python-peps/pep-0344.txt

PEP: 344
Title: Exception Chaining and Embedded Tracebacks
Version: $Revision$
Last-Modified: $Date$
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 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

    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 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.

    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

    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.

    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'.

    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 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.

    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."

    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 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.

        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):
            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 '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

    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.

    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 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.


Open Issues

    Walter D<>rwald [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.
    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


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.


Acknowledgements

    Brett Cannon, Greg Ewing, Guido van Rossum, Jeremy Hylton, Phillip
    J. Eby, Raymond Hettinger, Walter D<>rwald, 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 D<>rwald 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.