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PEP: 654
Title: Exception Groups and except*
Author: Irit Katriel <iritkatriel@gmail.com>,
Yury Selivanov <yury@edgedb.com>,
Guido van Rossum <guido@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 22-Feb-2021
Post-History: 22-Feb-2021
Abstract
========
This document proposes language extensions that allow programs to raise and handle
multiple unrelated exceptions simultaneously:
* A new standard exception type, the ``ExceptionGroup``, which represents a
group of unrelated exceptions being propagated together.
* A new syntax ``except*`` for handling ``ExceptionGroups``.
Motivation
==========
The interpreter is currently able to propagate at most one exception at a
time. The chaining features introduced in :pep:`3134` link together
exceptions that are related to each other as the cause or context, but
there are situations where multiple unrelated exceptions need to be propagated
together as the stack unwinds. Several real world use cases are listed below.
* **Concurrent errors**. Libraries for async concurrency provide APIs to invoke
multiple tasks and return their results in aggregate. There isn't currently
a good way for such libraries to handle situations where multiple tasks
raise exceptions. The Python standard library's ``asyncio.gather()`` [1]_
function provides two options: raise the first exception, or return the
exceptions in the results list. The Trio [2]_
library has a ``MultiError`` exception type which it raises to report a
collection of errors. Work on this PEP was initially motivated by the
difficulties in handling ``MultiErrors`` [9]_, which are detailed in a design
document for an improved version, ``MultiError2`` [3]_.
That document demonstrates how difficult it is to create an effective API
for reporting and handling multiple errors without the language changes we
are proposing (see also the `Programming Without 'except \*'`_ section.)
Implementing a better task spawning API in asyncio, inspired by Trio
nurseries, was the main motivation for this PEP. That work is currently
blocked on Python not having native language level support for exception
groups.
* **Multiple failures when retrying an operation.** The Python standard
library's ``socket.create_connection`` function may attempt to connect to
different addresses, and if all attempts fail it needs to report that to the
user. It is an open issue how to aggregate these errors, particularly when
they are different (see issue 29980 [4]_.)
* **Multiple user callbacks fail.** Python's ``atexit.register()`` function
allows users to register functions that are called on system exit. If any of
them raise exceptions, only the last one is reraised, but it would be better
to reraised all of them together (see ``atexit`` documentation [5]_.)
Similarly, the pytest library allows users to register finalizers which
are executed at teardown. If more than one of these finalizers raises an
exception, only the first is reported to the user. This can be improved with
``ExceptionGroups``, as explained in this issue by pytest developer Ran Benita
(see pytest issue 8217 [6]_.)
* **Multiple errors in a complex calculation.** The Hypothesis library performs
automatic bug reduction (simplifying code that demonstrates a bug). In the
process it may find variations that generate different errors, and
(optionally) reports all of them (see the Hypothesis documentation [7]_.)
An ``ExceptionGroup`` mechanism as we are proposing here can resolve some of
the difficulties with debugging that are mentioned in the link above, and
which are due to the loss of context/cause information (communicated
by Hypothesis Core Developer Zac Hatfield-Dodds).
* **Errors in wrapper code.** The Python standard library's
``tempfile.TemporaryDirectory`` context manager
had an issue where an exception raised during cleanup in ``__exit__``
effectively masked an exception that the user's code raised inside the context
manager scope. While the user's exception was chained as the context of the
cleanup error, it was not caught by the user's except clause
(see issue 40857 [8]_.)
The issue was resolved by making the cleanup code ignore errors, thus
sidestepping the multiple exception problem. With the features we propose
here, it would be possible for ``__exit__`` to raise an ``ExceptionGroup``
containing its own errors along with the user's errors, and this would allow
the user to catch their own exceptions by their types.
Rationale
=========
Grouping several exceptions together can be done without changes to the
language, simply by creating a container exception type.
Trio [2]_ is an example of a library that has made use of this technique in its
``MultiError`` [9]_ type. However, such an approach requires calling code to catch
the container exception type, and then to inspect it to determine the types of
errors that had occurred, extract the ones it wants to handle, and reraise the
rest.
Changes to the language are required in order to extend support for
exception groups in the style of existing exception handling mechanisms. At
the very least we would like to be able to catch an exception group only if
it contains an exception of a type that we choose to handle. Exceptions of
other types in the same group need to be automatically reraised,
otherwise it is too easy for user code to inadvertently swallow exceptions
that it is not handling.
We considered whether it is possible to modify the semantics of ``except``
for this purpose, in a backwards-compatible manner, and found that it is not.
See the `Rejected Ideas`_ section for more on this.
The purpose of this PEP, then, is to add the ``except*`` syntax for handling
exception groups in the interpreter, which in turn requires that
``ExceptionGroup`` is added as a builtin type. The desired semantics of
``except*`` are sufficiently different from the current exception
handling semantics that we are not proposing to modify the behavior of the
``except`` keyword but rather to add the new ``except*`` syntax.
Our premise is that exception groups and ``except*`` will be used
selectively, only when they are needed. We do not expect them to become
the default mechanism for exception handling. The decision to raise
exception groups from a library needs to be considered carefully and
regarded as an API-breaking change. We expect that this will normally be
done by introducing a new API rather than modifying an existing one.
