New submission from Colin Watson <[email protected]>:
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 (this is real code used in several places in
https://git.launchpad.net/launchpad):
try:
self.getInputValue()
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. The change was mentioned
in https://bugs.python.org/issue2380 and seems to have been intentional: I'm
not requesting that the previous behaviour be restored, since it's a fairly
rare porting issue and by now well-established in Python 3.
However, the relevant sentences of documentation in
https://docs.python.org/2/reference/compound_stmts.html#try and
https://docs.python.org/3/reference/compound_stmts.html#the-try-statement are
identical aside from punctuation, and they both read:
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.
I think this admits a recursive reading: I certainly read it that way. It
should make it clear that nested tuples are not allowed.
----------
assignee: docs@python
components: Documentation
messages: 383243
nosy: cjwatson, docs@python
priority: normal
severity: normal
status: open
title: "except" documentation still suggests nested tuples are allowed
versions: Python 3.10
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<https://bugs.python.org/issue42669>
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