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PEP 601: Fix footnotes (#2736) 

Co-authored-by: CAM Gerlach <CAM.Gerlach@Gerlach.CAM>
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Hugo van Kemenade 2022-07-28 12:41:30 +03:00 committed by GitHub
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@ -25,21 +25,21 @@ is a conditional return (or break etc.) inside a finally block.".
Abstract
========
This PEP proposes to forbid return, break and continue statements within
a finally suite where they would break out of the finally. Their use in
This PEP proposes to forbid ``return``, ``break`` and ``continue`` statements within
a ``finally`` suite where they would break out of the ``finally``. Their use in
such a location silently cancels any active exception being raised through
the finally, leading to unclear code and possible bugs.
the ``finally``, leading to unclear code and possible bugs.
Continue is currently not supported in a finally in Python 3.7 (due to
``continue`` is currently not supported in a ``finally`` in Python 3.7 (due to
implementation issues) and the proposal is to not add support for it in
Python 3.8. For return and break the proposal is to deprecate their use
Python 3.8. For ``return`` and ``break`` the proposal is to deprecate their use
in Python 3.9, emit a compilation warning in Python 3.10 and then forbid
their use after that.
Motivation
==========
The use of return, break and continue within a finally suite leads to behaviour
The use of ``return``, ``break`` and ``continue`` within a ``finally`` suite leads to behaviour
which is not at all obvious. Consider the following function::
def foo():
@ -49,9 +49,9 @@ which is not at all obvious. Consider the following function::
return
This will return cleanly (without an exception) even though it has infinite
recursion and raises an exception within the try. The reason is that the return
within the finally will silently cancel any exception that propagates through
the finally suite. Such behaviour is unexpected and not at all obvious.
recursion and raises an exception within the ``try``. The reason is that the ``return``
within the ``finally`` will silently cancel any exception that propagates through
the ``finally`` suite. Such behaviour is unexpected and not at all obvious.
This function is equivalent to::
def foo():
@ -61,8 +61,8 @@ This function is equivalent to::
pass
return
Break and continue have similar behaviour (they silence exceptions) if they
jump to code outside the finally suite. For example::
``break`` and ``continue`` have similar behaviour (they silence exceptions) if they
jump to code outside the ``finally`` suite. For example::
def bar():
while True:
@ -83,35 +83,35 @@ This behaviour goes against the following parts of The Zen of Python:
If this behaviour of silencing exceptions is really needed then the explicit
form of a try-except can be used instead, and this makes the code clearer.
Independent to the semantics, implementing return/break/continue within a
finally suite is non-trivial as it requires to correctly track any active
exceptions at runtime (an executing finally suite may or may not have an
Independent to the semantics, implementing ``return``/``break``/``continue`` within a
``finally`` suite is non-trivial as it requires to correctly track any active
exceptions at runtime (an executing ``finally`` suite may or may not have an
active exception) and cancel them as appropriate. CPython did have a bug in
this for the case of continue and so originally disallowed it [1]. Requiring
correct behaviour for return/break/continue within a finally puts an
this for the case of ``continue`` and so originally disallowed it [1]_. Requiring
correct behaviour for ``return``/``break``/``continue`` within a ``finally`` puts an
unnecessary burden on alternative implementations of Python.
Other languages
===============
Java allows to return from within a finally block, but its use is discouraged
according to [2], [3], [4]. The Java compiler later on included a linting
option -Xlint:finally to warn against the use of return within a finally block.
Java allows to return from within a ``finally`` block, but its use is discouraged
according to [2]_, [3]_, [4]_. The Java compiler later on included a linting
option ``-Xlint:finally`` to warn against the use of return within a ``finally`` block.
The Eclipse editor also warns about this use.
Ruby allows return from inside ensure (Python's finally), but it should be an
explicit return. It is discouraged and handled by linters [5], [6].
explicit return. It is discouraged and handled by linters [5]_, [6]_.
Like Ruby, JavaScript also allows use of return/break/continue within a finally
but it is seen as unsafe and it is handled by eslint [7].
Like Ruby, JavaScript also allows use of ``return``/``break``/``continue`` within a ``finally``
but it is seen as unsafe and it is handled by eslint [7]_.
C# forbids the use of ending statements like return/goto/break within a finally
[8], [9].
C# forbids the use of ending statements like ``return``/``goto``/``break`` within a ``finally``
[8]_, [9]_.
Rationale
=========
Since the behaviour of return/break/continue within a finally is unclear, the
Since the behaviour of ``return``/``break``/``continue`` within a ``finally`` is unclear, the
pattern is rarely used, and there is a simple alternative to writing equivalent
code (which is more explicit), forbidding the syntax is the most straightforward
approach.
