Close issue #6210: Implement PEP 409
diff --git a/Doc/ACKS.txt b/Doc/ACKS.txt
index f9e4d3b..b64c650 100644
--- a/Doc/ACKS.txt
+++ b/Doc/ACKS.txt
@@ -62,6 +62,7 @@
* Stefan Franke
* Jim Fulton
* Peter Funk
+ * Ethan Furman
* Lele Gaifax
* Matthew Gallagher
* Gabriel Genellina
diff --git a/Doc/c-api/exceptions.rst b/Doc/c-api/exceptions.rst
index c7252ed..fd7aee7 100644
--- a/Doc/c-api/exceptions.rst
+++ b/Doc/c-api/exceptions.rst
@@ -421,17 +421,24 @@
.. c:function:: PyObject* PyException_GetCause(PyObject *ex)
- Return the cause (another exception instance set by ``raise ... from ...``)
- associated with the exception as a new reference, as accessible from Python
- through :attr:`__cause__`. If there is no cause associated, this returns
- *NULL*.
+ Return the cause (either an exception instance, or :const:`None`,
+ set by ``raise ... from ...``) associated with the exception as a new
+ reference, as accessible from Python through :attr:`__cause__`.
+
+ If there is no cause associated, this returns *NULL* (from Python
+ ``__cause__ is Ellipsis``). If the cause is :const:`None`, the default
+ exception display routines stop showing the context chain.
.. c:function:: void PyException_SetCause(PyObject *ex, PyObject *ctx)
Set the cause associated with the exception to *ctx*. Use *NULL* to clear
- it. There is no type check to make sure that *ctx* is an exception instance.
- This steals a reference to *ctx*.
+ it. There is no type check to make sure that *ctx* is either an exception
+ instance or :const:`None`. This steals a reference to *ctx*.
+
+ If the cause is set to :const:`None` the default exception display
+ routines will not display this exception's context, and will not follow the
+ chain any further.
.. _unicodeexceptions:
diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst
index 3f1a30d..7e3a0c3 100644
--- a/Doc/library/exceptions.rst
+++ b/Doc/library/exceptions.rst
@@ -34,6 +34,24 @@
defining exceptions is available in the Python Tutorial under
:ref:`tut-userexceptions`.
+When raising (or re-raising) an exception in an :keyword:`except` clause
+:attr:`__context__` is automatically set to the last exception caught; if the
+new exception is not handled the traceback that is eventually displayed will
+include the originating exception(s) and the final exception.
+
+This implicit exception chain can be made explicit by using :keyword:`from`
+with :keyword:`raise`. The single argument to :keyword:`from` must be an
+exception or :const:`None`, and it will bet set as :attr:`__cause__` on the
+raised exception. If :attr:`__cause__` is an exception it will be displayed
+instead of :attr:`__context__`; if :attr:`__cause__` is None,
+:attr:`__context__` will not be displayed by the default exception handling
+code. (Note: the default value for :attr:`__context__` is :const:`None`,
+while the default value for :attr:`__cause__` is :const:`Ellipsis`.)
+
+In either case, the default exception handling code will not display
+any of the remaining links in the :attr:`__context__` chain if
+:attr:`__cause__` has been set.
+
Base classes
------------
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index be06595..1526a0a 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -2985,10 +2985,11 @@
The Ellipsis Object
-------------------
-This object is commonly used by slicing (see :ref:`slicings`). It supports no
-special operations. There is exactly one ellipsis object, named
-:const:`Ellipsis` (a built-in name). ``type(Ellipsis)()`` produces the
-:const:`Ellipsis` singleton.
+This object is commonly used by slicing (see :ref:`slicings`), but may also
+be used in other situations where a sentinel value other than :const:`None`
+is needed. It supports no special operations. There is exactly one ellipsis
+object, named :const:`Ellipsis` (a built-in name). ``type(Ellipsis)()``
+produces the :const:`Ellipsis` singleton.
It is written as ``Ellipsis`` or ``...``.
diff --git a/Doc/whatsnew/3.3.rst b/Doc/whatsnew/3.3.rst
index 560331f..9023dfa 100644
--- a/Doc/whatsnew/3.3.rst
+++ b/Doc/whatsnew/3.3.rst
@@ -254,6 +254,9 @@
PEP 380: Syntax for Delegating to a Subgenerator
================================================
+:pep:`380` - Syntax for Delegating to a Subgenerator
+ PEP written by Greg Ewing.
