PEP 3134 – Exception Chaining and Embedded Tracebacks
- Ka-Ping Yee
- Standards Track
Table of Contents
- Numbering Note
- Implicit Exception Chaining
- Explicit Exception Chaining
- Traceback Attribute
- Enhanced Reporting
- C API
- Open Issue: Extra Information
- Open Issue: Suppressing Context
- Open Issue: Limiting Exception Types
- Open Issue: yield
- Open Issue: Garbage Collection
- Possible Future Compatible Changes
- Possible Future Incompatible Changes
This PEP started its life as PEP 344. Since it is now targeted for Python 3000, it has been moved into the 3xxx space.
This PEP proposes three standard attributes on exception instances: the
__context__ attribute for implicitly chained exceptions, the
attribute for explicitly chained exceptions, and the
attribute for the traceback. A new
raise ... from statement sets the
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,
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.
__traceback__ attribute to exception values makes all the
exception information accessible from a single place.
Raymond Hettinger  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  brought up chained exceptions again in June 2003, prompting
a long discussion.
Greg Ewing  identified the case of an exception occurring in a
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
Greg Ewing  and Guido van Rossum , 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  .
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 , including
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
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
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 .
As for other languages, Java and Ruby both discard the original exception when
another exception occurs in a
clause. Perl 5 lacks built-in structured exception handling. For Perl 6, RFC
number 88  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
@@) can be evaluated for exception matching.
Exceptions in C# contain a read-only
InnerException property that may point
to another exception. Its documentation  says that “When an exception X
is thrown as a direct result of a previous exception Y, the
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
fulfills the same purpose as
InnerException, but this PEP proposes a new
raise rather than extending the constructors of all exceptions. C#
also provides a
GetBaseException method that jumps directly to the end of
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
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()
compute(0, 0) causes a
function catches this exception and calls
log(exc), but the
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
ZeroDivisionError is lost. With the proposed change, the instance of
IOError has an additional
__context__ attribute that retains the
The following more elaborate example demonstrates the handling of a mixture of
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'
main() with the name of an existing file will trigger four
exceptions. The ultimate result will be an
AttributeError due to the
__context__ points to a
to the misspelling of
__context__ points to an
due to the file being read-only, whose
__context__ points to a
__context__ attribute is
The proposed semantics are as follows:
- Each thread has an exception context initially set to
- Whenever an exception is raised, if the exception instance does not already
__context__attribute, the interpreter sets it equal to the thread’s exception context.
- Immediately after an exception is raised, the thread’s exception context is set to the exception.
- Whenever the interpreter exits an
exceptblock by reaching the end or executing a
breakstatement, the thread’s exception context is set to
Explicit Exception Chaining
__cause__ attribute on exception objects is always initialized to
None. It is set by a new form of the
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
DatabaseError, with a
__cause__ attribute that reveals the
IOError as the original cause.
The following example illustrates the
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
sys.exc_info()  and pass both
the value and the traceback to
write_exception(). With the proposed
write_exception() simply gets one argument and obtains the
exception using the
The proposed semantics are as follows:
- Whenever an exception is caught, if the exception instance does not already
__traceback__attribute, the interpreter sets it to the newly caught traceback.
The default exception handler will be modified to report chained exceptions.
The chain of exceptions is traversed by following the
__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:
During handling of the above exception, another exception occurred:
between tracebacks, depending whether they are linked by
__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)
traceback module, the
print_last functions will be updated to accept an
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.
cgitb module should also be updated to display the entire chain of
PyErr_Set* calls for setting exceptions will not set the
__context__ attribute on exceptions.
always set the
traceback attribute to its
tb argument and the
__cause__ attributes to
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
A similar API function,
PyErr_SetCause(cause), will set the
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
exception instances. As of 2005-05-12, the Python standard library contains no
mention of such attributes.
Open Issue: Extra Information
Walter Dörwald  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
None in an
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
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 . 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.
__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
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 .
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 (
traceback) arguments to
__exit__with a single exception argument.
sys.exc_info()in favour of a single member,
sys.last_tracebackin favour of a single member,
- Deprecate the three-argument form of the
raisestatement in favour of the one-argument form.
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.
- Replace the three-argument
sys.excepthookwith a one-argument API, and changing the
cgitbmodule to match.
- Remove the three-argument form of the
traceback.print_exceptionto accept an
exceptionargument instead of the
__cause__ attributes and the new raise syntax
were implemented in revision 57783 .
Brett Cannon, Greg Ewing, Guido van Rossum, Jeremy Hylton, Phillip J. Eby, Raymond Hettinger, Walter Dörwald, and others.
This document has been placed in the public domain.
Last modified: 2022-01-21 11:03:51 GMT