Specification
=============
ExceptionGroup and BaseExceptionGroup
-------------------------------------
We propose to add two new builtin exception types:
``BaseExceptionGroup(BaseException)`` and
``ExceptionGroup(BaseExceptionGroup, Exception)``. They are assignable to
``Exception.__cause__`` and ``Exception.__context__``, and they can be
raised and handled as any exception with ``raise ExceptionGroup(...)`` and
``try: ... except ExceptionGroup: ...`` or ``raise BaseExceptionGroup(...)``
and ``try: ... except BaseExceptionGroup: ...``.
Both have a constructor that takes two positional-only arguments: a message
string and a sequence of the nested exceptions, which are exposed in the
fields ``message`` and ``errors``. For example:
``ExceptionGroup('issues', [ValueError('bad value'), TypeError('bad type')])``.
The difference between them is that ``ExceptionGroup`` can only wrap
``Exception`` subclasses while ``BaseExceptionGroup`` can wrap any
``BaseException`` subclass. A factory method that inspects the nested
execptions and selects between ``ExceptionGroup`` and ``BaseExceptionGroup``
makes the choice automatic. In the rest of the document, when we refer to
an exception group, we mean either an ``ExceptionGroup`` or a
``BaseExceptionGroup``. When it is necessary to make the distunction, we
use the class name. For brevity, we will use ``ExceptionGroup`` in code
examples that are relevant to both.
Since an exception group can be nested, it represents a tree of exceptions,
where the leaves are plain exceptions and each internal node represent a time
at which the program grouped some unrelated exceptions into a new group and
raised them together. The ``ExceptionGroup`` and ``BaseExceptionGroup``
classes are final, i.e., they cannot be further subclassed.
The ``BaseExceptionGroup.subgroup(condition)`` method gives us a way to obtain
an exception group that has the same metadata (cause, context, traceback) as
the original group, and the same nested structure of groups, but
contains only those exceptions for which the condition is true:
.. code-block::
>>> eg = ExceptionGroup(
... "one",
... [
... TypeError(1),
... ExceptionGroup(
... "two",
... [TypeError(2), ValueError(3)]
... ),
... ExceptionGroup(
... "three",
... [OSError(4)]
... )
... ]
... )
>>> traceback.print_exception(eg)
ExceptionGroup: one
------------------------------------------------------------
TypeError: 1
------------------------------------------------------------
ExceptionGroup: two
------------------------------------------------------------
TypeError: 2
------------------------------------------------------------
ValueError: 3
------------------------------------------------------------
ExceptionGroup: three
------------------------------------------------------------
OSError: 4
>>> type_errors = eg.subgroup(lambda e: isinstance(e, TypeError))
>>> traceback.print_exception(type_errors)
ExceptionGroup: one
------------------------------------------------------------
TypeError: 1
------------------------------------------------------------
ExceptionGroup: two
------------------------------------------------------------
TypeError: 2
>>>
Empty nested groups are omitted from the result, as in the
case of ``ExceptionGroup("three")`` in the example above. If none of the
leaf exceptions match the condition, ``subgroup`` returns ``None`` rather
than an empty group. The original ``eg``
is unchanged by ``subgroup``, but the value returned is not necessarily a full
new copy. Leaf exceptions are not copied, nor are exception groups which are
fully contained in the result. When it is necessary to partition a
group because the condition holds for some, but not all of its
contained exceptions, a new ``ExceptionGroup`` or ``BaseExceptionGroup``
instance is created, while the ``__cause__``, ``__context__`` and
``__traceback__`` fields are copied by reference, so they are shared with
the original ``eg``.
If both the subgroup and its complement are needed, the
``BaseExceptionGroup.split(condition)`` method can be used:
.. code-block::
>>> type_errors, other_errors = eg.split(lambda e: isinstance(e, TypeError))
>>> traceback.print_exception(type_errors)
ExceptionGroup: one
------------------------------------------------------------
TypeError: 1
------------------------------------------------------------
ExceptionGroup: two
------------------------------------------------------------
TypeError: 2
>>> traceback.print_exception(other_errors)
ExceptionGroup: one
------------------------------------------------------------
ExceptionGroup: two
------------------------------------------------------------
ValueError: 3
------------------------------------------------------------
ExceptionGroup: three
------------------------------------------------------------
OSError: 4
>>>
If a split is trivial (one side is empty), then None is returned for the
other side:
.. code-block::
>>> other_errors.split(lambda e: isinstance(e, SyntaxError))
(None, ExceptionGroup('one', [
ExceptionGroup('two', [
ValueError(3)
]),
ExceptionGroup('three', [
OSError(4)])]))
Since splitting by exception type is a very common use case, ``subgroup`` and
``split`` can take an exception type or tuple of exception types and treat it
as a shorthand for matching that type: ``eg.split(T)`` divides ``eg`` into the
subgroup of leaf exceptions that match the type ``T``, and the subgroup of those
that do not (using the same check as ``except`` for a match).