+
PEP 380 adds the ``yield from`` expression, allowing a generator to delegate
part of its operations to another generator. This allows a section of code
containing 'yield' to be factored out and placed in another generator.
@@ -267,6 +270,67 @@
Nick Coghlan)
+PEP 409: Suppressing exception context
+======================================
+
+:pep:`409` - Suppressing exception context
+ PEP written by Ethan Furman, implemented by Ethan Furman and Nick Coghlan.
+
+PEP 409 introduces new syntax that allows the display of the chained
+exception context to be disabled. This allows cleaner error messages in
+applications that convert between exception types::
+
+ >>> class D:
+ ... def __init__(self, extra):
+ ... self._extra_attributes = extra
+ ... def __getattr__(self, attr):
+ ... try:
+ ... return self._extra_attributes[attr]
+ ... except KeyError:
+ ... raise AttributeError(attr) from None
+ ...
+ >>> D({}).x
+ Traceback (most recent call last):
+ File "<stdin>", line 1, in <module>
+ File "<stdin>", line 8, in __getattr__
+ AttributeError: x
+
+Without the ``from None`` suffix to suppress the cause, the original
+exception would be displayed by default::
+
+ >>> class C:
+ ... def __init__(self, extra):
+ ... self._extra_attributes = extra
+ ... def __getattr__(self, attr):
+ ... try:
+ ... return self._extra_attributes[attr]
+ ... except KeyError:
+ ... raise AttributeError(attr)
+ ...
+ >>> C({}).x
+ Traceback (most recent call last):
+ File "<stdin>", line 6, in __getattr__
+ KeyError: 'x'
+
+ During handling of the above exception, another exception occurred:
+
+ Traceback (most recent call last):
+ File "<stdin>", line 1, in <module>
+ File "<stdin>", line 8, in __getattr__
+ AttributeError: x
+
+No debugging capability is lost, as the original exception context remains
+available if needed (for example, if an intervening library has incorrectly
+suppressed valuable underlying details)::
+
+ >>> try:
+ ... D({}).x
+ ... except AttributeError as exc:
+ ... print(repr(exc.__context__))
+ ...
+ KeyError('x',)
+
+
PEP 3155: Qualified name for classes and functions
==================================================
diff --git a/Include/pyerrors.h b/Include/pyerrors.h
index 1bd0442..1e42ebb 100644
--- a/Include/pyerrors.h
+++ b/Include/pyerrors.h
@@ -105,6 +105,7 @@
/* Cause manipulation (PEP 3134) */
PyAPI_FUNC(PyObject *) PyException_GetCause(PyObject *);
PyAPI_FUNC(void) PyException_SetCause(PyObject *, PyObject *);
+PyAPI_FUNC(int) _PyException_SetCauseChecked(PyObject *, PyObject *);
/* Context manipulation (PEP 3134) */
PyAPI_FUNC(PyObject *) PyException_GetContext(PyObject *);
diff --git a/Lib/test/test_exceptions.py b/Lib/test/test_exceptions.py
index a7683ac..91d85ef 100644
--- a/Lib/test/test_exceptions.py
+++ b/Lib/test/test_exceptions.py
@@ -387,19 +387,36 @@
def testChainingAttrs(self):
e = Exception()
- self.assertEqual(e.__context__, None)
- self.assertEqual(e.__cause__, None)
+ self.assertIsNone(e.__context__)
+ self.assertIs(e.__cause__, Ellipsis)
e = TypeError()
- self.assertEqual(e.__context__, None)
- self.assertEqual(e.__cause__, None)
+ self.assertIsNone(e.__context__)
+ self.assertIs(e.__cause__, Ellipsis)
class MyException(EnvironmentError):
pass
e = MyException()
- self.assertEqual(e.__context__, None)
- self.assertEqual(e.__cause__, None)
+ self.assertIsNone(e.__context__)
+ self.assertIs(e.__cause__, Ellipsis)
+
+ def testChainingDescriptors(self):
+ try:
+ raise Exception()
+ except Exception as exc:
+ e = exc
+
+ self.assertIsNone(e.__context__)
+ self.assertIs(e.__cause__, Ellipsis)
+
+ e.__context__ = NameError()
+ e.__cause__ = None
+ self.assertIsInstance(e.__context__, NameError)
+ self.assertIsNone(e.__cause__)
+
+ e.__cause__ = Ellipsis
+ self.assertIs(e.__cause__, Ellipsis)
def testKeywordArgs(self):
# test that builtin exception don't take keyword args,
diff --git a/Lib/test/test_raise.py b/Lib/test/test_raise.py
index 92c50c7..8ae9210 100644
--- a/Lib/test/test_raise.py
+++ b/Lib/test/test_raise.py
@@ -3,12 +3,27 @@
"""Tests for the raise statement."""
