PEP 344 – Exception Chaining and Embedded Tracebacks

PEP: 344
Title: Exception Chaining and Embedded Tracebacks
Author: Ka-Ping Yee
Status: Superseded
Type: Standards Track
Created: 12-May-2005
Python-Version: 2.5
Post-History

Numbering Note

This PEP has been renumbered to PEP 3134. The text below is the last version submitted under the old number.

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, xc_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:

Each thread has an exception context initially set to None.
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.
Immediately after an exception is raised, the thread’s exception context is set to the exception.
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(StandardError):
    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:

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:

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.