The Traceback of an Exception Group
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For regular exceptions, the traceback represents a simple path of frames,
from the frame in which the exception was raised to the frame in which it
was caught or, if it hasn't been caught yet, the frame that the program's
execution is currently in. The list is constructed by the interpreter, which
appends any frame from which it exits to the traceback of the 'current
exception' if one exists. To support efficient appends, the links in a
traceback's list of frames are from the oldest to the newest frame. Appending
a new frame is then simply a matter of inserting a new head to the linked
list referenced from the exception's ``__traceback__`` field. Crucially, the
traceback's frame list is immutable in the sense that frames only need to be
added at the head, and never need to be removed.
We do not need to make any changes to this data structure. The ``__traceback__``
field of the exception group instance represents the path that the contained
exceptions travelled through together after being joined into the
group, and the same field on each of the nested exceptions
represents the path through which this exception arrived at the frame of the
merge.
What we do need to change is any code that interprets and displays tracebacks,
because it now needs to continue into tracebacks of nested exceptions, as
in the following example:
.. code-block::
>>> def f(v):
... try:
... raise ValueError(v)
... except ValueError as e:
... return e
...
>>> try:
... raise ExceptionGroup("one", [f(1)])
... except ExceptionGroup as e:
... eg1 = e
...
>>> try:
... raise ExceptionGroup("two", [f(2), eg1])
... except ExceptionGroup as e:
... eg2 = e
...
>>> import traceback
>>> traceback.print_exception(eg2)
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
ExceptionGroup: two
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in f
ValueError: 2
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
ExceptionGroup: one
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in f
ValueError: 1
>>>
Handling Exception Groups
~~~~~~~~~~~~~~~~~~~~~~~~~
We expect that when programs catch and handle exception groups, they will
typically either query to check if it has leaf exceptions for which some
condition holds (using ``subgroup`` or ``split``) or format the exception
(using the ``traceback`` module's methods).
It is less likely to be useful to iterate over the individual leaf exceptions.
To see why, suppose that an application caught an exception group raised by
an ``asyncio.gather()`` call. At this stage, the context for each specific
exception is lost. Any recovery for this exception should have been performed
before it was grouped with other exceptions [10]_.
Furthermore, the application is likely to react in the same way to any number
of instances of a certain exception type, so it is more likely that we will
want to know whether ``eg.subgroup(T)`` is None or not, than we are to be
interested in the number of ``Ts`` in ``eg``.
However, there are situations where it is necessary to inspect the
individual leaf exceptions. For example, suppose that we have an
exception group ``eg`` and that we want to log the ``OSErrors`` that have a
specific error code and reraise everything else. We can do this by passing
a function with side effects to ``subgroup``, as follows:
.. code-block::
def log_and_ignore_ENOENT(err):
if isinstance(err, OSError) and err.errno == ENOENT:
log(err)
return False
else:
return True
try:
. . .
except ExceptionGroup as eg:
eg = eg.subgroup(log_and_ignore_ENOENT)
if eg is not None:
raise eg
In the previous example, when ``log_and_ignore_ENOENT`` is invoked on a leaf
exception, only part of this exception's traceback is accessible -- the part
referenced from its ``__traceback__`` field. If we need the full traceback,
we need to look at the concatenation of the tracebacks of the exceptions on
the path from the root to this leaf. We can get that with direct iteration,
recursively, as follows:
.. code-block::
def leaf_generator(exc, tbs=None):
if tbs is None:
tbs = []
tbs.append(exc.__traceback__)
if isinstance(exc, ExceptionGroup):
for e in exc.errors:
yield from leaf_generator(e, tbs)
else:
# exc is a leaf exception and its traceback
# is the concatenation of the traceback
# segments in tbs
yield exc, tbs
tbs.pop()
We can then process the full tracebacks of the leaf exceptions:
.. code-block::
>>> import traceback
>>>
>>> def g(v):
... try:
... raise ValueError(v)
... except Exception as e:
... return e
...
>>> def f():
... raise ExceptionGroup("eg", [g(1), g(2)])
...
>>> try:
... f()
... except BaseException as e:
... eg = e
...
>>> for (i, (exc, tbs)) in enumerate(leaf_generator(eg)):
... print(f"\n=== Exception #{i+1}:")
... traceback.print_exception(exc)
... print(f"The complete traceback for Exception #{i+1}:")
... for tb in tbs:
... traceback.print_tb(tb)
...
=== Exception #1:
Traceback (most recent call last):
File "<stdin>", line 3, in g
ValueError: 1
The complete traceback for Exception #1
File "<stdin>", line 2, in <module>
File "<stdin>", line 2, in f
File "<stdin>", line 3, in g
=== Exception #2:
Traceback (most recent call last):
File "<stdin>", line 3, in g
ValueError: 2
The complete traceback for Exception #2:
File "<stdin>", line 2, in <module>
File "<stdin>", line 2, in f
File "<stdin>", line 3, in g
>>>
except*
-------
We are proposing to introduce a new variant of the ``try..except`` syntax to
simplify working with exception groups. The ``*`` symbol indicates that multiple
exceptions can be handled by each ``except*`` clause:
.. code-block::
try:
...
except *SpamError:
...
except *FooError as e:
...
except *(BarError, BazError) as e:
...
