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PEP 490 – Chain exceptions at C level

Author:: Victor Stinner <vstinner at python.org>
Status:: Rejected
Type:: Standards Track
Created:: 25-Mar-2015
Python-Version:: 3.6

Table of Contents

Abstract
Rationale
Proposal
Backward compatibility
Alternatives
- No change
- New helpers to chain exceptions
Appendix
Rejection
Copyright

Abstract

Chain exceptions at C level, as already done at Python level.

Rationale

Python 3 introduced a new killer feature: exceptions are chained by default, PEP 3134.

Example:

try:
    raise TypeError("err1")
except TypeError:
    raise ValueError("err2")

Output:

Traceback (most recent call last):
  File "test.py", line 2, in <module>
    raise TypeError("err1")
TypeError: err1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test.py", line 4, in <module>
    raise ValueError("err2")
ValueError: err2

Exceptions are chained by default in Python code, but not in extensions written in C.

A new private _PyErr_ChainExceptions() function was introduced in Python 3.4.3 and 3.5 to chain exceptions. Currently, it must be called explicitly to chain exceptions and its usage is not trivial.

Example of _PyErr_ChainExceptions() usage from the zipimport module to chain the previous OSError to a new ZipImportError exception:

PyObject *exc, *val, *tb;
PyErr_Fetch(&exc, &val, &tb);
PyErr_Format(ZipImportError, "can't open Zip file: %R", archive);
_PyErr_ChainExceptions(exc, val, tb);

This PEP proposes to also chain exceptions automatically at C level to stay consistent and give more information on failures to help debugging. The previous example becomes simply:

PyErr_Format(ZipImportError, "can't open Zip file: %R", archive);

Proposal

Modify PyErr_*() functions to chain exceptions

Modify C functions raising exceptions of the Python C API to automatically chain exceptions: modify PyErr_SetString(), PyErr_Format(), PyErr_SetNone(), etc.

Modify functions to not chain exceptions

Keeping the previous exception is not always interesting when the new exception contains information of the previous exception or even more information, especially when the two exceptions have the same type.

Example of an useless exception chain with int(str):

TypeError: a bytes-like object is required, not 'type'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: int() argument must be a string, a bytes-like object or a number, not 'type'

The new TypeError exception contains more information than the previous exception. The previous exception should be hidden.

The PyErr_Clear() function can be called to clear the current exception before raising a new exception, to not chain the current exception with a new exception.

Modify functions to chain exceptions

Some functions save and then restore the current exception. If a new exception is raised, the exception is currently displayed into sys.stderr or ignored depending on the function. Some of these functions should be modified to chain exceptions instead.

Examples of function ignoring the new exception(s):

ptrace_enter_call(): ignore exception
subprocess_fork_exec(): ignore exception raised by enable_gc()
t_bootstrap() of the _thread module: ignore exception raised by trying to display the bootstrap function to sys.stderr
PyDict_GetItem(), _PyDict_GetItem_KnownHash(): ignore exception raised by looking for a key in the dictionary
_PyErr_TrySetFromCause(): ignore exception
PyFrame_LocalsToFast(): ignore exception raised by dict_to_map()
_PyObject_Dump(): ignore exception. _PyObject_Dump() is used to debug, to inspect a running process, it should not modify the Python state.
Py_ReprLeave(): ignore exception “because there is no way to report them”
type_dealloc(): ignore exception raised by remove_all_subclasses()
PyObject_ClearWeakRefs(): ignore exception?
call_exc_trace(), call_trace_protected(): ignore exception
remove_importlib_frames(): ignore exception
do_mktuple(), helper used by Py_BuildValue() for example: ignore exception?
flush_io(): ignore exception
sys_write(), sys_format(): ignore exception
_PyTraceback_Add(): ignore exception
PyTraceBack_Print(): ignore exception

Examples of function displaying the new exception to sys.stderr:

atexit_callfuncs(): display exceptions with PyErr_Display() and return the latest exception, the function calls multiple callbacks and only returns the latest exception
sock_dealloc(): log the ResourceWarning exception with PyErr_WriteUnraisable()
slot_tp_del(): display exception with PyErr_WriteUnraisable()
_PyGen_Finalize(): display gen_close() exception with PyErr_WriteUnraisable()
slot_tp_finalize(): display exception raised by the __del__() method with PyErr_WriteUnraisable()
PyErr_GivenExceptionMatches(): display exception raised by PyType_IsSubtype() with PyErr_WriteUnraisable()

Backward compatibility

A side effect of chaining exceptions is that exceptions store traceback objects which store frame objects which store local variables. Local variables are kept alive by exceptions. A common issue is a reference cycle between local variables and exceptions: an exception is stored in a local variable and the frame indirectly stored in the exception. The cycle only impacts applications storing exceptions.

The reference cycle can now be fixed with the new traceback.TracebackException object introduced in Python 3.5. It stores information required to format a full textual traceback without storing local variables.

The asyncio is impacted by the reference cycle issue. This module is also maintained outside Python standard library to release a version for Python 3.3. traceback.TracebackException will maybe be backported in a private asyncio module to fix reference cycle issues.

Alternatives

No change

A new private _PyErr_ChainExceptions() function is enough to chain manually exceptions.

Exceptions will only be chained explicitly where it makes sense.

New helpers to chain exceptions

Functions like PyErr_SetString() don’t chain automatically exceptions. To make the usage of _PyErr_ChainExceptions() easier, new private functions are added:

_PyErr_SetStringChain(exc_type, message)
_PyErr_FormatChain(exc_type, format, ...)
_PyErr_SetNoneChain(exc_type)
_PyErr_SetObjectChain(exc_type, exc_value)

Helper functions to raise specific exceptions like _PyErr_SetKeyError(key) or PyErr_SetImportError(message, name, path) don’t chain exceptions. The generic _PyErr_ChainExceptions(exc_type, exc_value, exc_tb) should be used to chain exceptions with these helper functions.

Appendix

PEPs

PEP 3134 – Exception Chaining and Embedded Tracebacks (Python 3.0): new __context__ and __cause__ attributes for exceptions
PEP 415 – Implement context suppression with exception attributes (Python 3.3): raise exc from None
PEP 409 – Suppressing exception context (superseded by the PEP 415)

Python C API

The header file Include/pyerror.h declares functions related to exceptions.

Functions raising exceptions:

PyErr_SetNone(exc_type)
PyErr_SetObject(exc_type, exc_value)
PyErr_SetString(exc_type, message)
PyErr_Format(exc, format, ...)

Helpers to raise specific exceptions:

PyErr_BadArgument()
PyErr_BadInternalCall()
PyErr_NoMemory()
PyErr_SetFromErrno(exc)
PyErr_SetFromWindowsErr(err)
PyErr_SetImportError(message, name, path)
_PyErr_SetKeyError(key)
_PyErr_TrySetFromCause(prefix_format, ...)

Manage the current exception:

PyErr_Clear(): clear the current exception, like except: pass
PyErr_Fetch(exc_type, exc_value, exc_tb)
PyErr_Restore(exc_type, exc_value, exc_tb)
PyErr_GetExcInfo(exc_type, exc_value, exc_tb)
PyErr_SetExcInfo(exc_type, exc_value, exc_tb)

Others function to handle exceptions:

PyErr_ExceptionMatches(exc): check to implement except exc: ...
PyErr_GivenExceptionMatches(exc1, exc2)
PyErr_NormalizeException(exc_type, exc_value, exc_tb)
_PyErr_ChainExceptions(exc_type, exc_value, exc_tb)