[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Éric Araujo]

Éric Araujo  added the comment:

[There are two separate events in the ticket log: I added my comment, then 
noticed I was the only nosy so I found all the prople recently removed by error 
and re-added them]

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Guido van Rossum]

Guido van Rossum  added the comment:

[Meta: Why did adding a comment add all those people (back?) to the nosy list?]

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Éric Araujo]

Change by Éric Araujo :


--
nosy: +Anthony Sottile, BTaskaya, Jeffrey.Kintscher, Julian, Terry Davis, 
barry, benjamin.peterson, ezio.melotti, georg.brandl, gvanrossum, ishimoto, 
jack1142, lukasz.langa, ncoghlan, pablogsal, r.david.murray, serhiy.storchaka, 
steven.daprano, thautwarm, ulope

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Éric Araujo]

Éric Araujo  added the comment:

PEP 8 used with statements as an example of use of backslashes, I am proposing 
this change: https://github.com/python/peps/pull/2244

--
nosy: +eric.araujo
versions: +Python 3.10 -Python 3.8

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Eryk Sun]

Change by Eryk Sun :


--
Removed message: https://bugs.python.org/msg405694

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Eryk Sun]

Change by Eryk Sun :


--
components: +Interpreter Core -IDLE
nosy:  -ahmedsayeed1982, terry.reedy

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Ahmed Sayeed]

Ahmed Sayeed  added the comment:

$ cat test.c
struct foo {
int len; https://www.webb-dev.co.uk/category/computers/
int items[];
};

struct foo *p;
http://www.compilatori.com/category/technology/
int main() {
return 0;
}
$ gcc test.c -g -O0 -o test 
http://www.acpirateradio.co.uk/category/computers/
$ ./gdb -q -nx --data-directory=data-directory ./test -ex 'python 
gdb.parse_and_eval("p").type.target()["items"].type.range()'
Reading symbols from ./test... http://www-look-4.com/category/computers/
/home/simark/src/binutils-gdb/gdb/gdbtypes.h:435: internal-error: 
LONGEST dynamic_prop::const_val() const: Assertion `m_kind == PROP_CONST' 
failed. http://www.mconstantine.co.uk/category/services/ 
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) 
http://www.logoarts.co.uk/category/computers/

This is because the Python code (typy_range) blindly reads the high
bound of the type of `items` as a constant value.  Since it is a 
http://www.iu-bloomington.com/category/computers/
flexible array member, it has no high bound, the property is undefined.
Since commit 8c2e4e0689 https://komiya-dental.com/category/computers/ 
("gdb: add accessors to struct dynamic_prop"),
the getters check that you are not 
http://www.go-mk-websites.co.uk/category/services/ getting a property value of 
the wrong
kind, so this causes a failed assertion. 
http://www.slipstone.co.uk/category/computers/

Fix it by checking if the property is indeed a constant value before 
http://embermanchester.uk/category/computers/
accessing it as such.  Otherwise, use 0.  This restores the previous GDB 
http://fishingnewsletters.co.uk/category/services/
behavior: because the structure was zero-initialized, 
http://connstr.net/category/computers/  this is what was
returned before.  But now this behavior is explicit and not accidental. But 
now this behavior is explicit and not accidental. But now this behavior is 
explicit and not  http://the-hunters.org/technology/new-robot/ accidental. But 
now this behavior is explicit and not accidental. But now this behavior is 
explicit and is explicit and not accidental.
http://joerg.li/category/computers/
Add a test, gdb.python/flexible-array-member.exp, that is derived from
gdb.base/flexible-array-member.exp. 
http://www.jopspeech.com/category/computers/  It tests the same things, but
through the Python API.  It also specifically tests getting the range
from the various kinds http://www.wearelondonmade.com/category/computers/ 
of flexible array member types (AFAIK it wasn't
possible to do the equivalent through the CLI). 
https://waytowhatsnext.com/category/computers/

gdb/ChangeLog:

--
components: +IDLE -Interpreter Core
nosy: +ahmedsayeed1982, terry.reedy -Anthony Sottile, BTaskaya, 
Jeffrey.Kintscher, Julian, Terry Davis, barry, benjamin.peterson, eric.araujo, 
ezio.melotti, georg.brandl, gvanrossum, ishimoto, jack1142, lukasz.langa, 
ncoghlan, pablogsal, r.david.murray, serhiy.storchaka, steven.daprano, 
thautwarm, ulope
versions: +Python 3.8

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Guido van Rossum]

Guido van Rossum  added the comment:

But it is undocumented and doesn’t work with -X oldparser.
-- 
--Guido (mobile)

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

This is already implemented in master with the new PEG parser so closing this.

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[thautwarm]

thautwarm  added the comment:

Maybe you should close this.

