python-peps/pep-3134.txt

PEP: 3134
Title: Exception Chaining and Embedded Tracebacks
Version: $Revision$
Last-Modified: $Date$
Author: Ka-Ping Yee
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 12-May-2005
Python-Version: 3.0
Post-History:


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 occurring in a ``finally``
block during unwinding triggered by an original exception, as distinct from
the case of an exception occurring 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
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
number 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(Exception):
        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.


Implementation
==============

The ``__traceback__`` and ``__cause__`` attributes and the new raise syntax
were implemented in revision 57783 [14]_.


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

.. [14] Patch to implement the bulk of the PEP
        http://svn.python.org/view/python/branches/py3k/Include/?rev=57783&view=rev



Copyright
=========

This document has been placed in the public domain.


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