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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.
To keep things simpler, the C API calls for setting an exception
will not automatically set the exception's '__context__'. Guido
van Rossum has 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 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.
Exceptions in C# contain a read-only 'InnerException' property that
may point to another exception [9]. According to its documentation,
"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 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:
def setcause(self, cause):
self.__cause__ = cause
return self
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
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.
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.
Open Issues
Walter D<>rwald [10] 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.
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] MSDN .NET Framework Library, "Exception.InnerException Property"
http://msdn.microsoft.com/library/en-us/cpref/html/frlrfsystemexceptionclassinnerexceptiontopic.asp
[10] 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.
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