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[thautwarm]

thautwarm  added the comment:

I can confirm Guido's words, now parentheses for continuation across lines are 
already supported.

Even without parentheses, multiline with items can be supported. I just 
implemented it here: 
https://github.com/thautwarm/cpython/blob/bpo-12782/Grammar/python.gram#L180-L187

  from contextlib import contextmanager

  @contextmanager
  def f(x):
try:
yield x
finally:
pass

# Ok
  with f('c') as a,
   f('a') as b:
   pass


# Ok
  with f('c') as a,
   f('a') as b,
   f('a') as c:
   pass


  # ERROR
  with f('c') as a,
   f('a') as b,
   f('a') as c:
 x = 1 + 1

  # message:
File "/home/thaut/github/cpython/../a.py", line 49
  x = 1 + 1
 ^
IndentationError: unindent does not match any outer indentation 
level

  # ERROR
  with f('c') as a,
   f('a') as b,
   f('a') as c:
  x = 1 + 1

  File "/home/thaut/github/cpython/../a.py", line 49
x = 1 + 1
  
  IndentationError: unexpected indent



The grammar is:


with_stmt[stmt_ty]:
| ...
| 'with' a=(',' [NEWLINE ~ INDENT?]).with_item+ ':' tc=[TYPE_COMMENT] 
NEWLINE b=statements DEDENT {
_Py_With(a, b, NEW_TYPE_COMMENT(p, tc), EXTRA) }
| ...

The restriction here is, since the second 'with_item', until the end of 
'statements', the expression and statements have to keep the same indentation.

with item1,
   item2,
   ...:
   block

The indentation of 'item2', ..., 'block' should be the same.

This implementation leverages the new PEG and how the lexer deals with 
indent/dedent.

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Guido van Rossum]

Guido van Rossum  added the comment:

If we introduce a PEG-based parser, we can do this without hacking the 
tokenizer. See https://github.com/gvanrossum/pegen/issues/229

I'd propose to aim for Python 3.10 (if the PEG parser happens).

--
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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Guido van Rossum]

Change by Guido van Rossum :


--
versions:  -Python 3.8

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[jack1142]

Change by jack1142 :


--
nosy: +jack1142

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Batuhan]

Change by Batuhan :


--
nosy: +BTaskaya

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Jeffrey Kintscher]

Change by Jeffrey Kintscher :


--
nosy: +Jeffrey.Kintscher

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Nick Coghlan]

Nick Coghlan  added the comment:

Reviewing the thread, we never actually commented on thautwarm's proposal in 
https://bugs.python.org/issue12782#msg327875 that aims to strip out any INDENT, 
NEWLINE, and DEDENT tokens that appear between the opening "with" keyword and 
the statement header terminating ":".

The problem with that is that line continuations are actually handled by the 
tokenizer, *not* the compiler, and the tokenizer already switches off the 
INDENT/NEWLINE/DEDENT token generation based on the following rules:

* tracking opening & closing of triple-quoted strings 
* tracking opening & closing of parentheses ("()"), brackets ("[]"), and braces 
("{}")
* detecting a backslash immediately followed by a newline

By design, the tokenizer is generally unaware of which NAME tokens are actually 
keywords - it's only aware of async & await at the moment as part of the 
backwards compatibility dance that allowed those to be gradually converted to 
full keywords over the course of a couple of releases.

Hence why INDENT/NEWLINE/DEDENT never appear inside expressions in the Grammar: 
the tokenization rules mean that those tokens will never appear in those 
locations.

And it isn't simply a matter of making the tokenizer aware of the combination 
of "with" and ":" as a new pairing that ignores linebreaks between them, as ":" 
can appear in many subexpressions (e.g. lambda functions, slice notation, and 
the new assignments expressions), and it's only the full parser that has enough 
context to tell which colon is the one that actually ends the statement header.

Thus the design requirement is to come up with a grammar rule that allows this 
existing code to continue to compile and run correctly:

```
>>> from contextlib import nullcontext
>>> with (nullcontext()) as example:
... pass
... 
>>> 
```

While also enabling new code constructs like the following:


with (nullcontext() as example):
pass

with (nullcontext(), nullcontext()):
pass

with (nullcontext() as example, nullcontext()):
pass

with (nullcontext(), nullcontext() as example):
pass

with (nullcontext() as example1, nullcontext() as example2):
pass

If we can get the Grammar to allow those additional placements of parentheses, 
then the existing tokenizer will take care of the rest.

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

What do you mean with re-raise? The issue is not closed. If you have some 
proposal to overcome the limitations, the best approach is to comment here in 
the issue.

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Terry Davis]

Terry Davis  added the comment:

I'd like to re-raise this issue. Should I cross-post to "discuss.python.org - 
Ideas"?

