first parameter of the method (normally ``self`` for
+instance methods, and ``cls`` for class methods). For static methods and normal
+functions, ``<class>`` will be ``None``, resulting in a ``TypeError`` being
+raised during the preamble.
+
+Note: The relationship between ``super`` and ``__super__`` is similar to that
+between ``import`` and ``__import__``.
+
+Much of this was discussed in the thread of the python-dev list, "Fixing super
+anyone?" [1]_.
+
+
+Open Issues
+-----------
+
+
+Determining the class object to use
+'''''''''''''''''''''''''''''''''''
+
+The exact mechanism for associating the method with the defining class is not
+specified in this PEP, and should be chosen for maximum performance. For
+CPython, it is suggested that the class instance be held in a C-level variable
+on the function object which is bound to one of ``NULL`` (not part of a class),
+``Py_None`` (static method) or a class object (instance or class method).
+
+
+Should ``super`` actually become a keyword?
+'''''''''''''''''''''''''''''''''''''''''''
+
+With this proposal, ``super`` would become a keyword to the same extent that
+``None`` is a keyword. It is possible that further restricting the ``super``
+name may simplify implementation, however some are against the actual keyword-
+ization of super. The simplest solution is often the correct solution and the
+simplest solution may well not be adding additional keywords to the language
+when they are not needed. Still, it may solve other open issues.
+
+
+Closed Issues
+-------------
+
+super used with __call__ attributes
+'''''''''''''''''''''''''''''''''''
+
+It was considered that it might be a problem that instantiating super instances
+the classic way, because calling it would lookup the __call__ attribute and
+thus try to perform an automatic super lookup to the next class in the MRO.
+However, this was found to be false, because calling an object only looks up
+the __call__ method directly on the object's type. The following example shows
+this in action.
+
+::
+
+ class A(object):
+ def __call__(self):
+ return '__call__'
+ def __getattribute__(self, attr):
+ if attr == '__call__':
+ return lambda: '__getattribute__'
+ a = A()
+ assert a() == '__call__'
+ assert a.__call__() == '__getattribute__'
+
+In any case, with the renaming of ``__builtin__.super`` to
+``__builtin__.__super__`` this issue goes away entirely.
+
+
+Reference Implementation
+========================
+
+It is impossible to implement the above specification entirely in Python. This
+reference implementation has the following differences to the specification:
+
+1. New ``super`` semantics are implemented using bytecode hacking.
+
+2. Assignment to ``super`` is not a ``SyntaxError``. Also see point #4.
+
+3. Classes must either use the metaclass ``autosuper_meta`` or inherit from
+ the base class ``autosuper`` to acquire the new ``super`` semantics.
+
+4. ``super`` is not an implicit local variable. In particular, for inner
+ functions to be able to use the super instance, there must be an assignment
+ of the form ``super = super`` in the method.
+
+The reference implementation assumes that it is being run on Python 2.5+.
+
+::
+
+ #!/usr/bin/env python
+ #
+ # autosuper.py
+
+ from array import array
+ import dis
+ import new
+ import types
+ import __builtin__
+ __builtin__.__super__ = __builtin__.super
+ del __builtin__.super
+
+ # We need these for modifying bytecode
+ from opcode import opmap, HAVE_ARGUMENT, EXTENDED_ARG
+
+ LOAD_GLOBAL = opmap['LOAD_GLOBAL']
+ LOAD_NAME = opmap['LOAD_NAME']
+ LOAD_CONST = opmap['LOAD_CONST']
+ LOAD_FAST = opmap['LOAD_FAST']
+ LOAD_ATTR = opmap['LOAD_ATTR']
+ STORE_FAST = opmap['STORE_FAST']