In a traditional ``try-except`` statement there is only one exception to handle,
so the body of at most one ``except`` clause executes; the first one that matches
the exception. With the new syntax, an ``except*`` clause can match a subgroup
of the exception group that was raised, while the remaining part is matched
by following ``except*`` clauses. In other words, a single exception group can
cause several ``except*`` clauses to execute, but each such clause executes at
most once (for all matching exceptions from the group) and each exception is
either handled by exactly one clause (the first one that matches its type)
or is reraised at the end.
For example, suppose that the body of the ``try`` block above raises
``eg = ExceptionGroup('msg', [FooError(1), FooError(2), BazError()])``.
The ``except*`` clauses are evaluated in order by calling ``split`` on the
``unhandled`` exception group, which is initially equal to ``eg`` and then shrinks
as exceptions are matched and extracted from it. In the first ``except*`` clause,
``unhandled.split(SpamError)`` returns ``(None, unhandled)`` so the body of this
block is not executed and ``unhandled`` is unchanged. For the second block,
``unhandled.split(FooError)`` returns a non-trivial split ``(match, rest)`` with
``match = ExceptionGroup('msg', [FooError(1), FooError(2)])``
and ``rest = ExceptionGroup('msg', [BazError()])``. The body of this ``except*``
block is executed, with the value of ``e`` and ``sys.exc_info()`` set to ``match``.
Then, ``unhandled`` is set to ``rest``.
Finally, the third block matches the remaining exception so it is executed
with ``e`` and ``sys.exc_info()`` set to ``ExceptionGroup('msg', [BazError()])``.
Exceptions are matched using a subclass check. For example:
.. code-block::
try:
low_level_os_operation()
except *OSError as eg:
for e in eg.errors:
print(type(e).__name__)
could output:
.. code-block::
BlockingIOError
ConnectionRefusedError
OSError
InterruptedError
BlockingIOError
The order of ``except*`` clauses is significant just like with the regular
``try..except``:
.. code-block::
>>> try:
... raise ExceptionGroup("problem", [BlockingIOError()])
... except *OSError as e: # Would catch the error
... print(repr(e))
... except *BlockingIOError: # Would never run
... print('never')
...
ExceptionGroup('problem', [BlockingIOError()])
Recursive Matching
~~~~~~~~~~~~~~~~~~
The matching of ``except*`` clauses against an exception group is performed
recursively, using the ``split()`` method:
.. code-block::
>>> try:
... raise ExceptionGroup(
... "eg",
... [
... ValueError('a'),
... TypeError('b'),
... ExceptionGroup(
... "nested",
... [TypeError('c'), KeyError('d')])
... ]
... )
... except *TypeError as e1:
... print(f'e1 = {e1!r}')
... except *Exception as e2:
... print(f'e2 = {e2!r}')
...
e1 = ExceptionGroup('eg', [TypeError('b'), ExceptionGroup('nested', [TypeError('c')])])
e2 = ExceptionGroup('eg', [ValueError('a'), ExceptionGroup('nested', [KeyError('d')])])
>>>
Unmatched Exceptions
~~~~~~~~~~~~~~~~~~~~
If not all exceptions in an exception group were matched by the ``except*``
clauses, the remaining part of the group is propagated on:
.. code-block::
>>> try:
... try:
... raise ExceptionGroup(
... "msg", [
... ValueError('a'), TypeError('b'),
... TypeError('c'), KeyError('e')
... ]
... )
... except *ValueError as e:
... print(f'got some ValueErrors: {e!r}')
... except *TypeError as e:
... print(f'got some TypeErrors: {e!r}')
... except ExceptionGroup as e:
... print(f'propagated: {e!r}')
...
got some ValueErrors: ExceptionGroup('msg', [ValueError('a')])
got some TypeErrors: ExceptionGroup('msg', [TypeError('b'), TypeError('c')])
propagated: ExceptionGroup('msg', [KeyError('e')])
>>>
Naked Exceptions
~~~~~~~~~~~~~~~~
If the exception raised inside the ``try`` body is not of type ``ExceptionGroup``
or ``BaseExceptionGroup``, we call it a ``naked`` exception. If its type matches
one of the ``except*`` clauses, it is caught and wrapped by an ``ExceptionGroup``
(or ``BaseExceptionGroup`` if it is not an ``Exception`` subclass) with an empty
message string. This is to make the type of ``e`` consistent and statically known:
.. code-block::
>>> try:
... raise BlockingIOError
... except *OSError as e:
... print(repr(e))
...