--
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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[thautwarm]

thautwarm  added the comment:

How about:

with_stmt: 'with' (with_items | '(' with_items ')') ':' suite
ignored: INDENT | NEWLINE | DEDENT
with_items: with_item (ignored* ',' ignored* with_item)*

--
nosy: +thautwarm

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Anthony Sottile]

Change by Anthony Sottile :


--
nosy: +Anthony Sottile

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

Now that I think about this a bit better, this may be actually a problem as:

 with (yield something) as x:

is a more than valid use case that will be broken with the simple grammar rule 
:(

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


--
pull_requests:  -9036

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


--
keywords: +patch
pull_requests: +9036

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

The DFA for the rule

with_stmt: 'with' ( with_item (',' with_item)* | '(' with_item (',' with_item)* 
[','] ')' ) ':' suite

is:

DFA for with_stmt   
  [512/2103]
  State 0
'with' -> 1
  State 1
'(' -> 2
with_item -> 3
  State 2
with_item -> 4
  State 3
',' -> 5
':' -> 6
  State 4
')' -> 7
',' -> 8
  State 5
with_item -> 3
  State 6
suite -> 10
  State 7
':' -> 6
  State 8
')' -> 7
with_item -> 4
  State 9
',' -> 5
':' -> 6
  State 10 (final)
  State 11
')' -> 7
',' -> 8

It works because the transition from State 1 into a "(" is going to prioritize 
the path:

0 -> 1 -> "(" -> 2

instead if 

0 -> 1 -> with_item -> 3

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

The Python grammar is already not LL(1) strictly. Take for example the 
production for "argument":

argument: ( test [comp_for] | test '=' test | '**' test | '*' test )

obviously the first sets of test and test are the same and is ambiguous, but 
the NDFAs are still able to produce DFAs that can generate a concrete syntax 
tree that allows the AST generation to disambiguate that the second test is a 
NAME and not any other thing. 

The rule with_stmt: 'with' ( with_item (',' with_item)* | '(' with_item (',' 
with_item)* [','] ')' ) ':' suite

will generate a similar scenario. The NDFAs will generate DFAs that will 
ultimately allow us to just skip the more external group of parenthesis when 
generating the nodes. This makes valid all these expressions:

 with (manager() as x, manager() as y):
 pass
 with (manager() as x, manager() as y,):
 pass
 with (manager()):
 pass
 with (manager() as x):
 pass
 with (((manager(:
 pass
 with manager( as x):

but not this one:

 with (((manager( as x:

the reason is that it assigns the first LPAR to the second production and it 
fails when searching for the one that is at the end. I think this limitation is 
OK.

If you want to play with that. here is a prototype of the implementation with 
some tests:

https://github.com/pablogsal/cpython/tree/parenthesized_with

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Nick Coghlan]

Nick Coghlan  added the comment:

Especially since the dynamic flexibility of ExitStack comes at a genuine 
runtime cost when unwinding the resource stack.

I also (very!) belatedly noticed that I never answered Julian's request for 
clarification about the potential grammar ambiguity, so going into detail about 
that now:

The first item in the grammar after the 'with' keyword is a 'test' node, which 
can already start with a parenthesis, which means a naive attempt at allowing 
grouping parentheses will likely fail to generate a valid LL(1) parser.

That doesn't mean a more sophisticated change isn't possible (and Pablo has 
apparently implemented one) - it just means that the required grammar update is 
going to be more complicated than just changing:

with_stmt: 'with' with_item (',' with_item)*  ':' suite

to be:

with_stmt: 'with' (with_items | '(' with_items ')') ':' suite
with_items: with_item (',' with_item)*

(That would need too much lookahead to decide whether an opening parenthesis 
belongs to the first 'with_item' in 'with_items' or if it's starting the 
alternative multi-line grouping construct)

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Łukasz Langa]

Łukasz Langa  added the comment:

This was closed without enough explanation. Suggesting people should use 
ExitStack due to a Python grammar deficiency is suboptimal to say the least.

This problem is coming back to users of Black due to Black's removal of 
backslashes. It's the only piece of our grammar where backslashes are required 
for readability which shows there's something wrong.

The syntax ambiguity that Nick is raising fortunately shouldn't be a problem 
because a single tuple is an invalid context manager. In other contexts if the 
organizational parentheses are matched by the with-statement and not by the 
underlying `test`, that's also fine since they were organizational this doesn't 
make the `test` invalid.

Pablo has a working patch for this, we intend to fix this wart for Python 3.8.

--
nosy: +lukasz.langa
resolution: wont fix -> 
stage:  -> patch review
status: closed -> open
versions: +Python 3.8 -Python 3.4

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Łukasz Langa]

Change by Łukasz Langa :


--
assignee:  -> pablogsal
nosy: +pablogsal

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Ulrich Petri]

Ulrich Petri added the comment:

So this would basically be:

with ExitStack() as stack:
cm1 = stack.enter_context(a_long_name.with_a_long_method())
cm2 = stack.enter_context(another_long_variable.with_a_long_method())

Seems like a very non-obvious and inelegant solution...

