[Python-checkins] bpo-42669: Document that `except` rejects nested tuples (GH-23822) (GH-23871)

ericvsmith webhook-mailer at python.org
Sun Dec 20 16:18:46 EST 2020

commit: 81f706d2db0f57c4fdd747df6e0a4cffcbc54704
branch: 3.8
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: ericvsmith <ericvsmith at users.noreply.github.com>
date: 2020-12-20T16:18:40-05:00

bpo-42669: Document that `except` rejects nested tuples (GH-23822) (GH-23871)

In Python 2, it was possible to use `except` with a nested tuple, and occasionally natural.  For example, `zope.formlib.interfaces.InputErrors` is a tuple of several exception classes, and one might reasonably think to do something like this:

        return True
    except (InputErrors, SomethingElse):
        return False

As of Python 3.0, this raises `TypeError: catching classes that do not inherit from BaseException is not allowed` instead: one must instead either break it up into multiple `except` clauses or flatten the tuple.  However, the reference documentation was never updated to match this new restriction.  Make it clear that the definition is no longer recursive.

Automerge-Triggered-By: GH:ericvsmith
(cherry picked from commit c95f8bc2700b42f4568886505a819816c9b0ba28)

Co-authored-by: Colin Watson <cjwatson at debian.org>

Co-authored-by: Colin Watson <cjwatson at debian.org>

M Doc/reference/compound_stmts.rst

diff --git a/Doc/reference/compound_stmts.rst b/Doc/reference/compound_stmts.rst
index 6900c7ac7d071..61037d97a0c3c 100644
--- a/Doc/reference/compound_stmts.rst
+++ b/Doc/reference/compound_stmts.rst
@@ -254,7 +254,8 @@ present, must be last; it matches any exception.  For an except clause with an
 expression, that expression is evaluated, and the clause matches the exception
 if the resulting object is "compatible" with the exception.  An object is
 compatible with an exception if it is the class or a base class of the exception
-object or a tuple containing an item compatible with the exception.
+object, or a tuple containing an item that is the class or a base class of
+the exception object.
 If no except clause matches the exception, the search for an exception handler
 continues in the surrounding code and on the invocation stack.  [#]_
diff --git a/Misc/ACKS b/Misc/ACKS
index 8afc0a8e46d57..6ae882479d436 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1792,6 +1792,7 @@ Zachary Ware
 Barry Warsaw
 Steve Waterbury
 Bob Watson
+Colin Watson
 David Watson
 Aaron Watters
 Henrik Weber

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