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Add "raise ... from" statement. 

Add reference to Perl 6 exception RFC.
This commit is contained in:
Ka-Ping Yee 2005-05-16 06:58:12 +00:00
parent 22bae2bbd0
commit 3d514b0e42
1 changed files with 75 additions and 40 deletions
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pep-0344.txt
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@ -15,7 +15,8 @@ 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.
'__traceback__' attribute for the traceback. A new "raise ... from"
statement sets the '__cause__' attribute.
Motivation
@ -68,10 +69,11 @@ History
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 @@ Rationale
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 @@ Rationale
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 @@ Rationale
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
@ -207,13 +207,15 @@ Implicit Exception Chaining
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
@ -229,7 +231,11 @@ Explicit Exception Chaining
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 @@ Enhanced Reporting
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 @@ C API
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 @@ 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 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.
@ -442,10 +474,13 @@ References
[8] Guido van Rossum discusses automatic chaining in PyErr_Set*
http://mail.python.org/pipermail/python-dev/2003-June/036180.html
[9] MSDN .NET Framework Library, "Exception.InnerException Property"
[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
[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