--
nosy: +ulope

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Barry A. Warsaw]

Changes by Barry A. Warsaw :


--
resolution:  -> wont fix
status: open -> closed

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Barry A. Warsaw]

Barry A. Warsaw added the comment:

Let's just Won't Fix this.  Use a contextlib.ExitStack.

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Steven D'Aprano]

Steven D'Aprano added the comment:

This has come up on Python-Ideas again:

http://permalink.gmane.org/gmane.comp.python.ideas/32002

https://mail.python.org/pipermail/python-ideas/2015-February/032000.html

--
nosy: +steven.daprano

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

There is a discussion about this on Python-Ideas:

http://comments.gmane.org/gmane.comp.python.ideas/17597
http://mail.python.org/pipermail/python-ideas/2012-October/017610.html

--
nosy: +serhiy.storchaka

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Ezio Melotti]

Ezio Melotti added the comment:

If this can't be fixed I think it should be at least documented in the FAQs.

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Atsuo Ishimoto]

Atsuo Ishimoto added the comment:

In Python 3.3, we have contextlib.ExitStack() for multiple contexts. 
So, perhaps we can close this issue?

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Ezio Melotti]

Changes by Ezio Melotti :


--
nosy: +barry
versions: +Python 3.4 -Python 3.3

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Atsuo Ishimoto]

Changes by Atsuo Ishimoto :


--
nosy: +ishimoto

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Julian Berman]

Julian Berman  added the comment:

>  we explicitly advise against importing too many modules in a single import 
> statement, but importing multiple names from a single location is often a 
> useful thing to do.

Cool. I imagined this had to do with it.

> there may be a grammar ambiguity problem in this case, since (unlike 
> from-import) with statements allow arbitrary subexpressions.

Sorry, can you possibly clarify where the ambiguity might come in?

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Nick Coghlan]

Nick Coghlan  added the comment:

As Georg noted, only individual expressions get parentheses based continuations 
automatically. For statement level use of comma separation, it's decided on a 
case-by-cases basis as to whether we think it is a legitimate usage based on 
our style guidelines.

That's why 'from location import (name1, name2)' is allowed, but 'import 
(name1, name2)' is not: we explicitly advise against importing too many modules 
in a single import statement, but importing multiple names from a single 
location is often a useful thing to do.

However, while the multiple context expression use case is reasonable, there 
may be a grammar ambiguity problem in this case, since (unlike from-import) 
with statements allow arbitrary subexpressions.

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[R. David Murray]

R. David Murray  added the comment:

Following on to Georg's comment about expressions, as a workaround, note
that:

  with (
 open('abc')) as foo:

works.

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nosy: +r.david.murray

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Ezio Melotti]

Changes by Ezio Melotti :


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nosy: +ezio.melotti

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Georg Brandl]

Georg Brandl  added the comment:

> is the only other example I can think of 

One similar example would be "raise" in Python 2.

> all compound statements uniformly allowed the same continuation syntax.

This is not true: only "import-as" allows this syntax.  All other uses of 
parentheses for continuation are continuations of *expressions*.

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nosy: +georg.brandl

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Éric Araujo]

Éric Araujo  added the comment:

I agree it’s unfortunate that we have to use backslashes to have multi-line 
with statements.

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Antoine Pitrou]

Changes by Antoine Pitrou :


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nosy: +benjamin.peterson, ncoghlan
type: behavior -> feature request
versions:  -Python 2.7

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[issue12782] Multiple context expressions do not support parentheses for continuation across lines

[Julian Berman]

New submission from Julian Berman :

Using multiple `with` statements across multiple lines does not support using 
parens to break them up:


with (open("a_really_long_foo") as foo,
  open("a_really_long_bar") as bar):
pass

Traceback (most recent call last):
  File "", line 1, in 
  File "demo.py", line 19
with (open("a_really_long_foo") as foo,
^
SyntaxError: invalid syntax


Also, without convoluting things, import also does not support doing so, and is 
the only other example I can think of of a compound statement that forces you 
to either be redundant or bite your teeth and use \, despite the fact that PEP 
328 gave us parens for from imports.

(I did not find a discussion as to why import didn't grow it as well, so please 
correct me as I'm sure it must have been discussed before).

It's understandably a lot rarer to need multiple lines when importing, but it'd 
be nice if all compound statements uniformly allowed the same continuation 
syntax.

--
components: Interpreter Core
messages: 142411
nosy: Julian
priority: normal
severity: normal
status: open
title: Multiple context expressions do not support parentheses for continuation 
across lines
type: behavior
versions: Python 2.7, Python 3.3

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