Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Neil Schemenauer]

On 2018-01-04, Guido van Rossum wrote:
> We should interview you for the paper we may be writing for HOPL.

History of Programming Languages?

I did some more digging this afternoon, trying to find source code
between versions 1.0.1 and 0.9.1.  No luck though.  It looks like
0.9.1 might have been the last one you uploaded to alt.sources.
Later 0.9.X releases were uploaded to ftp.cwi.nl and
wuarchive.wustle.edu.  No one seems to have an archive of those.

I think all my old PCs have been sent to the scrapyard.  I might
have some old hard disk images somewhere.  Maybe on a writable DVD
or CDR.  Probably unreadable at this point.  I don't know exactly
which version of Python I first downloaded.  No earlier than the
fall of 1992 and maybe 1993 but it could have been pre-1.0.  I do
recall running a DOS port at some point.

Here is the announcement of 0.9.4alpha:

http://legacy.python.org/search/hypermail/python-1992/0270.html

The Misc/HISTORY file has quite a lot of details.  It shows that
'continue' was added in 0.9.2.

Back on topic, it looks like allowing 'continue' will be trival once
Serhiy's unwind stack PR lands.  Just a few lines of code and I
think everything works.  If Mark implements his alternative
"wordcode for finally blocks gets copied" thing, it will make things
more complicated but not much more so than handling 'break' and
'return'.  So, those three should probably be all allowed or all
forbidden.

Regards,

  Neil
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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Guido van Rossum]

We should interview you for the paper we may be writing for HOPL.

On Wed, Jan 3, 2018 at 6:05 PM, Neil Schemenauer 
wrote:

> On 2018-01-03, Guido van Rossum wrote:
> > I'm sorry, I don't think more research can convince me either way.
> > I want all three of return/break/continue to work inside finally
> > clauses, despite there being few use cases.
>
> That's fine.  The history of 'continue' inside 'finally' is
> interesting.  The restriction dates back to at least when Jeremy
> committed the AST-based compiler (I have fond memories of hacking on
> it with Armin Rigo and others at a Python core sprint).  Going
> further back, I looked at 1.5.2 and there is the comment in
> compile.c:
>
> TO DO:
> ...
> XXX Allow 'continue' inside try-finally
>
> So if we allow 'continue' we will be knocking off a nearly 20 year
> old todo item. ;-)
>
> For giggles, I unpacked a Python 0.9.1 tarball.  The source code is
> all under 'src' in that version.  There doesn't seem to be a
> restriction on 'continue' but only because the grammar doesn't
> include it!  Without doing more research, I think the restriction
> could be as old as the 'continue' keyword.
>
> BTW, the bytecode structure for try/except shown in the compile.c
> comments is very simlar to what is currently generated.  It is quite
> remarkable how well your initial design and implementation have stood
> the test of time.  Thank you for making it open source.
>
> Regards,
>
>   Neil
>



-- 
--Guido van Rossum (python.org/~guido)
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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Neil Schemenauer]

On 2018-01-03, Guido van Rossum wrote:
> I'm sorry, I don't think more research can convince me either way.
> I want all three of return/break/continue to work inside finally
> clauses, despite there being few use cases.

That's fine.  The history of 'continue' inside 'finally' is
interesting.  The restriction dates back to at least when Jeremy
committed the AST-based compiler (I have fond memories of hacking on
it with Armin Rigo and others at a Python core sprint).  Going
further back, I looked at 1.5.2 and there is the comment in
compile.c:

TO DO:
...
XXX Allow 'continue' inside try-finally

So if we allow 'continue' we will be knocking off a nearly 20 year
old todo item. ;-)

For giggles, I unpacked a Python 0.9.1 tarball.  The source code is
all under 'src' in that version.  There doesn't seem to be a
restriction on 'continue' but only because the grammar doesn't
include it!  Without doing more research, I think the restriction
could be as old as the 'continue' keyword.

BTW, the bytecode structure for try/except shown in the compile.c
comments is very simlar to what is currently generated.  It is quite
remarkable how well your initial design and implementation have stood
the test of time.  Thank you for making it open source.

Regards,

  Neil
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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Guido van Rossum]

I'm sorry, I don't think more research can convince me either way. I want
all three of return/break/continue to work inside finally clauses, despite
there being few use cases.

On Wed, Jan 3, 2018 at 2:30 PM, Neil Schemenauer 
wrote:

> On 2018-01-03, Serhiy Storchaka wrote:
> > I haven't found 'finally' clauses in
> > https://github.com/gevent/gevent/blob/master/src/gevent/
> libev/corecffi.py.
> > Perhaps this code was changed in recent versions.
>
> Yes, I was looking at was git revision bcf4f65e.  I reran my AST
> checker and found this:
>
> ./src/gevent/_ffi/loop.py: 181: return inside finally
>
> > In any case we now know that this combination is occurred (but
> > very rarely) in the wild.
>
> Looks like it.  If we do want to seriously consider changing the
> grammar, I will download more packages of PyPI and check them.
>
> BTW, ./src/gevent/threadpool.py doesn't compile with 3.7 because it
> uses 'async' as a variable name.  So either they didn't notice the
> deprecation warnings or they didn't care to update their code.
>
> Regards,
>
>   Neil
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-- 
--Guido van Rossum (python.org/~guido)
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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Neil Schemenauer]

On 2018-01-03, Serhiy Storchaka wrote:
> I haven't found 'finally' clauses in
> https://github.com/gevent/gevent/blob/master/src/gevent/libev/corecffi.py.
> Perhaps this code was changed in recent versions.

