Add "raise ... from" statement.

Add reference to Perl 6 exception RFC.

pep-0344.txt

@@ -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
+    The Python-Dev discussions revealed interest in exception chaining
-    exceptions because the unexpected raising of a secondary exception
+    for two quite different purposes.  To handle the unexpected raising
-    and the intentional translation of an exception are two different
+    of a secondary exception, the exception must be retained implicitly.
-    situations deserving quite different interpretations.
+    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
+    for these three attributes because they are set by the Python VM.
-    by the Python VM.  The name '__cause__' is not set automatically by
+    Only in very special cases should they be set by normal assignment.
-    the VM, but it seems confusing and collision-prone to use 'cause'.
 
     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
+    exception handling.  For Perl 6, RFC 88 [9] proposes an exception
-    mechanism that retains chained exceptions in an array named @@.
+    mechanism that implicitly retains chained exceptions in an array
-    In that RFC, the most recently raised exception is exposed for
+    named @@.  In that RFC, the most recently raised exception is
-    matching, as in this PEP; also, arbitrary expressions (possibly
+    exposed for matching, as in this PEP; also, arbitrary expressions
-    involving @@) can be evaluated for exception matching.
+    (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
+    the same purpose as InnerException, but this PEP proposes a new form
-    single method to the base Exception class rather than extending the
+    of 'raise' rather than extending the constructors of all exceptions.
-    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
+    to None.  It is set by a new form of the 'raise' statement:
-    defined on the base Exception class.  For convenience, this method
-    returns the exception itself:
 
-        def setcause(self, cause):
+        raise EXCEPTION from CAUSE
-            self.__cause__ = cause
+
-            return self
+    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
+    The proposed changes should not break any code unless it sets or
-    or uses attributes named '__context__', '__cause__', 'setcause', or
+    uses attributes named '__context__', '__cause__', or '__traceback__'
-    '__traceback__' on exception instances.  As of 2005-05-12, the
+    on exception instances.  As of 2005-05-12, the Python standard
-    Python standard library contains no mention of such attributes.
+    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
+    since setting exc.__context__ to None in an 'except' or 'finally'
-    set again the moment that exc is raised.
+    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