@ -119,19 +119,19 @@ approach.
Specification
=============
This is a change to the compiler, not the grammar. The compiler should
check for the following in a finally suite:
This is a change to the compiler, not the grammar. The compiler should
check for the following in a ``finally`` suite:
* A return in any statement, at any level of nesting.
* A ``return`` in any statement, at any level of nesting.
* A break/continue in any statement, at any level of nesting, that would
transfer control flow outside the finally suite.
* A ``break``/``continue`` in any statement, at any level of nesting, that would
transfer control flow outside the ``finally`` suite.
Upon finding such a case it should emit the appropriate exception:
* For continue, a SyntaxError (this is the current behaviour of 3.7).
* For ``continue``, a ``SyntaxError`` (this is the current behaviour of 3.7).
* For return/break, a SyntaxWarning in 3.10, and a SyntaxError after that.
* For ``return``/``break``, a ``SyntaxWarning`` in 3.10, and a ``SyntaxError`` after that.
For example, the following are all forbidden by this proposal::
@ -160,8 +160,8 @@ For example, the following are all forbidden by this proposal::
for x in range(10):
return
The following is still allowed because the continue doesn't escape the
finally::
The following is still allowed because the ``continue`` doesn't escape the
``finally``::
try:
pass
@ -169,17 +169,17 @@ finally::
for x in range(10):
continue
Note that yielding from within a finally remains acceptable by this PEP
because resuming the generator will resume the finally and eventually
Note that yielding from within a ``finally`` remains acceptable by this PEP
because resuming the generator will resume the ``finally`` and eventually
raise any active exceptions (so they are never silenced by yielding).
Backwards Compatibility
=======================
This is a backwards incompatible change, for return and break.
This is a backwards incompatible change, for ``return`` and ``break``.
The following locations in the CPython standard library (at
v3.8.0b1-651-g7fcc2088a5) use return within finally:
v3.8.0b1-651-g7fcc2088a5) use ``return`` within ``finally``:
* Lib/subprocess.py:921 - the use here looks like a bug
@ -189,11 +189,11 @@ v3.8.0b1-651-g7fcc2088a5) use return within finally:
* Lib/multiprocessing/connection.py:318 - the use here looks legitimate
but the intention is not clear
* Lib/test/test_sys_settrace.py:837 - a test for return within finally
* Lib/test/test_sys_settrace.py:837 - a test for ``return`` within ``finally``
* Lib/test/test_sys_settrace.py:1346 - a test for return within finally
* Lib/test/test_sys_settrace.py:1346 - a test for ``return`` within ``finally``
There are no uses of break within a finally (that break out of the finally)
There are no uses of ``break`` within a ``finally`` (that break out of the ``finally``)
in the standard library.
Security Implications
@ -207,20 +207,20 @@ How to Teach This
This feature is very rarely used so forbidding it will likely only impact
advanced users, not beginners and probably not any existing teaching
material. Since this is the removal of a feature teaching users will be
one by the raising of a SyntaxError if/when the forbidden feature is used.
material. Since this is the removal of a feature teaching users will be
one by the raising of a ``SyntaxError`` if/when the forbidden feature is used.
Reference Implementation
========================
There is currently no reference implementation, although the way continue
is currently handled in a finally (raising a SyntaxError) can be extended
to return and break.
is currently handled in a ``finally`` (raising a ``SyntaxError``) can be extended
to ``return`` and ``break``.
References
==========
.. [1] https://bugs.python.org/issue37830
.. [1] https://github.com/python/cpython/issues/82011
.. [2] https://stackoverflow.com/questions/48088/returning-from-a-finally-block-in-java
@ -228,9 +228,9 @@ References
.. [4] https://wiki.sei.cmu.edu/confluence/display/java/ERR04-J.+Do+not+complete+abruptly+from+a+finally+block
.. [5] https://github.com/rubocop-hq/rubocop/issues/5949
.. [5] https://github.com/rubocop/rubocop/issues/5949
.. [6] https://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Lint/EnsureReturn
.. [6] https://www.rubydoc.info/gems/rubocop/0.74.0/RuboCop/Cop/Lint/EnsureReturn
.. [7] https://eslint.org/docs/rules/no-unsafe-finally
@ -243,14 +243,3 @@ Copyright
This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.
..
Local Variables:
mode: indented-text
indent-tabs-mode: nil
sentence-end-double-space: t
fill-column: 70
coding: utf-8
End: