Replace MultiError with (Base)ExceptionGroup

Closes #2211.

docs/source/design.rst

@@ -465,9 +465,6 @@ There are two notable sub-modules that are largely independent of
 the rest of Trio, and could (possibly should?) be extracted into their
 own independent packages:
 
-* ``_multierror.py``: Implements :class:`MultiError` and associated
-  infrastructure.
-
 * ``_ki.py``: Implements the core infrastructure for safe handling of
   :class:`KeyboardInterrupt`.
 

docs/source/reference-core.rst

@@ -641,7 +641,7 @@ crucial things to keep in mind:
 
   * Any unhandled exceptions are re-raised inside the parent task. If
     there are multiple exceptions, then they're collected up into a
-    single :exc:`MultiError` exception.
+    single :exc:`BaseExceptionGroup` or :exc:`ExceptionGroup` exception.
 
 Since all tasks are descendents of the initial task, one consequence
 of this is that :func:`run` can't finish until all tasks have
@@ -712,14 +712,9 @@ limitation. Consider code like::
 what? In some sense, the answer should be "both of these at once", but
 in Python there can only be one exception at a time.
 
-Trio's answer is that it raises a :exc:`MultiError` object. This is a
+Trio's answer is that it raises a :exc:`BaseExceptionGroup` object. This is a
 special exception which encapsulates multiple exception objects –
-either regular exceptions or nested :exc:`MultiError`\s. To make these
-easier to work with, Trio installs a custom `sys.excepthook` that
-knows how to print nice tracebacks for unhandled :exc:`MultiError`\s,
-and it also provides some helpful utilities like
-:meth:`MultiError.catch`, which allows you to catch "part of" a
-:exc:`MultiError`.
+either regular exceptions or nested :exc:`BaseExceptionGroup`\s.
 
 
 Spawning tasks without becoming a parent
@@ -837,104 +832,6 @@ The nursery API
    See :meth:`~Nursery.start`.
 
 
-Working with :exc:`MultiError`\s
-++++++++++++++++++++++++++++++++
-
-.. autoexception:: MultiError
-
-   .. attribute:: exceptions
-
-      The list of exception objects that this :exc:`MultiError`
-      represents.
-
-   .. automethod:: filter
-
-   .. automethod:: catch
-      :with:
-
-Examples:
-
-Suppose we have a handler function that discards :exc:`ValueError`\s::
-
-    def handle_ValueError(exc):
-        if isinstance(exc, ValueError):
-            return None
-        else:
-            return exc
-
-Then these both raise :exc:`KeyError`::
-
-    with MultiError.catch(handle_ValueError):
-         raise MultiError([KeyError(), ValueError()])
-
-    with MultiError.catch(handle_ValueError):
-         raise MultiError([
-             ValueError(),
-             MultiError([KeyError(), ValueError()]),
-         ])
-
-And both of these raise nothing at all::
-
-    with MultiError.catch(handle_ValueError):
-         raise MultiError([ValueError(), ValueError()])
-
-    with MultiError.catch(handle_ValueError):
-         raise MultiError([
-             MultiError([ValueError(), ValueError()]),
-             ValueError(),
-         ])
-
-You can also return a new or modified exception, for example::
-
-    def convert_ValueError_to_MyCustomError(exc):
-        if isinstance(exc, ValueError):
-            # Similar to 'raise MyCustomError from exc'
-            new_exc = MyCustomError(...)
-            new_exc.__cause__ = exc
-            return new_exc
-        else:
-            return exc
-
-In the example above, we set ``__cause__`` as a form of explicit
-context chaining. :meth:`MultiError.filter` and
-:meth:`MultiError.catch` also perform implicit exception chaining – if
-you return a new exception object, then the new object's
-``__context__`` attribute will automatically be set to the original
-exception.
-
-We also monkey patch :class:`traceback.TracebackException` to be able
-to handle formatting :exc:`MultiError`\s. This means that anything that
-formats exception messages like :mod:`logging` will work out of the
-box::
-
-    import logging
-
-    logging.basicConfig()
-
-    try:
-        raise MultiError([ValueError("foo"), KeyError("bar")])
-    except:
-        logging.exception("Oh no!")
-        raise
-
-Will properly log the inner exceptions:
-
-.. code-block:: none
-
-    ERROR:root:Oh no!
-    Traceback (most recent call last):
-      File "<stdin>", line 2, in <module>
-    trio.MultiError: ValueError('foo',), KeyError('bar',)
-
-    Details of embedded exception 1:
-
-      ValueError: foo
-
-    Details of embedded exception 2:
-
-      KeyError: 'bar'
-
-
 .. _task-local-storage:
 
 Task-local storage

docs/source/tutorial.rst

@@ -13,14 +13,13 @@ Tutorial
    still probably read this, because Trio is different.)
 
    Trio turns Python into a concurrent language. It takes the core
-   async/await syntax introduced in 3.5, and uses it to add three
+   async/await syntax introduced in 3.5, and uses it to add two
    new pieces of semantics:
 
    - cancel scopes: a generic system for managing timeouts and
      cancellation
    - nurseries: which let your program do multiple things at the same
      time
-   - MultiErrors: for when multiple things go wrong at once
 
    Of course it also provides a complete suite of APIs for doing
    networking, file I/O, using worker threads,
@@ -57,8 +56,6 @@ Tutorial
    and demonstrate start()
    then point out that you can just use serve_tcp()
 
-   exceptions and MultiError
-
    example: catch-all logging in our echo server
 
    review of the three (or four) core language extensions
@@ -1149,9 +1146,6 @@ TODO: explain :exc:`Cancelled`
 TODO: explain how cancellation is also used when one child raises an
 exception
 
-TODO: show an example :exc:`MultiError` traceback and walk through its
-structure
-
 TODO: maybe a brief discussion of :exc:`KeyboardInterrupt` handling?
 
 ..

newsfragments/2211.removal.rst

@@ -0,0 +1,4 @@
+``trio.MultiError`` has been removed in favor of the built-in ``BaseExceptionGroup``
+(and its derivative ``ExceptionGroup``), falling back to the backport_ on Python < 3.11.
+
+.. _backport: https://pypi.org/project/exceptiongroup/

trio/__init__.py

@@ -22,7 +22,6 @@
     Cancelled,
     BusyResourceError,
     ClosedResourceError,
-    MultiError,
     run,
     open_nursery,
     CancelScope,

trio/_core/__init__.py

@@ -17,8 +17,6 @@
     EndOfChannel,
 )
 
-from ._multierror import MultiError
-
 from ._ki import (
     enable_ki_protection,
     disable_ki_protection,