ExceptionGroup('', [BlockingIOError()])
However, if a naked exception is not caught, it propagates in its original
naked form:
.. code-block::
>>> try:
... try:
... raise ValueError(12)
... except *TypeError as e:
... print('never')
... except ValueError as e:
... print(f'caught ValueError: {e!r}')
...
caught ValueError: ValueError(12)
>>>
Raising exceptions in an ``except*`` block
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a traditional ``except`` block, there are two ways to raise exceptions:
``raise e`` to explicitly raise an exception object ``e``, or naked ``raise`` to
reraise the 'current exception'. When ``e`` is the current exception, the two
forms are not equivalent because a reraise does not add the current frame to
the stack:
.. code-block::
def foo(): | def foo():
try: | try:
1 / 0 | 1 / 0
except ZeroDivisionError as e: | except ZeroDivisionError:
raise e | raise
|
foo() | foo()
|
Traceback (most recent call last): | Traceback (most recent call last):
File "/Users/guido/a.py", line 7 | File "/Users/guido/b.py", line 7
foo() | foo()
File "/Users/guido/a.py", line 5 | File "/Users/guido/b.py", line 3
raise e | 1/0
File "/Users/guido/a.py", line 3 | ZeroDivisionError: division by zero
1/0 |
ZeroDivisionError: division by zero |
This holds for exception groups as well, but the situation is now more complex
because there can be exceptions raised and reraised from multiple ``except*``
clauses, as well as unhandled exceptions that need to propagate.
The interpreter needs to combine all those exceptions into a result, and
raise that.
The reraised exceptions and the unhandled exceptions are subgroups of the
original group, and share its metadata (cause, context, traceback).
On the other hand, each of the explicitly raised exceptions has its own
metadata - the traceback contains the line from which it was raised, its
cause is whatever it may have been explicitly chained to, and its context is the
value of ``sys.exc_info()`` in the ``except*`` clause of the raise.
In the aggregated exception group, the reraised and unhandled exceptions have
the same relative structure as in the original exception, as if they were split
off together in one ``subgroup`` call. For example, in the snippet below the
inner ``try-except*`` block raises an ``ExceptionGroup`` that contains all
``ValueErrors`` and ``TypeErrors`` merged back into the same shape they had in
the original ``ExceptionGroup``:
.. code-block::
>>> try:
... try:
... raise ExceptionGroup(
... "eg",
... [
... ValueError(1),
... TypeError(2),
... OSError(3),
... ExceptionGroup(
... "nested",
... [OSError(4), TypeError(5), ValueError(6)])
... ]
... )
... except *ValueError as e:
... print(f'*ValueError: {e!r}')
... raise
... except *OSError as e:
... print(f'*OSError: {e!r}')
... except ExceptionGroup as e:
... print(repr(e))
...
*ValueError: ExceptionGroup('eg', [ValueError(1), ExceptionGroup('nested', [ValueError(6)])])
*OSError: ExceptionGroup('eg', [OSError(3), ExceptionGroup('nested', [OSError(4)])])
ExceptionGroup('eg', [ValueError(1), TypeError(2), ExceptionGroup('nested', [TypeError(5), ValueError(6)])])
>>>
When exceptions are raised explicitly, they are independent of the original
exception group, and cannot be merged with it (they have their own cause,
context and traceback). Instead, they are combined into a new ``ExceptionGroup``
(or ``BaseExceptionGroup``), which also contains the reraised/unhandled
subgroup described above.
In the following example, the ``ValueErrors`` were raised so they are in their
own ``ExceptionGroup``, while the ``OSErrors`` were reraised so they were
merged with the unhandled ``TypeErrors``.
.. code-block::
>>> try:
... try:
... raise ExceptionGroup(
... "eg",
... [
... ValueError(1),
... TypeError(2),
... OSError(3),
... ExceptionGroup(
... "nested",
... [OSError(4), TypeError(5), ValueError(6)])
... ]
... )
... except *ValueError as e:
... print(f'*ValueError: {e!r}')
... raise e
... except *OSError as e:
... print(f'*OSError: {e!r}')
... raise
... except ExceptionGroup as e:
... traceback.print_exception(e)
...
*ValueError: ExceptionGroup('eg', [ValueError(1), ExceptionGroup('nested', [ValueError(6)])])
*OSError: ExceptionGroup('eg', [OSError(3), ExceptionGroup('nested', [OSError(4)])])
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 12, in <module>
File "<stdin>", line 3, in <module>
ExceptionGroup: eg
------------------------------------------------------------
ValueError: 1
------------------------------------------------------------
ExceptionGroup: nested
------------------------------------------------------------
ValueError: 6
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup: eg
------------------------------------------------------------
TypeError: 2
------------------------------------------------------------
OSError: 3
------------------------------------------------------------
ExceptionGroup: nested
------------------------------------------------------------
OSError: 4
------------------------------------------------------------
TypeError: 5
>>>
Chaining
~~~~~~~~
Explicitly raised exception groups are chained as with any exceptions. The
following example shows how part of ``ExceptionGroup`` "one" became the
context for ``ExceptionGroup`` "two", while the other part was combined with
it into the new ``ExceptionGroup``.
.. code-block::
>>> try:
... try:
... raise ExceptionGroup("one", [ValueError('a'), TypeError('b')])
... except *ValueError:
... raise ExceptionGroup("two", [KeyError('x'), KeyError('y')])
... except BaseException as e:
... traceback.print_exception(e)
...
