Mailman 3 May 2005 - Python-checkins

python/dist/src/Modules posixmodule.c, 2.334, 2.335
by montanaro＠users.sourceforge.net 16 May '05

16 May '05

Update of /cvsroot/python/python/dist/src/Modules In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv27235/Modules Modified Files: posixmodule.c Log Message: Add O_SHLOCK & O_EXLOCK. Closes patch #1103951. Index: posixmodule.c =================================================================== RCS file: /cvsroot/python/python/dist/src/Modules/posixmodule.c,v retrieving revision 2.334 retrieving revision 2.335 diff -u -d -r2.334 -r2.335 --- posixmodule.c 8 Mar 2005 09:10:29 -0000 2.334 +++ posixmodule.c 16 May 2005 02:42:21 -0000 2.335 @@ -7718,6 +7718,12 @@ #ifdef O_LARGEFILE if (ins(d, "O_LARGEFILE", (long)O_LARGEFILE)) return -1; #endif +#ifdef O_SHLOCK + if (ins(d, "O_SHLOCK", (long)O_SHLOCK)) return -1; +#endif +#ifdef O_EXLOCK + if (ins(d, "O_EXLOCK", (long)O_EXLOCK)) return -1; +#endif /* MS Windows */ #ifdef O_NOINHERIT

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python/dist/src/Doc/lib libos.tex,1.153,1.154
by montanaro＠users.sourceforge.net 16 May '05

16 May '05

Update of /cvsroot/python/python/dist/src/Doc/lib In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv27235/Doc/lib Modified Files: libos.tex Log Message: Add O_SHLOCK & O_EXLOCK. Closes patch #1103951. Index: libos.tex =================================================================== RCS file: /cvsroot/python/python/dist/src/Doc/lib/libos.tex,v retrieving revision 1.153 retrieving revision 1.154 diff -u -d -r1.153 -r1.154 --- libos.tex 13 Feb 2005 22:50:03 -0000 1.153 +++ libos.tex 16 May 2005 02:42:21 -0000 1.154 @@ -594,7 +594,9 @@ The following data items are available for use in constructing the -\var{flags} parameter to the \function{open()} function. +\var{flags} parameter to the \function{open()} function. Some items will +not be available on all platforms. For descriptions of their availability +and use, consult \manpage{open}{2}. \begin{datadesc}{O_RDONLY} \dataline{O_WRONLY} @@ -614,6 +616,8 @@ \dataline{O_NDELAY} \dataline{O_NONBLOCK} \dataline{O_NOCTTY} +\dataline{O_SHLOCK} +\dataline{O_EXLOCK} More options for the \var{flag} argument to the \function{open()} function. Availability: Macintosh, \UNIX. \end{datadesc}

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python/nondist/peps pep-0343.txt,1.9,1.10
by gvanrossum＠users.sourceforge.net 16 May '05

16 May '05

Update of /cvsroot/python/python/nondist/peps In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv9870 Modified Files: pep-0343.txt Log Message: Move the __enter__ call out of the try suite. I was nuts putting it inside. Index: pep-0343.txt =================================================================== RCS file: /cvsroot/python/python/nondist/peps/pep-0343.txt,v retrieving revision 1.9 retrieving revision 1.10 diff -u -d -r1.9 -r1.10 --- pep-0343.txt 15 May 2005 00:45:42 -0000 1.9 +++ pep-0343.txt 16 May 2005 00:36:01 -0000 1.10 @@ -160,8 +160,8 @@ abc = EXPR exc = (None, None, None) try: + VAR = abc.__enter__() try: - VAR = abc.__enter__() BLOCK except: exc = sys.exc_info()

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python/nondist/peps pep-0000.txt, 1.307, 1.308 pep-0340.txt, 1.27, 1.28
by gvanrossum＠users.sourceforge.net 16 May '05