-from test import support
+from test import support, script_helper
+import re
import sys
import types
import unittest
+try:
+ from resource import setrlimit, RLIMIT_CORE, error as resource_error
+except ImportError:
+ prepare_subprocess = None
+else:
+ def prepare_subprocess():
+ # don't create core file
+ try:
+ setrlimit(RLIMIT_CORE, (0, 0))
+ except (ValueError, resource_error):
+ pass
+
+
+
def get_tb():
try:
raise OSError()
@@ -77,6 +92,16 @@
nested_reraise()
self.assertRaises(TypeError, reraise)
+ def test_raise_from_None(self):
+ try:
+ try:
+ raise TypeError("foo")
+ except:
+ raise ValueError() from None
+ except ValueError as e:
+ self.assertTrue(isinstance(e.__context__, TypeError))
+ self.assertIsNone(e.__cause__)
+
def test_with_reraise1(self):
def reraise():
try:
@@ -139,6 +164,23 @@
class TestCause(unittest.TestCase):
+
+ def testCauseSyntax(self):
+ try:
+ try:
+ try:
+ raise TypeError
+ except Exception:
+ raise ValueError from None
+ except ValueError as exc:
+ self.assertIsNone(exc.__cause__)
+ raise exc from Ellipsis
+ except ValueError as exc:
+ e = exc
+
+ self.assertIs(e.__cause__, Ellipsis)
+ self.assertIsInstance(e.__context__, TypeError)
+
def test_invalid_cause(self):
try:
raise IndexError from 5
@@ -178,6 +220,44 @@
class TestTraceback(unittest.TestCase):
+
+ def get_output(self, code, filename=None):
+ """
+ Run the specified code in Python (in a new child process) and read the
+ output from the standard error or from a file (if filename is set).
+ Return the output lines as a list.
+ """
+ options = {}
+ if prepare_subprocess:
+ options['preexec_fn'] = prepare_subprocess
+ process = script_helper.spawn_python('-c', code, **options)
+ stdout, stderr = process.communicate()
+ exitcode = process.wait()
+ output = support.strip_python_stderr(stdout)
+ output = output.decode('ascii', 'backslashreplace')
+ if filename:
+ self.assertEqual(output, '')
+ with open(filename, "rb") as fp:
+ output = fp.read()
+ output = output.decode('ascii', 'backslashreplace')
+ output = re.sub('Current thread 0x[0-9a-f]+',
+ 'Current thread XXX',
+ output)
+ return output.splitlines(), exitcode
+
+ def test_traceback_verbiage(self):
+ code = """
+try:
+ raise ValueError
+except:
+ raise NameError from None
+"""
+ text, exitcode = self.get_output(code)
+ self.assertEqual(len(text), 3)
+ self.assertTrue(text[0].startswith('Traceback'))
+ self.assertTrue(text[1].startswith(' File '))
+ self.assertTrue(text[2].startswith('NameError'))
+
def test_sets_traceback(self):
try:
raise IndexError()
diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 4752d37..5bce2af 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -246,6 +246,21 @@
self.check_zero_div(blocks[0])
self.assertIn('inner_raise() # Marker', blocks[2])
+ def test_context_suppression(self):
+ try:
+ try:
+ raise Exception
+ except:
+ raise ZeroDivisionError from None
+ except ZeroDivisionError as _:
+ e = _
+ lines = self.get_report(e).splitlines()
+ self.assertEqual(len(lines), 4)
+ self.assertTrue(lines[0].startswith('Traceback'))
+ self.assertTrue(lines[1].startswith(' File'))
+ self.assertIn('ZeroDivisionError from None', lines[2])
+ self.assertTrue(lines[3].startswith('ZeroDivisionError'))
+
def test_cause_and_context(self):