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup: one
------------------------------------------------------------
ValueError: a
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 5, in <module>
ExceptionGroup: two
------------------------------------------------------------
KeyError: 'x'
------------------------------------------------------------
KeyError: 'y'
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup: one
------------------------------------------------------------
TypeError: b
Raising New Exceptions
~~~~~~~~~~~~~~~~~~~~~~
In the previous examples the explicit raises were of the exceptions that
were caught, so for completion we show a new exception being raised, with
chaining:
.. code-block::
>>> try:
... try:
... raise TypeError('bad type')
... except *TypeError as e:
... raise ValueError('bad value') from e
... except ExceptionGroup as e:
... traceback.print_exception(e)
...
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup
------------------------------------------------------------
ExceptionGroup
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
TypeError: bad type
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<stdin>", line 5, in <module>
ValueError: bad value
>>>
Note that exceptions raised in one ``except*`` clause are not eligible to match
other clauses from the same ``try`` statement:
.. code-block::
>>> try:
... try:
... raise TypeError(1)
... except *TypeError:
... raise ValueError(2) # <- not caught in the next clause
... except *ValueError:
... print('never')
... except ExceptionGroup as e:
... traceback.print_exception(e)
...
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup
------------------------------------------------------------
ExceptionGroup
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
TypeError: 1
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 5, in <module>
ValueError: 2
Raising a new instance of a naked exception does not cause this exception to
be wrapped by an exception group. Rather, the exception is raised as is, and
if it needs to be combined with other propagated exceptions, it becomes a
direct child of the new exception group created for that:
.. code-block::
>>> try:
... try:
... raise ExceptionGroup("eg", [ValueError('a')])
... except *ValueError:
... raise KeyError('x')
... except BaseException as e:
... traceback.print_exception(e)
...
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup: eg
------------------------------------------------------------
ValueError: a
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 5, in <module>
KeyError: 'x'
>>>
>>> try:
... try:
... raise ExceptionGroup("eg", [ValueError('a'), TypeError('b')])
... except *ValueError:
... raise KeyError('x')
... except BaseException as e:
... traceback.print_exception(e)
...
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup: eg
------------------------------------------------------------
ValueError: a
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 5, in <module>
KeyError: 'x'
------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
ExceptionGroup: eg
------------------------------------------------------------
TypeError: b
>>>
Finally, as an example of how the proposed semantics can help us work
effectively with exception groups, the following code ignores all ``EPIPE``
OS errors, while letting all other exceptions propagate.
.. code-block::
try:
low_level_os_operation()
except *OSError as errors:
raise errors.subgroup(lambda e: e.errno != errno.EPIPE) from None
Caught Exception Objects
~~~~~~~~~~~~~~~~~~~~~~~~
It is important to point out that the exception group bound to ``e`` in an
``except*`` clause is an ephemeral object. Raising it via ``raise`` or
``raise e`` will not cause changes to the overall shape of the original
exception group. Any modifications to ``e`` will likely be lost:
.. code-block::
>>> eg = ExceptionGroup("eg", [TypeError(12)])
>>> eg.foo = 'foo'
>>> try:
... raise eg
... except *TypeError as e:
... e.foo = 'bar'
... # ^----------- ``e`` is an ephemeral object that might get
>>> # destroyed after the ``except*`` clause.
>>> eg.foo
'foo'
Forbidden Combinations
~~~~~~~~~~~~~~~~~~~~~~
It is not possible to use both traditional ``except`` blocks and the new
``except*`` clauses in the same ``try`` statement. The following is a
``SyntaxError``:
.. code-block::
try:
...
except ValueError:
pass
except *CancelledError: # <- SyntaxError:
pass # combining ``except`` and ``except*``
# is prohibited
It is possible to catch the ``ExceptionGroup`` and ``BaseExceptionGroup``
types with ``except``, but not with ``except*`` because the latter is
ambiguous:
.. code-block::
try:
...
except ExceptionGroup: # <- This works
pass
try:
...
except *ExceptionGroup: # <- Runtime error
pass
try:
...
except *(TypeError, ExceptionGroup): # <- Runtime error
pass
An empty "match anything" ``except*`` block is not supported as its meaning may
be confusing:
.. code-block::
try:
...
except*: # <- SyntaxError
pass
``continue``, ``break``, and ``return`` are disallowed in ``except*`` clauses,
causing a ``SyntaxError``. This is because the exceptions in an
``ExceptionGroup`` are assumed to be independent, and the presence or absence
of one of them should not impact handling of the others, as could happen if we
allow an ``except*`` clause to change the way control flows through other
clauses.
Backwards Compatibility
=======================
Backwards compatibility was a requirement of our design, and the changes we
propose in this PEP will not break any existing code:
* The addition of the new builtin exception types ``ExceptionGroup`` and
``BaseExceptionGroup`` does not impact existing programs. The way that
existing exceptions are handled and displayed does not change in any way.
* The behaviour of ``except`` is unchanged so existing code will continue to work.
Programs will only be impacted by the changes proposed in this PEP once they
begin to use exception groups and ``except*``.
* An important concern was that ``except Exception:`` will continue to catch
almost all exceptions, and by making ``ExceptionGroup`` extend ``Exception``
we ensured that this will be the case. ``BaseExceptionGroups`` will not be
caught, which is appropriate because they include exceptions that would not
have been caught by ``except Exception``.