16 May '05

Update of /cvsroot/python/python/nondist/peps In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv9626 Modified Files: pep-0000.txt pep-0340.txt Log Message: Rejecting PEP 340 in favor of PEP 343. Index: pep-0000.txt =================================================================== RCS file: /cvsroot/python/python/nondist/peps/pep-0000.txt,v retrieving revision 1.307 retrieving revision 1.308 diff -u -d -r1.307 -r1.308 --- pep-0000.txt 15 May 2005 19:49:01 -0000 1.307 +++ pep-0000.txt 16 May 2005 00:34:33 -0000 1.308 @@ -117,7 +117,6 @@ S 336 Make None Callable McClelland S 337 Logging Usage in the Standard Library Dubner S 338 Executing modules inside packages with '-m' Coghlan - S 340 Anonymous Block Statements GvR S 341 Unifying try-except and try-finally Birkenfeld S 342 Enhanced Iterators GvR S 343 Anonymous Block Redux GvR @@ -212,6 +211,7 @@ SR 317 Eliminate Implicit Exception Instantiation Taschuk SR 326 A Case for Top and Bottom Values Carlson, Reedy SR 329 Treating Builtins as Constants in the Standard Library Hettinger + SR 340 Anonymous Block Statements GvR SR 666 Reject Foolish Indentation Creighton @@ -379,7 +379,7 @@ S 337 Logging Usage in the Standard Library Dubner S 338 Executing modules inside packages with '-m' Coghlan I 339 How to Change CPython's Bytecode Cannon - S 340 Anonymous Block Statements GvR + SR 340 Anonymous Block Statements GvR S 341 Unifying try-except and try-finally Birkenfeld S 342 Enhanced Iterators GvR S 343 Anonymous Block Redux GvR Index: pep-0340.txt =================================================================== RCS file: /cvsroot/python/python/nondist/peps/pep-0340.txt,v retrieving revision 1.27 retrieving revision 1.28 diff -u -d -r1.27 -r1.28 --- pep-0340.txt 11 May 2005 22:09:37 -0000 1.27 +++ pep-0340.txt 16 May 2005 00:34:33 -0000 1.28 @@ -3,7 +3,7 @@ Version: $Revision$ Last-Modified: $Date$ Author: Guido van Rossum -Status: Draft +Status: Rejected Type: Standards Track Content-Type: text/plain Created: 27-Apr-2005 @@ -33,6 +33,11 @@ but the two proposals are really independent and with Steven Bethard's help I have moved it to a separate PEP.) +Rejection Notice + + I am rejecting this PEP in favor of PEP 343. See the motivational + section in that PEP for the reasoning behind this rejection. + Motivation and Summary (Thanks to Shane Hathaway -- Hi Shane!)

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python/nondist/peps pep-0344.txt,1.3,1.4
by ping＠users.sourceforge.net 15 May '05