+ LOAD_DEREF = opmap['LOAD_DEREF']
+ STORE_DEREF = opmap['STORE_DEREF']
+ CALL_FUNCTION = opmap['CALL_FUNCTION']
+ STORE_GLOBAL = opmap['STORE_GLOBAL']
+ DUP_TOP = opmap['DUP_TOP']
+ POP_TOP = opmap['POP_TOP']
+ NOP = opmap['NOP']
+ JUMP_FORWARD = opmap['JUMP_FORWARD']
+ ABSOLUTE_TARGET = dis.hasjabs
+
+ def _oparg(code, opcode_pos):
+ return code[opcode_pos+1] + (code[opcode_pos+2] << 8)
+
+ def _bind_autosuper(func, cls):
+ co = func.func_code
+ name = func.func_name
+ newcode = array('B', co.co_code)
+ codelen = len(newcode)
+ newconsts = list(co.co_consts)
+ newvarnames = list(co.co_varnames)
+
+ # Check if the global 'super' keyword is already present
+ try:
+ sn_pos = list(co.co_names).index('super')
+ except ValueError:
+ sn_pos = None
+
+ # Check if the varname 'super' keyword is already present
+ try:
+ sv_pos = newvarnames.index('super')
+ except ValueError:
+ sv_pos = None
+
+ # Check if the callvar 'super' keyword is already present
+ try:
+ sc_pos = list(co.co_cellvars).index('super')
+ except ValueError:
+ sc_pos = None
+
+ # If 'super' isn't used anywhere in the function, we don't have anything to do
+ if sn_pos is None and sv_pos is None and sc_pos is None:
+ return func
+
+ c_pos = None
+ s_pos = None
+ n_pos = None
+
+ # Check if the 'cls_name' and 'super' objects are already in the constants
+ for pos, o in enumerate(newconsts):
+ if o is cls:
+ c_pos = pos
+
+ if o is __super__:
+ s_pos = pos
+
+ if o == name:
+ n_pos = pos
+
+ # Add in any missing objects to constants and varnames
+ if c_pos is None:
+ c_pos = len(newconsts)
+ newconsts.append(cls)
+
+ if n_pos is None:
+ n_pos = len(newconsts)
+ newconsts.append(name)
+
+ if s_pos is None:
+ s_pos = len(newconsts)
+ newconsts.append(__super__)
+
+ if sv_pos is None:
+ sv_pos = len(newvarnames)
+ newvarnames.append('super')
+
+ # This goes at the start of the function. It is:
+ #
+ # super = __super__(cls, self)
+ #
+ # If 'super' is a cell variable, we store to both the
+ # local and cell variables (i.e. STORE_FAST and STORE_DEREF).
+ #
+ preamble = [
+ LOAD_CONST, s_pos & 0xFF, s_pos >> 8,
+ LOAD_CONST, c_pos & 0xFF, c_pos >> 8,
+ LOAD_FAST, 0, 0,
+ CALL_FUNCTION, 2, 0,
+ ]
+
+ if sc_pos is None:
+ # 'super' is not a cell variable - we can just use the local variable
+ preamble += [
+ STORE_FAST, sv_pos & 0xFF, sv_pos >> 8,
+ ]
+ else:
+ # If 'super' is a cell variable, we need to handle LOAD_DEREF.
+ preamble += [
+ DUP_TOP,
+ STORE_FAST, sv_pos & 0xFF, sv_pos >> 8,
+ STORE_DEREF, sc_pos & 0xFF, sc_pos >> 8,
+ ]
+
+ preamble = array('B', preamble)
+
+ # Bytecode for loading the local 'super' variable.
+ load_super = array('B', [
+ LOAD_FAST, sv_pos & 0xFF, sv_pos >> 8,
+ ])
+
+ preamble_len = len(preamble)
+ need_preamble = False
+ i = 0
+
+ while i < codelen:
+ opcode = newcode[i]
+ need_load = False
+ remove_store = False
+
+ if opcode == EXTENDED_ARG:
+ raise TypeError("Cannot use 'super' in function with EXTENDED_ARG opcode")
+
+ # If the opcode is an absolute target it needs to be adjusted
+ # to take into account the preamble.
+ elif opcode in ABSOLUTE_TARGET:
+ oparg = _oparg(newcode, i) + preamble_len
+ newcode[i+1] = oparg & 0xFF
+ newcode[i+2] = oparg >> 8
+
+ # If LOAD_GLOBAL(super) or LOAD_NAME(super) then we want to change it into
+ # LOAD_FAST(super)
+ elif (opcode == LOAD_GLOBAL or opcode == LOAD_NAME) and _oparg(newcode, i) == sn_pos:
+ need_preamble = need_load = True
+
+ # If LOAD_FAST(super) then we just need to add the preamble
+ elif opcode == LOAD_FAST and _oparg(newcode, i) == sv_pos:
+ need_preamble = need_load = True
+
+ # If LOAD_DEREF(super) then we change it into LOAD_FAST(super) because
+ # it's slightly faster.