# When both a cause and a context are set, only the cause should be
# displayed and the context should be muted.
diff --git a/Lib/traceback.py b/Lib/traceback.py
index 8d4e96e..35858af 100644
--- a/Lib/traceback.py
+++ b/Lib/traceback.py
@@ -120,14 +120,14 @@
seen.add(exc)
its = []
cause = exc.__cause__
- if cause is not None and cause not in seen:
- its.append(_iter_chain(cause, None, seen))
- its.append([(_cause_message, None)])
- else:
+ if cause is Ellipsis:
context = exc.__context__
if context is not None and context not in seen:
its.append(_iter_chain(context, None, seen))
its.append([(_context_message, None)])
+ elif cause is not None and cause not in seen:
+ its.append(_iter_chain(cause, False, seen))
+ its.append([(_cause_message, None)])
its.append([(exc, custom_tb or exc.__traceback__)])
# itertools.chain is in an extension module and may be unavailable
for it in its:
diff --git a/Misc/ACKS b/Misc/ACKS
index 48ef080..e9077c3 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -338,6 +338,7 @@
Tadayoshi Funaba
Gyro Funch
Peter Funk
+Ethan Furman
Geoff Furnish
Ulisses Furquim
Hagen Fürstenau
diff --git a/Misc/NEWS b/Misc/NEWS
index bc9fe5d..be6f700 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,10 @@
Core and Builtins
-----------------
+- PEP 409, Issue #6210: "raise X from None" is now supported as a means of
+ suppressing the display of the chained exception context. The chained
+ context still remains available as the __context__ attribute.
+
- Issue #10181: New memoryview implementation fixes multiple ownership
and lifetime issues of dynamically allocated Py_buffer members (#9990)
as well as crashes (#8305, #7433). Many new features have been added
diff --git a/Objects/exceptions.c b/Objects/exceptions.c
index e9522e8..bc43799 100644
--- a/Objects/exceptions.c
+++ b/Objects/exceptions.c
@@ -266,7 +266,24 @@
PyObject *res = PyException_GetCause(self);
if (res)
return res; /* new reference already returned above */
- Py_RETURN_NONE;
+ Py_INCREF(Py_Ellipsis);
+ return Py_Ellipsis;
+}
+
+int
+_PyException_SetCauseChecked(PyObject *self, PyObject *arg) {
+ if (arg == Py_Ellipsis) {
+ arg = NULL;
+ } else if (arg != Py_None && !PyExceptionInstance_Check(arg)) {
+ PyErr_SetString(PyExc_TypeError, "exception cause must be None, "
+ "Ellipsis or derive from BaseException");
+ return -1;
+ } else {
+ /* PyException_SetCause steals a reference */
+ Py_INCREF(arg);
+ }
+ PyException_SetCause(self, arg);
+ return 0;
}
static int
@@ -274,18 +291,8 @@
if (arg == NULL) {
PyErr_SetString(PyExc_TypeError, "__cause__ may not be deleted");
return -1;
- } else if (arg == Py_None) {
- arg = NULL;
- } else if (!PyExceptionInstance_Check(arg)) {
- PyErr_SetString(PyExc_TypeError, "exception cause must be None "
- "or derive from BaseException");
- return -1;
- } else {
- /* PyException_SetCause steals this reference */
- Py_INCREF(arg);
}
- PyException_SetCause(self, arg);
- return 0;
+ return _PyException_SetCauseChecked(self, arg);
}
diff --git a/Python/ceval.c b/Python/ceval.c
index 06bff4c..017dc4a 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -3567,22 +3567,23 @@
if (cause) {
PyObject *fixed_cause;
+ int result;
if (PyExceptionClass_Check(cause)) {
fixed_cause = PyObject_CallObject(cause, NULL);
if (fixed_cause == NULL)
goto raise_error;
- Py_DECREF(cause);
- }
- else if (PyExceptionInstance_Check(cause)) {
+ Py_CLEAR(cause);
+ } else {
+ /* Let "exc.__cause__ = cause" handle all further checks */
fixed_cause = cause;
+ cause = NULL; /* Steal the reference */
}
- else {
- PyErr_SetString(PyExc_TypeError,
- "exception causes must derive from "
- "BaseException");
+ /* We retain ownership of the reference to fixed_cause */
+ result = _PyException_SetCauseChecked(value, fixed_cause);
+ Py_DECREF(fixed_cause);
+ if (result < 0) {
goto raise_error;
}
- PyException_SetCause(value, fixed_cause);
}
PyErr_SetObject(type, value);
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index a642c0b..f4e7e7b 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -1698,7 +1698,11 @@
else if (PyExceptionInstance_Check(value)) {
cause = PyException_GetCause(value);
context = PyException_GetContext(value);
- if (cause) {
+ if (cause && cause == Py_None) {
+ /* print neither cause nor context */
+ ;
+ }
+ else if (cause) {
res = PySet_Contains(seen, cause);
if (res == -1)
PyErr_Clear();