15 May '05

Update of /cvsroot/python/python/nondist/peps In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv2622 Modified Files: pep-0344.txt Log Message: Update Rationale. Index: pep-0344.txt =================================================================== RCS file: /cvsroot/python/python/nondist/peps/pep-0344.txt,v retrieving revision 1.3 retrieving revision 1.4 diff -u -d -r1.3 -r1.4 --- pep-0344.txt 15 May 2005 23:29:56 -0000 1.3 +++ pep-0344.txt 15 May 2005 23:53:56 -0000 1.4 @@ -102,22 +102,29 @@ of tracebacks (from oldest to most recent frame) and because the actual thrown exception is easier to find on the last line. - Java and Ruby both discard the current exception when an exception - occurs in a 'catch'/'rescue' or 'finally'/'ensure' clause. Perl 5 - lacks built-in structured exception handling. For Perl 6, RFC 88 - proposes an exception mechanism that retains all the 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. + 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 proposes an exception + mechanism that 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. This property is not set by the VM - automatically. Instead, the constructors of exception objects all - accept an optional 'innerException' argument to explicitly set this - property. The C# documentation says "an exception that is thrown as - a direct result of a previous exception should include a reference - to the previous exception in the InnerException property." + may point to another exception [9]. According to its documentation, + "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 adding a + single method to the base Exception class rather than extending the + constructors of all exceptions. The reason all three of these attributes are presented together in one proposal is that the '__traceback__' attribute provides @@ -202,7 +209,11 @@ The '__cause__' attribute on exception objects is always initialized to None. It is set by calling the 'setcause' method, a new method defined on the base Exception class. For convenience, this method - returns the exception itself. + returns the exception itself: + + def setcause(self, cause): + self.__cause__ = cause + return self In the following example, a database provides implementations for a few different kinds of storage, with file storage as one kind. The @@ -301,12 +312,10 @@ C API - To keep things simpler, the PyErr_Set* calls for setting exceptions - will not set the '__context__' attribute on exceptions. The BDFL has - has expressed qualms with making such changes to PyErr_Set* [8]. - - PyErr_NormalizeException will always set the 'traceback' attribute - to its 'tb' argument and the '__context__' attribute to None. + 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 @@ -319,25 +328,28 @@ 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 except for code that - currently sets or depends on the values of attributes named - '__context__', '__cause__', or '__traceback__' on exception - instances. As of 2005-05-12, the Python standard library contains - no mention of such attributes. + The proposed changes should not break any code unless the code sets + or uses attributes named '__context__', '__cause__', 'setcause', or + '__traceback__' on exception instances. As of 2005-05-12, the + Python standard library contains no mention of such attributes. Open Issues - Walter Dörwald [9] expressed a desire to attach extra information - to an exception during its upward propagation, without changing its + Walter Dörwald [10] 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. - It is not clear whether the '__context__' feature proposed here would - be sufficient to cover all the use cases that Raymond Hettinger [1] - originally had in mind. + It is not clear whether the '__context__' and '__cause__' features + proposed here would be sufficient to cover all the use cases that + Raymond Hettinger [1] originally had in mind. + + As written, this PEP makes it impossible to suppress '__context__', + since setting exc.__context__ to None will only result in it being + set again the moment that exc is raised. The exception context is lost when a 'yield' statement is executed; resuming the frame after the 'yield' does not restore the context. @@ -430,7 +442,10 @@ [8] Guido van Rossum discusses automatic chaining in PyErr_Set* http://mail.python.org/pipermail/python-dev/2003-June/036180.html - [9] Walter Dörwald suggests wrapping exceptions to add details + [9] MSDN .NET Framework Library, "Exception.InnerException Property" + http://msdn.microsoft.com/library/en-us/cpref/html/frlrfsystemexceptionclas… + + [10] Walter Dörwald suggests wrapping exceptions to add details http://mail.python.org/pipermail/python-dev/2003-June/036148.html

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python/nondist/peps pep-0344.txt,1.2,1.3
by ping＠users.sourceforge.net 15 May '05

