[Python-checkins] python/nondist/peps pep-0344.txt,1.4,1.5
ping@users.sourceforge.net
ping at users.sourceforge.net
Mon May 16 08:58:15 CEST 2005
Update of /cvsroot/python/python/nondist/peps
In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv689
Modified Files:
pep-0344.txt
Log Message:
Add "raise ... from" statement.
Add reference to Perl 6 exception RFC.
Index: pep-0344.txt
===================================================================
RCS file: /cvsroot/python/python/nondist/peps/pep-0344.txt,v
retrieving revision 1.4
retrieving revision 1.5
diff -u -d -r1.4 -r1.5
--- pep-0344.txt 15 May 2005 23:53:56 -0000 1.4
+++ pep-0344.txt 16 May 2005 06:58:12 -0000 1.5
@@ -15,7 +15,8 @@
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.
+ '__traceback__' attribute for the traceback. A new "raise ... from"
+ statement sets the '__cause__' attribute.
Motivation
@@ -68,10 +69,11 @@
Rationale
- 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.
+ 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',
@@ -84,9 +86,8 @@
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'.
+ 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
@@ -98,7 +99,7 @@
raised exception for compatibility with current behaviour.
This PEP proposes that tracebacks display the outermost exception
- last, because it would be consistent with the chronological order
+ 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.
@@ -109,22 +110,21 @@
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.
+ 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 [9]. According to its documentation,
+ 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 adding a
- single method to the base Exception class rather than extending the
- constructors of all exceptions.
+ the same purpose as InnerException, but this PEP proposes a new form
+ of 'raise' 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
@@ -207,14 +207,16 @@
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:
+ to None. It is set by a new form of the 'raise' statement:
- def setcause(self, cause):
- self.__cause__ = cause
- return self
+ 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
@@ -229,7 +231,11 @@
try:
self.file = open(filename)
except IOError, exc:
- raise DatabaseError('failed to open').setcause(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
@@ -297,7 +303,10 @@
exc, link = exc.__context__, 'During handling...'
for exc, link in reversed(chain):
print_exc(exc)
- print '\n' + link + '\n'
+ if link:
+ print
+ print link
+ print
In the 'traceback' module, the format_exception, print_exception,
print_exc, and print_last functions will be updated to accept an
@@ -320,7 +329,8 @@
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. A similar API function,
+ PyErr_SetCause(cause), will set the '__cause__' attribute.
Compatibility
@@ -328,28 +338,50 @@
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 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.
+ 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 Issues
- Walter Dörwald [10] expressed a desire to attach 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.
- 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.
+ since setting exc.__context__ to None in an 'except' or 'finally'
+ clause will only result in it being set again when exc is raised.
+
+ 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
The exception context is lost when a 'yield' statement is executed;
resuming the frame after the 'yield' does not restore the context.
@@ -441,11 +473,14 @@
[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
- [9] MSDN .NET Framework Library, "Exception.InnerException Property"
+ [10] MSDN .NET Framework Library, "Exception.InnerException Property"
http://msdn.microsoft.com/library/en-us/cpref/html/frlrfsystemexceptionclassinnerexceptiontopic.asp
- [10] Walter Dörwald suggests wrapping exceptions to add details
+ [11] Walter Dörwald suggests wrapping exceptions to add details
http://mail.python.org/pipermail/python-dev/2003-June/036148.html
More information about the Python-checkins
mailing list