+ elif opcode == LOAD_DEREF and _oparg(newcode, i) == sc_pos:
+ need_preamble = need_load = True
+
+ if need_load:
+ newcode[i:i+3] = load_super
+
+ i += 1
+
+ if opcode >= HAVE_ARGUMENT:
+ i += 2
+
+ # No changes needed - get out.
+ if not need_preamble:
+ return func
+
+ # Our preamble will have 3 things on the stack
+ co_stacksize = max(3, co.co_stacksize)
+
+ # Conceptually, our preamble is on the `def` line.
+ co_lnotab = array('B', co.co_lnotab)
+
+ if co_lnotab:
+ co_lnotab[0] += preamble_len
+
+ co_lnotab = co_lnotab.tostring()
+
+ # Our code consists of the preamble and the modified code.
+ codestr = (preamble + newcode).tostring()
+
+ codeobj = new.code(co.co_argcount, len(newvarnames), co_stacksize,
+ co.co_flags, codestr, tuple(newconsts), co.co_names,
+ tuple(newvarnames), co.co_filename, co.co_name,
+ co.co_firstlineno, co_lnotab, co.co_freevars,
+ co.co_cellvars)
+
+ func.func_code = codeobj
+ func.func_class = cls
+ return func
+
+ class autosuper_meta(type):
+ def __init__(cls, name, bases, clsdict):
+ UnboundMethodType = types.UnboundMethodType
+
+ for v in vars(cls):
+ o = getattr(cls, v)
+ if isinstance(o, UnboundMethodType):
+ _bind_autosuper(o.im_func, cls)
+
+ class autosuper(object):
+ __metaclass__ = autosuper_meta
+
+ if __name__ == '__main__':
+ class A(autosuper):
+ def f(self):
+ return 'A'
+
+ class B(A):
+ def f(self):
+ return 'B' + super.f()
+
+ class C(A):
+ def f(self):
+ def inner():
+ return 'C' + super.f()
+
+ # Needed to put 'super' into a cell
+ super = super
+ return inner()
+
+ class D(B, C):
+ def f(self, arg=None):
+ var = None
+ return 'D' + super.f()
+
+ assert D().f() == 'DBCA'
+
+Disassembly of B.f and C.f reveals the different preambles used when ``super``
+is simply a local variable compared to when it is used by an inner function.
+
+::
+
+ >>> dis.dis(B.f)
+
+ 214 0 LOAD_CONST 4 (<type 'super'>)
+ 3 LOAD_CONST 2 (<class '__main__.B'>)
+ 6 LOAD_FAST 0 (self)
+ 9 CALL_FUNCTION 2
+ 12 STORE_FAST 1 (super)
+
+ 215 15 LOAD_CONST 1 ('B')
+ 18 LOAD_FAST 1 (super)
+ 21 LOAD_ATTR 1 (f)
+ 24 CALL_FUNCTION 0
+ 27 BINARY_ADD
+ 28 RETURN_VALUE
+
+::
+
+ >>> dis.dis(C.f)
+
+ 218 0 LOAD_CONST 4 (<type 'super'>)
+ 3 LOAD_CONST 2 (<class '__main__.C'>)
+ 6 LOAD_FAST 0 (self)
+ 9 CALL_FUNCTION 2
+ 12 DUP_TOP
+ 13 STORE_FAST 1 (super)
+ 16 STORE_DEREF 0 (super)
+
+ 219 19 LOAD_CLOSURE 0 (super)
+ 22 LOAD_CONST 1 (<code object inner at 00C160A0, file "autosuper.py", line 219>)
+ 25 MAKE_CLOSURE 0
+ 28 STORE_FAST 2 (inner)
+
+ 223 31 LOAD_FAST 1 (super)
+ 34 STORE_DEREF 0 (super)
+
+ 224 37 LOAD_FAST 2 (inner)
+ 40 CALL_FUNCTION 0
+ 43 RETURN_VALUE
+
+Note that in the final implementation, the preamble would not be part of the
+bytecode of the method, but would occur immediately following unpacking of
+parameters.