15 May '05

Update of /cvsroot/python/python/nondist/peps In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv30968 Modified Files: pep-0344.txt Log Message: Add __cause__ and language comparisons. Index: pep-0344.txt =================================================================== RCS file: /cvsroot/python/python/nondist/peps/pep-0344.txt,v retrieving revision 1.2 retrieving revision 1.3 diff -u -d -r1.2 -r1.3 --- pep-0344.txt 15 May 2005 19:49:01 -0000 1.2 +++ pep-0344.txt 15 May 2005 23:29:56 -0000 1.3 @@ -12,19 +12,25 @@ Abstract - This PEP proposes two standard attributes on exception instances: - the 'context' attribute for chained exceptions, and the 'traceback' - attribute for the traceback. + 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. Motivation - Sometimes, during the handling of one exception (exception A), - another exception (exception B) can occur. In today's Python + 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. But in order to debug the problem, it is - useful to know about both exceptions. The 'context' attribute - retains this information. + 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, @@ -35,12 +41,8 @@ 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. - - The reason both of these attributes are presented together in one - proposal is that the 'traceback' attribute provides convenient - access to the traceback on chained exceptions. + Adding the '__traceback__' attribute to exception values makes all + the exception information accessible from a single place. History @@ -48,51 +50,83 @@ 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 - and a long discussion followed. Other suggested attribute names - include 'cause', 'antecedent', 'reason', 'original', 'chain', - 'chainedexc', 'exc_chain', 'excprev', 'previous', and 'precursor'. - This PEP suggests 'context' because the intended meaning is more - specific than temporal precedence and less specific than causation: - an exception occurs in the *context* of handling another exception. + 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. This PEP handles - both situations in the same way; it is assumed to be unnecessary to - add mechanisms for distinguishing them since the programmer can tell - them apart by reading the traceback. + block that is handling the original exception. Greg Ewing [4] and Guido van Rossum [5], and probably others, have - previously mentioned adding the traceback attribute to Exception + 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]. -*** Add rationale for: choice of name, handling both finally/except, - handling finally/except the same, displaying outermost last. +Rationale -*** Compare to Java: exceptions are lost in catch or finally clauses. + This PEP distinguishes implicit chaining from explicit chaining of + exceptions because the unexpected raising of a secondary exception + and the intentional translation of an exception are two different + situations deserving quite different interpretations. -*** Compare to Ruby: exceptions are lost just like Java. + 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 '__context__' and '__traceback__' because the attributes are set + by the Python VM. The name '__cause__' is not set automatically by + the VM, but it seems confusing and collision-prone to use 'cause'. -*** Compare to C#: + 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. -*** Compare to E: + 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. -*** COmpare to Perl: RFC 88 a mess. + This PEP proposes that tracebacks display the outermost exception + last, because it 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. -*** Note http://pclt.cis.yale.edu/pclt/exceptions.htm + Java and Ruby both discard the current exception when an exception + occurs in a 'catch'/'rescue' or 'finally'/'ensure' clause. Perl 5 + lacks built-in structured exception handling. For Perl 6, RFC 88 + proposes an exception mechanism that retains all the 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. This property is not set by the VM + automatically. Instead, the constructors of exception objects all + accept an optional 'innerException' argument to explicitly set this + property. The C# documentation says "an exception that is thrown as + a direct result of a previous exception should include a reference + to the previous exception in the InnerException property." -Exception Chaining + 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. - Here is an example to illustrate the 'context' attribute. + +Implicit Exception Chaining + + Here is an example to illustrate the '__context__' attribute. def compute(a, b): try: @@ -112,11 +146,11 @@ 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 + instance of IOError has an additional '__context__' attribute that retains the ZeroDivisionError. The following more elaborate example demonstrates the handling of a - mix of 'finally' and 'except' clauses: + mixture of 'finally' and 'except' clauses: def main(filename): file = open(filename) # oops, forgot the 'w' @@ -142,19 +176,18 @@ 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', which has a context attribute pointing to - a NameError due to the misspelling of 'ex', which has a context - attribute pointing to an IOError due to the file being read-only, - which has a context attribute pointing to a ZeroDivisionError, which - has a context attribute of None. + 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. + 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. @@ -164,9 +197,33 @@ 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 calling the 'setcause' method, a new method + defined on the base Exception class. For convenience, this method + returns the exception itself. + + 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').setcause(exc) + + Traceback Attribute - The following example illustrates the 'traceback' attribute. + The following example illustrates the '__traceback__' attribute. def do_logged(file, work): try: @@ -181,7 +238,7 @@ ... type = exc.__class__ message = str(exc) - lines = format_tb(exc.traceback) + lines = format_tb(exc.__traceback__) file.write(... type ... message ... lines ...) ... @@ -189,63 +246,84 @@ 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. + obtains the exception using the '__traceback__' attribute. The proposed semantics are as follows: - 1. Whenever an exception is raised, if the exception instance does - not already have a 'traceback' attribute, the interpreter sets - it to the newly raised traceback. + 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. In keeping with the chronological order of tracebacks, - the most recently raised exception is displayed last. 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 the following line between tracebacks: + 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): + chain = [] + link = None + while exc: + chain.append((exc, link)) + if exc.__cause__: + exc, link = exc.__cause__, 'The above exception...' + else: + exc, link = exc.__context__, 'During handling...' + for exc, link in reversed(chain): + print_exc(exc) + print '\n' + link + '\n' + In the 'traceback' module, the format_exception, print_exception, print_exc, and print_last functions will be updated to accept an - optional 'context' argument, True by default. When this argument is + 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 will be updated to display the entire chain of - exceptions. + The 'cgitb' module should also be updated to display the entire + chain of exceptions. C API To keep things simpler, the PyErr_Set* calls for setting exceptions - will not set the 'context' attribute on exceptions. Guido van Rossum + will not set the '__context__' attribute on exceptions. The BDFL has has expressed qualms with making such changes to PyErr_Set* [8]. PyErr_NormalizeException will always set the 'traceback' attribute - to its 'tb' argument and the 'context' attribute to None. + to its 'tb' argument and the '__context__' attribute 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. + then set its '__context__' attribute. Compatibility - Chained exceptions expose their outermost type so that they will - continue to match the same 'except' clauses as they do now. - - The proposed changes should not break any code except for code - that currently sets and relies on the values of attributes named - 'context' or 'traceback' on exceptions. + Chained exceptions expose the type of the most recent exception, so + they will still match the same 'except' clauses as they do now. - As of 2005-05-12, the Python standard library contains no mention - of such attributes. + The proposed changes should not break any code except for code that + currently sets or depends on the values of 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 Issues @@ -257,13 +335,14 @@ establishing conventions for other informational attributes on exceptions. - It is not clear whether the 'context' feature proposed here would + It is not clear whether the '__context__' feature proposed here would be sufficient to cover all the use cases that Raymond Hettinger [1] originally had in mind. The exception context is lost when a 'yield' statement is executed; resuming the frame after the 'yield' does not restore the context. - This is not a new problem, as demonstrated by the following example: + 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: @@ -279,24 +358,25 @@ TypeError: exceptions must be classes, instances, or strings (deprecated), not NoneType - For now, addressing this problem is out of the scope of this PEP. - 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. - - Deprecating sys.exc_type, sys.exc_value, sys.exc_traceback, and + - 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. - - Deprecating sys.last_type, sys.last_value, and sys.last_traceback + - Deprecate sys.last_type, sys.last_value, and sys.last_traceback in favour of a single member, sys.last_exception. - - Deprecating the three-argument form of the 'raise' statement in + - Deprecate the three-argument form of the 'raise' statement in favour of the one-argument form. - - Upgrading cgitb.html() to accept a single value as its first + - Upgrade cgitb.html() to accept a single value as its first argument as an alternative to a (type, value, traceback) tuple. @@ -304,17 +384,17 @@ These changes might be worth considering for Python 3000. - - Removing sys.exc_type, sys.exc_value, sys.exc_traceback, and + - Remove sys.exc_type, sys.exc_value, sys.exc_traceback, and sys.exc_info(). - - Removing sys.last_type, sys.last_value, and sys.last_traceback. + - Remove sys.last_type, sys.last_value, and sys.last_traceback. - - Replacing the three-argument sys.excepthook with a one-argument + - Replace the three-argument sys.excepthook with a one-argument API, and changing the 'cgitb' module to match. - - Removing the three-argument form of the 'raise' statement. + - Remove the three-argument form of the 'raise' statement. - - Upgrading traceback.print_exception to accept an 'exception' + - Upgrade traceback.print_exception to accept an 'exception' argument instead of the type, value, and traceback arguments.