Once programs begin to use these features, there will be migration issues to
consider:
* An ``except T:`` clause that wraps code which is now potentially raising
an exception group may need to become ``except *T:``, and its body may
need to be updated. This means that raising an exception group is an
API-breaking change and will likely be done in new APIs rather than
added to existing ones.
* Libraries that need to support older Python versions will not be able to use
``except*`` or raise exception groups.
How to Teach This
=================
Exception groups and ``except*`` will be documented as part of the language
standard. Libraries that raise exception groups such as ``asyncio`` will need
to specify this in their documentation and clarify which API calls need to be
wrapped with ``try-except*`` rather than ``try-except``.
Reference Implementation
========================
We developed these concepts (and the examples for this PEP) with
the help of the reference implementation [11]_.
It has the builtin ``ExceptionGroup`` along with the changes to the traceback
formatting code, in addition to the grammar, compiler and interpreter changes
required to support ``except*``. ``BaseExceptionGroup`` will be added
soon.
Two opcodes were added: one implements the exception type match check via
``ExceptionGroup.split()``, and the other is used at the end of a ``try-except``
construct to merge all unhandled, raised and reraised exceptions (if any).
The raised/reraised exceptions are collected in a list on the runtime stack.
For this purpose, the body of each ``except*`` clause is wrapped in a traditional
``try-except`` which captures any exceptions raised. Both raised and reraised
exceptions are collected in the same list. When the time comes to merge them
into a result, the raised and reraised exceptions are distinguished by comparing
their metadata fields (context, cause, traceback) with those of the originally
raised exception. As mentioned above, the reraised exceptions have the same
metadata as the original, while the raised ones do not.
Rejected Ideas
==============
Make Exception Groups Iterable
------------------------------
We considered making exception groups iterable, so that ``list(eg)`` would
produce a flattened list of the leaf exceptions contained in the group.
We decided that this would not be a sound API, because the metadata
(cause, context and traceback) of the individual exceptions in a group is
incomplete and this could create problems.
Furthermore, as we explained in the `Handling Exception Groups`_ section, we
find it unlikely that iteration over leaf exceptions will have many use cases.
We did, however, provide there the code for a traversal algorithm that
correctly constructs each leaf exceptions' metadata. If it does turn out to
be useful in practice, we can add that utility to the standard library.
Traceback Representation
------------------------
We considered options for adapting the traceback data structure to represent
trees, but it became apparent that a traceback tree is not meaningful once
separated from the exceptions it refers to. While a simple-path traceback can
be attached to any exception by a ``with_traceback()`` call, it is hard to
imagine a case where it makes sense to assign a traceback tree to an exception
group. Furthermore, a useful display of the traceback includes information
about the nested exceptions. For these reasons we decided that it is best to
leave the traceback mechanism as it is and modify the traceback display code.
Extend ``except`` to Handle Exception Groups
---------------------------------------------
We considered extending the semantics of ``except`` to handle
exception groups, instead of introducing ``except*``. There were two
backwards compatibility concerns with this. The first is the type of the
caught exception. Consider this example:
.. code-block::
try:
. . .
except OSError as err:
if err.errno != ENOENT:
raise
If the value assigned to err is an exception group containing all of
the ``OSErrors`` that were raised, then the attribute access ``err.errno``
no longer works. So we would need to execute the body of the ``except``
clause multiple times, once for each exception in the group. However, this
too is a potentially breaking change because at the moment we write ``except``
clauses with the knowledge that they are only executed once. If there is
a non-idempotent operation there, such as releasing a resource, the
repetition could be harmful.
A New ``except`` Alternative
----------------------------
We considered introducing a new keyword (such as ``catch``) which can be used
to handle both naked exceptions and exception groups. Its semantics would
be the same as those of ``except*`` when catching an exception group, but
it would not wrap a naked exception to create an exception group. This
would have been part of a long term plan to replace ``except`` by ``catch``,
but we decided that deprecating ``except`` in favour of an enhanced keyword
would be too confusing for users at this time, so it is more appropriate
to introduce the ``except*`` syntax for exception groups while ``except``
continues to be used for simple exceptions.
Applying an ``except*`` Clause on One Exception at a Time
---------------------------------------------------------
We explained above that it is unsafe to execute an ``except`` clause in
existing code more than once, because the code may not be idempotent.
We considered doing this in the new ``except*`` clauses,
where the backwards compatibility considerations do not exist.
The idea is to always execute an ``except*`` clause on a single exception,
possibly executing the same clause multiple times when it matches multiple
exceptions. We decided instead to execute each ``except*`` clause at most
once, giving it an exception group that contains all matching exceptions. The
reason for this decision was the observation that when a program needs to know
the particular context of an exception it is handling, the exception is
handled before it is grouped and raised together with other exceptions.
