bpo-42669: Document that `except` rejects nested tuples #23822

cjwatson · 2020-12-17T13:30:43Z

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:

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. However, the reference documentation was never updated to match this new restriction. Make it clear that the definition is no longer recursive.

https://bugs.python.org/issue42669

Automerge-Triggered-By: GH:ericvsmith

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: 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. However, the reference documentation was never updated to match this new restriction. Make it clear that the definition is no longer recursive.

ericvsmith · 2020-12-20T02:18:25Z

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.  (Nested tuples are not allowed.)


With this reworded sentence, I don't think the parenthesized statement is needed.

ericvsmith · 2020-12-20T02:18:25Z

Misc/NEWS.d/next/Documentation/2020-12-17-14-23-16.bpo-42669.fjuDXY.rst

@@ -0,0 +1 @@
+Explain that :keyword:`except` no longer accepts nested tuples.  (This was changed in Python 3.0, but not previously documented.)


I don't think we need the blurb for this change. It's not substantive enough to be in the News file.

Eric V. Smith thinks this isn't a substantive enough change for that, so I'll defer.

cjwatson · 2020-12-20T18:11:01Z

I have made the requested changes; please review again.

bedevere-bot · 2020-12-20T18:11:04Z

Thanks for making the requested changes!

: please review the changes made to this pull request.

ericvsmith

Looks good. Thanks!

miss-islington · 2020-12-20T18:24:09Z

@cjwatson: Status check is done, and it's a success ✅ .

miss-islington · 2020-12-20T18:24:13Z

Thanks @cjwatson for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8, 3.9.
🐍🍒⛏🤖

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: 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. 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 c95f8bc) Co-authored-by: Colin Watson <cjwatson@debian.org>

bedevere-bot · 2020-12-20T18:24:25Z

GH-23870 is a backport of this pull request to the 3.9 branch.

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: 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. 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 c95f8bc) Co-authored-by: Colin Watson <cjwatson@debian.org>

bedevere-bot · 2020-12-20T18:24:35Z

GH-23871 is a backport of this pull request to the 3.8 branch.

…H-23870) 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: 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. 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 c95f8bc) Co-authored-by: Colin Watson <cjwatson@debian.org> Co-authored-by: Colin Watson <cjwatson@debian.org>

…H-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: 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. 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 c95f8bc) Co-authored-by: Colin Watson <cjwatson@debian.org> Co-authored-by: Colin Watson <cjwatson@debian.org>

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: 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. 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

the-knights-who-say-ni added the CLA signed label Dec 17, 2020

bedevere-bot added the awaiting review label Dec 17, 2020

📜🤖 Added by blurb_it.

aaa4548

ericvsmith reviewed Dec 20, 2020

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cjwatson added 2 commits Dec 20, 2020

Remove redundant parenthesised statement

9f4986c

Remove blurb

80fcc52

Eric V. Smith thinks this isn't a substantive enough change for that, so I'll defer.

bedevere-bot added awaiting change review and removed awaiting review labels Dec 20, 2020

ericvsmith added skip news 🤖 automerge PR will be merged once it's been approved and all CI passed labels Dec 20, 2020

ericvsmith approved these changes Dec 20, 2020

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bedevere-bot added awaiting merge and removed awaiting change review labels Dec 20, 2020

ericvsmith added needs backport to 3.8 needs backport to 3.9 labels Dec 20, 2020

miss-islington merged commit c95f8bc into python:master Dec 20, 2020
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bedevere-bot removed the needs backport to 3.9 label Dec 20, 2020

bedevere-bot removed the needs backport to 3.8 label Dec 20, 2020

bpo-42669: Document that `except` rejects nested tuples #23822

bpo-42669: Document that `except` rejects nested tuples #23822

cjwatson commented Dec 17, 2020 •

edited by miss-islington

ericvsmith Dec 20, 2020

ericvsmith Dec 20, 2020

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miss-islington commented Dec 20, 2020

miss-islington commented Dec 20, 2020

bedevere-bot commented Dec 20, 2020

bedevere-bot commented Dec 20, 2020

		@@ -0,0 +1 @@
		Explain that :keyword:`except` no longer accepts nested tuples. (This was changed in Python 3.0, but not previously documented.)

bpo-42669: Document that except rejects nested tuples #23822

bpo-42669: Document that except rejects nested tuples #23822

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cjwatson commented Dec 17, 2020 • edited by miss-islington

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bedevere-bot commented Dec 20, 2020

bpo-42669: Document that `except` rejects nested tuples #23822

bpo-42669: Document that `except` rejects nested tuples #23822

cjwatson commented Dec 17, 2020 •

edited by miss-islington