Yes, I was looking at was git revision bcf4f65e.  I reran my AST
checker and found this:

./src/gevent/_ffi/loop.py: 181: return inside finally

> In any case we now know that this combination is occurred (but
> very rarely) in the wild.

Looks like it.  If we do want to seriously consider changing the
grammar, I will download more packages of PyPI and check them.

BTW, ./src/gevent/threadpool.py doesn't compile with 3.7 because it
uses 'async' as a variable name.  So either they didn't notice the
deprecation warnings or they didn't care to update their code.

Regards,

  Neil
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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Serhiy Storchaka]

02.01.18 22:31, Neil Schemenauer пише:

Serhiy Storchaka  wrote:

Currently 'break' and 'return' are never used inside 'finally'
clause in the stdlib.


See the _recv_bytes() function:

Lib/multiprocessing/connection.py: 316


Thank you Neil! I missed this case because ran only fast tests, without 
enabling network tests.



I would want to see a third-party code that uses them.


These are the only ones I found so far:

../gevent/src/gevent/libev/corecffi.py: 147


I haven't found 'finally' clauses in 
https://github.com/gevent/gevent/blob/master/src/gevent/libev/corecffi.py. 
Perhaps this code was changed in recent versions.


In any case we now know that this combination is occurred (but very 
rarely) in the wild.


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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Neil Schemenauer]

Generally I think programming language implementers don't get to
decide how the language works. You just have to implement it as
specified, inconvenient as that might be.

However, from a languge design prespective, I think there is a good
argument that this is a corner of the language we should consider
changing.  First, I analyzed over one million lines of Python code
with my AST walker and only found this construct being used in four
different places.  It seems to be extremely rare.

Second, the existance of a pylint warning for it suggests that it is
confusing. I did a little more searching using the pylint warning
and found these pages:


https://stackoverflow.com/questions/35505624/break-statement-in-finally-block-swallows-exception

http://thegreyblog.blogspot.ca/2011/02/do-not-return-in-finally-block-return.html

So, given the above and that the implementation (both compiler and
bytecode evaluator) is pretty complicated, I vote that we should
disallow it.

Regards,

  Neil
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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Neil Schemenauer]

Serhiy Storchaka  wrote:
> Currently 'break' and 'return' are never used inside 'finally'
> clause in the stdlib.

See the _recv_bytes() function:

Lib/multiprocessing/connection.py: 316

> I would want to see a third-party code that uses them.

These are the only ones I found so far:

./gevent/src/gevent/libev/corecffi.py: 147
./gevent/src/gevent/threadpool.py: 226

I have an AST walker script that finds them.

Regards,

  Neil
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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Guido van Rossum]

I don't think the language definition should be judgmental here. The
semantics are unambiguous.

On Dec 28, 2017 11:38 AM, "Serhiy Storchaka"  wrote:

> 28.12.17 16:38, Guido van Rossum пише:
>
>> Looks to me the prohibition was to prevent a crash. It makes more sense
>> to fix it.
>>
>
> The crash can be fixed by just removing the check after finishing
> issue17611.
>
> But is there any use case for 'continue'/'break'/'return' inside 'finally'
> clause? The code like
>
> try:
> return 1
> finally:
> return 2
>
> or
>
> try:
> continue
> finally:
> break
>
> looks at least confusing. Currently 'break' and 'return' are never used
> inside 'finally' clause in the stdlib. I would want to see a third-party
> code that uses them.
>
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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Serhiy Storchaka]

28.12.17 16:38, Guido van Rossum пише:
Looks to me the prohibition was to prevent a crash. It makes more sense 
to fix it.


The crash can be fixed by just removing the check after finishing 
issue17611.


But is there any use case for 'continue'/'break'/'return' inside 
'finally' clause? The code like


try:
return 1
finally:
return 2

or

try:
continue
finally:
break

looks at least confusing. Currently 'break' and 'return' are never used 
inside 'finally' clause in the stdlib. I would want to see a third-party 
code that uses them.


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Re: [Python-Dev] 'continue'/'break'/'return' inside 'finally' clause

[Guido van Rossum]

Looks to me the prohibition was to prevent a crash. It makes more sense to
fix it.

On Dec 28, 2017 03:39, "Serhiy Storchaka"  wrote:

Currently 'continue' is prohibited inside 'finally' clause, but 'break' and
'return' are allowed. What is the r

'continue' was prohibited in https://bugs.python.org/issue1542451. Should
we prohibit also 'break' and 'return' or allow
'continue'? 'break' and 'return' are never used inside 'finally' clause in
the stdlib.

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