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python/nondist/peps pep-0000.txt, 1.306, 1.307 pep-0344.txt, 1.1, 1.2
by ping＠users.sourceforge.net 15 May '05

15 May '05

Update of /cvsroot/python/python/nondist/peps In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv21258 Modified Files: pep-0000.txt pep-0344.txt Log Message: Add PEP 344. Shorten title to make it fit in PEP 0. Index: pep-0000.txt =================================================================== RCS file: /cvsroot/python/python/nondist/peps/pep-0000.txt,v retrieving revision 1.306 retrieving revision 1.307 diff -u -d -r1.306 -r1.307 --- pep-0000.txt 14 May 2005 00:08:19 -0000 1.306 +++ pep-0000.txt 15 May 2005 19:49:01 -0000 1.307 @@ -121,6 +121,7 @@ S 341 Unifying try-except and try-finally Birkenfeld S 342 Enhanced Iterators GvR S 343 Anonymous Block Redux GvR + S 344 Exception Chaining and Embedded Tracebacks Yee S 754 IEEE 754 Floating Point Special Values Warnes Finished PEPs (done, implemented in CVS) @@ -382,6 +383,7 @@ S 341 Unifying try-except and try-finally Birkenfeld S 342 Enhanced Iterators GvR S 343 Anonymous Block Redux GvR + S 344 Exception Chaining and Embedded Tracebacks Yee SR 666 Reject Foolish Indentation Creighton S 754 IEEE 754 Floating Point Special Values Warnes I 3000 Python 3.0 Plans Kuchling, Cannon Index: pep-0344.txt =================================================================== RCS file: /cvsroot/python/python/nondist/peps/pep-0344.txt,v retrieving revision 1.1 retrieving revision 1.2 diff -u -d -r1.1 -r1.2 --- pep-0344.txt 15 May 2005 19:30:38 -0000 1.1 +++ pep-0344.txt 15 May 2005 19:49:01 -0000 1.2 @@ -1,5 +1,5 @@ PEP: 344 -Title: Exception Chaining and the Traceback Attribute +Title: Exception Chaining and Embedded Tracebacks Version: $Revision$ Last-Modified: $Date$ Author: Ka-Ping Yee

