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Renumber 344 -> 3134 and 367 -> 3135.

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Guido van Rossum 2007-06-07 22:17:56 +00:00
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13
pep-0000.txt
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@ -91,18 +91,18 @@ Index by Category
S 286 Enhanced Argument Tuples von Loewis S 286 Enhanced Argument Tuples von Loewis
S 335 Overloadable Boolean Operators Ewing S 335 Overloadable Boolean Operators Ewing
S 337 Logging Usage in the Standard Library Dubner 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 345 Metadata for Python Software Packages 1.2 Jones
S 362 Function Signature Object Cannon, Seo S 362 Function Signature Object Cannon, Seo
S 364 Transitioning to the Py3K Standard Library Warsaw S 364 Transitioning to the Py3K Standard Library Warsaw
S 365 Adding the pkg_resources module Eby S 365 Adding the pkg_resources module Eby
S 366 Main module explicit relative imports Coghlan S 366 Main module explicit relative imports Coghlan
S 367 New Super Spealman
S 3101 Advanced String Formatting Talin S 3101 Advanced String Formatting Talin
S 3108 Standard Library Reorganization Cannon S 3108 Standard Library Reorganization Cannon
S 3116 New I/O Stutzbach, Verdone, GvR S 3116 New I/O Stutzbach, Verdone, GvR
S 3118 Revising the buffer protocol Oliphant, Banks S 3118 Revising the buffer protocol Oliphant, Banks
S 3124 Overloading, Generic Functions, Interfaces Eby 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 S 3141 A Type Hierarchy for Numbers Yasskin
Finished PEPs (done, implemented in Subversion) Finished PEPs (done, implemented in Subversion)
@ -251,6 +251,7 @@ Index by Category
SW 334 Simple Coroutines via SuspendIteration Evans SW 334 Simple Coroutines via SuspendIteration Evans
SR 336 Make None Callable McClelland SR 336 Make None Callable McClelland
SR 340 Anonymous Block Statements GvR SR 340 Anonymous Block Statements GvR
SR 344 Exception Chaining and Embedded Tracebacks Yee
SW 346 User Defined ("with") Statements Coghlan SW 346 User Defined ("with") Statements Coghlan
SR 348 Exception Reorganization for Python 3.0 Cannon SR 348 Exception Reorganization for Python 3.0 Cannon
SD 349 Allow str() to return unicode strings Schemenauer SD 349 Allow str() to return unicode strings Schemenauer
@ -260,6 +261,7 @@ Index by Category
SR 355 Path - Object oriented filesystem paths Lindqvist SR 355 Path - Object oriented filesystem paths Lindqvist
SW 359 The "make" Statement Bethard SW 359 The "make" Statement Bethard
SR 363 Syntax For Dynamic Attribute Access North SR 363 Syntax For Dynamic Attribute Access North
SR 367 New Super Spealman, Delaney
SR 666 Reject Foolish Indentation Creighton SR 666 Reject Foolish Indentation Creighton
SR 754 IEEE 754 Floating Point Special Values Warnes SR 754 IEEE 754 Floating Point Special Values Warnes
SR 3103 A Switch/Case Statement GvR SR 3103 A Switch/Case Statement GvR
@ -440,7 +442,7 @@ Numerical Index
SF 341 Unifying try-except and try-finally Brandl SF 341 Unifying try-except and try-finally Brandl
SF 342 Coroutines via Enhanced Generators GvR, Eby SF 342 Coroutines via Enhanced Generators GvR, Eby
SF 343 Anonymous Block Redux and Generator Enhancements GvR 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 S 345 Metadata for Python Software Packages 1.2 Jones
SW 346 User Defined ("with") Statements Coghlan SW 346 User Defined ("with") Statements Coghlan
PA 347 Migrating the Python CVS to Subversion von Löwis PA 347 Migrating the Python CVS to Subversion von Löwis
@ -463,7 +465,7 @@ Numerical Index
S 364 Transitioning to the Py3K Standard Library Warsaw S 364 Transitioning to the Py3K Standard Library Warsaw
S 365 Adding the pkg_resources module Eby S 365 Adding the pkg_resources module Eby
S 366 Main module explicit relative imports Coghlan 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 666 Reject Foolish Indentation Creighton
SR 754 IEEE 754 Floating Point Special Values Warnes SR 754 IEEE 754 Floating Point Special Values Warnes
P 3000 Python 3000 GvR P 3000 Python 3000 GvR
@ -504,6 +506,8 @@ Numerical Index
SA 3131 Supporting Non-ASCII Identifiers von Löwis SA 3131 Supporting Non-ASCII Identifiers von Löwis
SF 3132 Extended Iterable Unpacking Brandl SF 3132 Extended Iterable Unpacking Brandl
SR 3133 Introducing Roles Winter 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 S 3141 A Type Hierarchy for Numbers Yasskin
@ -544,6 +548,7 @@ Owners
Cole, Dave djc@object-craft.com.au Cole, Dave djc@object-craft.com.au
Craig, Christopher python-pep@ccraig.org Craig, Christopher python-pep@ccraig.org
Creighton, Laura lac@strakt.com Creighton, Laura lac@strakt.com
Delaney, Tim timothy.c.delaney@gmail.com
Dörwald, Walter Dörwald, Walter
Drake, Fred fdrake@acm.org Drake, Fred fdrake@acm.org
Dubner, Michael P. dubnerm@mindless.com Dubner, Michael P. dubnerm@mindless.com

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@ -3,13 +3,19 @@ Title: Exception Chaining and Embedded Tracebacks
Version: $Revision$ Version: $Revision$
Last-Modified: $Date$ Last-Modified: $Date$
Author: Ka-Ping Yee Author: Ka-Ping Yee
Status: Active Status: Replaced
Type: Standards Track Type: Standards Track
Content-Type: text/plain Content-Type: text/plain
Created: 12-May-2005 Created: 12-May-2005
Python-Version: 2.5 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 Abstract
This PEP proposes three standard attributes on exception instances: This PEP proposes three standard attributes on exception instances:

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@ -4,13 +4,19 @@ Version: $Revision$
Last-Modified: $Date$ Last-Modified: $Date$
Author: Calvin Spealman <ironfroggy@gmail.com>, Author: Calvin Spealman <ironfroggy@gmail.com>,
Tim Delaney <timothy.c.delaney@gmail.com> Tim Delaney <timothy.c.delaney@gmail.com>
Status: Draft Status: Replaced
Type: Standards Track Type: Standards Track
Content-Type: text/x-rst Content-Type: text/x-rst
Created: 28-Apr-2007 Created: 28-Apr-2007
Python-Version: 2.6 Python-Version: 2.6
Post-History: 28-Apr-2007, 29-Apr-2007 (1), 29-Apr-2007 (2), 14-May-2007 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 Abstract
======== ========

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@ -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
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sentence-end-double-space: t
fill-column: 70
coding: utf-8
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PEP: 3135
Title: New Super
Version: $Revision$
Last-Modified: $Date$
Author: Calvin Spealman <ironfroggy@gmail.com>,
Tim Delaney <timothy.c.delaney@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.
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