+
+
+Alternative Proposals
+=====================
+
+No Changes
+----------
+
+Although its always attractive to just keep things how they are, people have
+sought a change in the usage of super calling for some time, and for good
+reason, all mentioned previously.
+
+- Decoupling from the class name (which might not even be bound to the
+ right class anymore!)
+- Simpler looking, cleaner super calls would be better
+
+Dynamic attribute on super type
+-------------------------------
+
+The proposal adds a dynamic attribute lookup to the super type, which will
+automatically determine the proper class and instance parameters. Each super
+attribute lookup identifies these parameters and performs the super lookup on
+the instance, as the current super implementation does with the explicit
+invokation of a super instance upon a class and instance.
+
+This proposal relies on sys._getframe(), which is not appropriate for anything
+except a prototype implementation.
+
+
+super(__this_class__, self)
+---------------------------
+
+This is nearly an anti-proposal, as it basically relies on the acceptance of
+the __this_class__ PEP, which proposes a special name that would always be
+bound to the class within which it is used. If that is accepted, __this_class__
+could simply be used instead of the class' name explicitly, solving the name
+binding issues [2]_.
+
+self.__super__.foo(\*args)
+--------------------------
+
+The __super__ attribute is mentioned in this PEP in several places, and could
+be a candidate for the complete solution, actually using it explicitly instead
+of any super usage directly. However, double-underscore names are usually an
+internal detail, and attempted to be kept out of everyday code.
+
+super(self, \*args) or __super__(self, \*args)
+----------------------------------------------
+
+This solution only solves the problem of the type indication, does not handle
+differently named super methods, and is explicit about the name of the
+instance. It is less flexable without being able to enacted on other method
+names, in cases where that is needed. One use case this fails is where a base-
+class has a factory classmethod and a subclass has two factory classmethods,
+both of which needing to properly make super calls to the one in the base-
+class.
+
+super.foo(self, \*args)
+-----------------------
+
+This variation actually eliminates the problems with locating the proper
+instance, and if any of the alternatives were pushed into the spotlight, I
+would want it to be this one.
+
+super or super()
+----------------
+
+This proposal leaves no room for different names, signatures, or application
+to other classes, or instances. A way to allow some similar use alongside the
+normal proposal would be favorable, encouraging good design of multiple
+inheritence trees and compatible methods.
+
+super(\*p, \*\*kw)
+------------------
+
+There has been the proposal that directly calling ``super(*p, **kw)`` would
+be equivalent to calling the method on the ``super`` object with the same name
+as the method currently being executed i.e. the following two methods would be
+equivalent:
+
+::
+
+ def f(self, *p, **kw):
+ super.f(*p, **kw)
+
+::
+
+ def f(self, *p, **kw):
+ super(*p, **kw)
+
+There is strong sentiment for and against this, but implementation and style
+concerns are obvious. Guido has suggested that this should be excluded from
+this PEP on the principle of KISS (Keep It Simple Stupid).
+
+
+
+History
+=======
+29-Apr-2007 - Changed title from "Super As A Keyword" to "New Super"
+ - Updated much of the language and added a terminology section
+ for clarification in confusing places.
+ - Added reference implementation and history sections.
+
+06-May-2007 - Updated by Tim Delaney to reflect discussions on the python-3000
+ and python-dev mailing lists.
+
+References
+==========
+
+.. [1] Fixing super anyone?
+ (http://mail.python.org/pipermail/python-3000/2007-April/006667.html)
+
+.. [2] PEP 3130: Access to Module/Class/Function Currently Being Defined (this)
+ (http://mail.python.org/pipermail/python-ideas/2007-April/000542.html)
+
+
+Copyright
+=========
+
+This document has been placed in the public domain.
+
+
+
+..
+ Local Variables:
+ mode: indented-text
+ indent-tabs-mode: nil
+ sentence-end-double-space: t
+ fill-column: 70
+ coding: utf-8
+ End:
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