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python/nondist/peps pep-0344.txt,NONE,1.1
by ping＠users.sourceforge.net 15 May '05

15 May '05

Update of /cvsroot/python/python/nondist/peps In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv18411 Added Files: pep-0344.txt Log Message: Add PEP 344: Exception Chaining and the Traceback Attribute. --- NEW FILE: pep-0344.txt --- PEP: 344 Title: Exception Chaining and the Traceback Attribute Version: $Revision: 1.1 $ Last-Modified: $Date: 2005/05/15 19:30:38 $ Author: Ka-Ping Yee Status: Active Type: Standards Track Content-Type: text/plain Created: 12-May-2005 Python-Version: 2.5 Abstract This PEP proposes two standard attributes on exception instances: the 'context' attribute for chained exceptions, and the 'traceback' attribute for the traceback. Motivation Sometimes, during the handling of one exception (exception A), another exception (exception B) can occur. In today's Python (version 2.4), if this happens, exception B is propagated outward and exception A is lost. But in order to debug the problem, it is useful to know about both exceptions. The 'context' attribute retains this information. 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. The reason both of these attributes are presented together in one proposal is that the 'traceback' attribute provides convenient access to the traceback on chained exceptions. 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 and a long discussion followed. Other suggested attribute names include 'cause', 'antecedent', 'reason', 'original', 'chain', 'chainedexc', 'exc_chain', 'excprev', 'previous', and 'precursor'. This PEP suggests 'context' because the intended meaning is more specific than temporal precedence and less specific than causation: an exception occurs in the *context* of handling another exception. 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. This PEP handles both situations in the same way; it is assumed to be unnecessary to add mechanisms for distinguishing them since the programmer can tell them apart by reading the traceback. Greg Ewing [4] and Guido van Rossum [5], and probably others, have previously mentioned adding the 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]. *** Add rationale for: choice of name, handling both finally/except, handling finally/except the same, displaying outermost last. *** Compare to Java: exceptions are lost in catch or finally clauses. *** Compare to Ruby: exceptions are lost just like Java. *** Compare to C#: *** Compare to E: *** COmpare to Perl: RFC 88 a mess. *** Note http://pclt.cis.yale.edu/pclt/exceptions.htm 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 mix 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', which has a context attribute pointing to a NameError due to the misspelling of 'ex', which has a context attribute pointing to an IOError due to the file being read-only, which has a context attribute pointing to a ZeroDivisionError, which has a context attribute of 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. 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 raised, if the exception instance does not already have a 'traceback' attribute, the interpreter sets it to the newly raised traceback. Enhanced Reporting The default exception handler will be modified to report chained exceptions. In keeping with the chronological order of tracebacks, the most recently raised exception is displayed last. 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 the following line between tracebacks: During handling of the above exception, another exception occurred: In the 'traceback' module, the format_exception, print_exception, print_exc, and print_last functions will be updated to accept an optional 'context' 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 will be updated to display the entire chain of exceptions. C API To keep things simpler, the PyErr_Set* calls for setting exceptions will not set the 'context' attribute on exceptions. Guido van Rossum has expressed qualms with making such changes to PyErr_Set* [8]. PyErr_NormalizeException will always set the 'traceback' attribute to its 'tb' argument and the 'context' attribute 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. Compatibility Chained exceptions expose their outermost type so that they will continue to match the same 'except' clauses as they do now. The proposed changes should not break any code except for code that currently sets and relies on the values of attributes named 'context' or 'traceback' on exceptions. As of 2005-05-12, the Python standard library contains no mention of such attributes. Open Issues Walter Dörwald [9] 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. It is not clear whether the 'context' feature proposed here would be sufficient to cover all the use cases that Raymond Hettinger [1] originally had in mind. The exception context is lost when a 'yield' statement is executed; resuming the frame after the 'yield' does not restore the context. This 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 For now, addressing this problem is out of the scope of this PEP. 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. - Deprecating sys.exc_type, sys.exc_value, sys.exc_traceback, and sys.exc_info() in favour of a single member, sys.exception. - Deprecating sys.last_type, sys.last_value, and sys.last_traceback in favour of a single member, sys.last_exception. - Deprecating the three-argument form of the 'raise' statement in favour of the one-argument form. - Upgrading 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. - Removing sys.exc_type, sys.exc_value, sys.exc_traceback, and sys.exc_info(). - Removing sys.last_type, sys.last_value, and sys.last_traceback. - Replacing the three-argument sys.excepthook with a one-argument API, and changing the 'cgitb' module to match. - Removing the three-argument form of the 'raise' statement. - Upgrading 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] Walter Dörwald suggests wrapping exceptions to add details http://mail.python.org/pipermail/python-dev/2003-June/036148.html Copyright This document has been placed in the public domain.

