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PEP 654 -- Exception Groups and except*

PEP:654
Title:Exception Groups and except*
Author:Irit Katriel <iritkatriel at gmail.com>, Yury Selivanov <yury at edgedb.com>, Guido van Rossum <guido at python.org>
Status:Draft
Type:Standards Track
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:

>>> 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:

>>> 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:

>>> 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:

>>> 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:

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:

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:

>>> 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:

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:

try:
    low_level_os_operation()
except *OSError as eg:
    for e in eg.errors:
        print(type(e).__name__)

could output:

BlockingIOError
ConnectionRefusedError
OSError
InterruptedError
BlockingIOError

The order of except* clauses is significant just like with the regular try..except:

>>> 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:

>>> 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:

>>> 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:

>>> 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:

>>> 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:

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:

>>> 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.

>>> 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.

>>> 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:

>>> 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:

>>> 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:

>>> 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.

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:

>>> 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:

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:

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:

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:

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:

try:
    dct[key]
except KeyError:
    # handle the exception

It is unlikely that asyncio users would want to do something like this:

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:

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):

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:

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].
Source: https://github.com/python/peps/blob/master/pep-0654.rst