For example, ``KeyError`` is an exception that typically relates to a certain
operation. Any recovery code would be local to the place where the error
occurred, and would use the traditional ``except``:
.. code-block::
try:
dct[key]
except KeyError:
# handle the exception
It is unlikely that asyncio users would want to do something like this:
.. code-block::
try:
async with asyncio.TaskGroup() as g:
g.create_task(task1); g.create_task(task2)
except *KeyError:
# handling KeyError here is meaningless, there's
# no context to do anything with it but to log it.
When a program handles a collection of exceptions that were aggregated into
an exception group, it would not typically attempt to recover from any
particular failed operation, but will rather use the types of the errors to
determine how they should impact the program's control flow or what logging
or cleanup is required. This decision is likely to be the same whether the group
contains a single or multiple instances of something like a ``KeyboardInterrupt``
or ``asyncio.CancelledError``. Therefore, it is more convenient to handle all
exceptions matching an ``except*`` at once. If it does turn out to be necessary,
the handler can inpect the exception group and process the individual
exceptions in it.
Not Matching Naked Exceptions in ``except*``
--------------------------------------------
We considered the option of making ``except *T`` match only exception groups
that contain ``Ts``, but not naked ``Ts``. To see why we thought this would
not be a desirable feature, return to the distinction in the previous paragraph
between operation errors and control flow exceptions. If we don't know whether
we should expect naked exceptions or exception groups from the body of a
``try`` block, then we're not in the position of handling operation errors.
Rather, we are likely calling a fairly generic function and will be handling
errors to make control flow decisions. We are likely to do the same thing
whether we catch a naked exception of type ``T`` or an exception group
with one or more ``Ts``. Therefore, the burden of having to explicitly handle
both is not likely to have semantic benefit.
If it does turn out to be necessary to make the distinction, it is always
possible to nest in the ``try-except*`` clause an additional ``try-except``
clause which intercepts and handles a naked exception before the ``except*``
clause has a chance to wrap it in an exception group. In this case the
overhead of specifying both is not additional burden - we really do need to
write a separate code block to handle each case:
.. code-block::
try:
try:
...
except SomeError:
# handle the naked exception
except *SomeError:
# handle the exception group
Allow mixing ``except:`` and ``except*:`` in the same ``try``
-------------------------------------------------------------
This option was rejected because it adds complexity without adding useful
semantics. Presumably the intention would be that an ``except T:`` block handles
only naked exceptions of type ``T``, while ``except *T:`` handles ``T`` in
exception groups. We already discussed above why this is unlikely
to be useful in practice, and if it is needed then the nested ``try-except``
block can be used instead to achieve the same result.
``try*`` instead of ``except*``
-------------------------------
Since either all or none of the clauses of a ``try`` construct are ``except*``,
we considered changing the syntax of the ``try`` instead of all the ``except*``
clauses. We rejected this because it would be less obvious. The fact that we
are handling exception groups of ``T`` rather than only naked ``Ts`` should be
specified in the same place where we state ``T``.
Programming Without 'except \*'
===============================
Consider the following simple example of the ``except *`` syntax (pretending
Trio natively supported this proposal):
.. code-block::
try:
async with trio.open_nursery() as nursery:
# Make two concurrent calls to child()
nursery.start_soon(child)
nursery.start_soon(child)
except *ValueError:
pass
Here is how this code would look in Python 3.9:
.. code-block::
def handle_ValueError(exc):
if isinstance(exc, ValueError):
return None
else:
return exc # reraise exc
with MultiError.catch(handle_ValueError):
async with trio.open_nursery() as nursery:
# Make two concurrent calls to child()
nursery.start_soon(child)
nursery.start_soon(child)
This example clearly demonstrates how unintuitive and cumbersome handling
of multiple errors is in current Python. The exception handling logic has
to be in a separate closure and is fairly low level, requiring the writer to
have non-trivial understanding of both Python exceptions mechanics and the
Trio APIs. Instead of using the ``try..except`` block we have to use a
``with`` block. We need to explicitly reraise exceptions we are not handling.
Handling more exception types or implementing more complex
exception handling logic will only further complicate the code to the point
of it being unreadable.
See Also
========
* An analysis of how exception groups will likely be used in asyncio
programs: [10]_.
* The issue where the ``except*`` concept was first formalized: [12]_.
* ``MultiError2`` design document: [3]_.
* Reporting Multiple Errors in the Hypothesis library: [7]_.
References
==========
.. [1] https://docs.python.org/3/library/asyncio-task.html#asyncio.gather
.. [2] https://trio.readthedocs.io/en/stable/
.. [3] https://github.com/python-trio/trio/issues/611
.. [4] https://bugs.python.org/issue29980
.. [5] https://docs.python.org/3/library/atexit.html#atexit.register
.. [6] https://github.com/pytest-dev/pytest/issues/8217
.. [7] https://hypothesis.readthedocs.io/en/latest/settings.html#hypothesis.settings.report_multiple_bugs
.. [8] https://bugs.python.org/issue40857
.. [9] https://trio.readthedocs.io/en/stable/reference-core.html#trio.MultiError
.. [10] https://github.com/python/exceptiongroups/issues/3#issuecomment-716203284
.. [11] https://github.com/iritkatriel/cpython/tree/exceptionGroup-stage5
.. [12] https://github.com/python/exceptiongroups/issues/4
.. [13] https://trio.readthedocs.io/en/stable/reference-core.html#nurseries-and-spawning
Copyright
=========
This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.
..
Local Variables:
mode: indented-text
indent-tabs-mode: nil
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