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python/dist/src/Objects typeobject.c, 2.265, 2.266
by arigo＠users.sourceforge.net 15 May '05

15 May '05

Update of /cvsroot/python/python/dist/src/Objects In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv4545 Modified Files: typeobject.c Log Message: Fixed a quite misleading comment: a "not" should not have been there. Index: typeobject.c =================================================================== RCS file: /cvsroot/python/python/dist/src/Objects/typeobject.c,v retrieving revision 2.265 retrieving revision 2.266 diff -u -d -r2.265 -r2.266 --- typeobject.c 3 Mar 2005 16:45:19 -0000 2.265 +++ typeobject.c 15 May 2005 15:32:08 -0000 2.266 @@ -5648,7 +5648,7 @@ return self; } if (su->ob_type != &PySuper_Type) - /* If su is not an instance of a subclass of super, + /* If su is an instance of a (strict) subclass of super, call its type */ return PyObject_CallFunction((PyObject *)su->ob_type, "OO", su->type, obj);

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python/dist/src/Lib/test test_mutants.py, 1.7, 1.8
by arigo＠users.sourceforge.net 15 May '05

15 May '05

Update of /cvsroot/python/python/dist/src/Lib/test In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv16135/Lib/test Modified Files: test_mutants.py Log Message: This test relied on short-circuiting details of dictobject.py to avoid crashing, and indirectly on the fact that hash codes in random.randrange(1000000000) were very unlikely to exhibit collisions. To see the problem, replace this number with 500 and observe the crash on either del target[key] or del keys[i]. The fix prevents recursive mutation, just as in the key insertion case. Index: test_mutants.py =================================================================== RCS file: /cvsroot/python/python/dist/src/Lib/test/test_mutants.py,v retrieving revision 1.7 retrieving revision 1.8 diff -u -d -r1.7 -r1.8 --- test_mutants.py 23 Jul 2002 19:03:57 -0000 1.7 +++ test_mutants.py 15 May 2005 13:29:26 -0000 1.8 @@ -69,14 +69,12 @@ elif keys: # Delete a key at random. + mutate = 0 # disable mutation until key deleted i = random.randrange(len(keys)) key = keys[i] del target[key] - # CAUTION: don't use keys.remove(key) here. Or do <wink>. The - # point is that .remove() would trigger more comparisons, and so - # also more calls to this routine. We're mutating often enough - # without that. del keys[i] + mutate = 1 # A horrid class that triggers random mutations of dict1 and dict2 when # instances are compared.

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