[issue37757] TargetScopeError not raised for comprehension scope conflict

[Nick Coghlan]

Nick Coghlan  added the comment:

Merged for 3.8b4 after Emily's review.

Thanks to all involved in the PEP update and change discussion!

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

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Nick Coghlan]

Nick Coghlan  added the comment:


New changeset 6ca030765db49525f16b8fabff4153238148b58d by Nick Coghlan in 
branch '3.8':
[3.8] bpo-37757: Disallow PEP 572 cases that expose implementation details 
(GH-15491)
https://github.com/python/cpython/commit/6ca030765db49525f16b8fabff4153238148b58d


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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Nick Coghlan]

Change by Nick Coghlan :


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pull_requests: +15178
pull_request: https://github.com/python/cpython/pull/15491

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Nick Coghlan]

Nick Coghlan  added the comment:


New changeset 5dbe0f59b7a4f39c7c606b48056bc29e406ebf78 by Nick Coghlan in 
branch 'master':
bpo-37757: Disallow PEP 572 cases that expose implementation details (GH-15131)
https://github.com/python/cpython/commit/5dbe0f59b7a4f39c7c606b48056bc29e406ebf78


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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Guido van Rossum]

Guido van Rossum  added the comment:

The decision has been made to get rid of TargetScopeError and instead just use 
SyntaxError. PEP 572 was updated already. We're just waiting for someone 
(Serhiy?) to review Nick's patch, PR #15131.

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Łukasz Langa]

Łukasz Langa  added the comment:

This is marked as a release blocker. The last beta is scheduled for Monday. 
Please decide how to proceed ASAP.

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nosy: +lukasz.langa
priority: deferred blocker -> release blocker

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Nick Coghlan]

Nick Coghlan  added the comment:

The outcome of the python-dev discussion was that we agreed to switch to 
raising a plain SyntaxError, as that's what we do everywhere else that this 
kind of problem comes up (e.g. conflicts between global and nonlocal 
declarations, or between those and parameter declarations, as well as the 
various other cases of "that statement/expression is fine in isolation, but you 
can't use it *here*").

Both PRs have been updated accordingly, and the PEP PR has been merged.

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Guido van Rossum]

Guido van Rossum  added the comment:

If you and Nick both feel strongly about this please take it to python-dev,
I'm sure we'll get closure quickly there.

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Barry A. Warsaw]

Barry A. Warsaw  added the comment:

> OTOH This would require a PEP change, formal review, etc.

It would be trivial though.  There are only two references to TargetScopeError 
in the PEP.  One talks about adding the exception and the other just mentions 
it almost in passing as a subclass of SyntaxError.

I think it would be better to just use SyntaxError.

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

> We report all sorts of things found by the bytecode compiler as SyntaxError.

Except of these which are reported with IndentationError or its subclass 
TabError.

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Guido van Rossum]

Guido van Rossum  added the comment:

But we don't do that with any of the other (many) errors detected by later 
passes of the compiler. Those report dozens of SyntaxErrors, with good 
descriptive messages. Users can search the web for those messages too.

Also, I doubt that many people will ever get a TargetScopeError. The examples 
are all esoteric -- yes, the compiler needs to reject them, but no, they are 
not things one is likely to try intentionally. (The exception may be the 
forbidden form you're adding, [... for ... in (i := ...)].)

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Nick Coghlan]

Nick Coghlan  added the comment:

I believe our main motivation for separating it out was the fact that even 
though TargetScopeError is a compile-time error, the affected code is 
syntactically fine - there are just issues with unambiguously inferring the 
intended read/write location for the affected target names. (Subclassing 
SyntaxError is then a pragmatic concession to the fact that "SyntaxError" also 
de facto means "CompilationError")

Searching for "Python TargetScopeError" will also get folks to relevant 
information far more quickly than searching for "Python SyntaxError" will.

Pre-seeding Stack Overflow with an answer to "What does TargetScopeError mean 
in Python?" would probably be a good idea though (similar to what I did for 
https://stackoverflow.com/questions/25445439/what-does-syntaxerror-missing-parentheses-in-call-to-print-mean-in-python
 )

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Guido van Rossum]

Guido van Rossum  added the comment:

[Barry]
> I know the PEP defines TargetScopeError as a subclass of SyntaxError, but it 
> doesn't really explain why a subclass is necessary.  I think seeing 
> "TargetScopeError" will be a head scratcher.  Why not just SyntaxError 
> without introducing a new exception?

Hm, that's not a bad point. We report all sorts of things found by the bytecode 
compiler as SyntaxError.

OTOH This would require a PEP change, formal review, etc. (IMO much more so 
than adding the new edge case that Nick and Damien discovered.) Maybe the best 
way of doing this would be to implement TargetScopeError now, then start the 
debate about killing it, and try to get that in before beta4?

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Barry A. Warsaw]

Barry A. Warsaw  added the comment:

I know the PEP defines TargetScopeError as a subclass of SyntaxError, but it 
doesn't really explain why a subclass is necessary.  I think seeing 
"TargetScopeError" will be a head scratcher.  Why not just SyntaxError without 
introducing a new exception?

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nosy: +barry

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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nosy: +serhiy.storchaka

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Nick Coghlan]

Nick Coghlan  added the comment:

Added a PEP update as well: https://github.com/python/peps/pull/1140

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Guido van Rossum]

Guido van Rossum  added the comment:

Thanks for being part of the village raising this child!

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Nick Coghlan]

Change by Nick Coghlan :


--
keywords: +patch
pull_requests: +14871
stage: needs patch -> patch review
pull_request: https://github.com/python/cpython/pull/15131

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[issue37757] TargetScopeError not raised for comprehension scope conflict

[Nick Coghlan]

New submission from Nick Coghlan :

While implementing PEP 572, Emily noted that the check for conflicts between 
assignment operators and comprehension iteration variables had not yet been 
implemented: https://bugs.python.org/issue35224#msg334331

Damien George came across this discrepancy while implementing assignment 
expressions for MicroPython.

The proposed discussion regarding whether or not the PEP should be changed 
didn't happen, and the PEP itself misses the genuinely confusing cases where 
even an assignment expression that *never executes* will still make the 
iteration variable leak:

>>> [i for i in range(5)]
[0, 1, 2, 3, 4]
>>> [i := 10 for i in range(5)]
[10, 10, 10, 10, 10]
>>> i
10
>>> [False and (i := 10) for i in range(5)]
[False, False, False, False, False]
>>> i
4

And that side effect happens even if the assignment expression is nested 
further down in an inner loop:

>>> [(i, j, k) for i in range(2) for j in range(2) for k in range(2)]
[(0, 0, 0), (0, 0, 1), (0, 1, 0), (0, 1, 1), (1, 0, 0), (1, 0, 1), (1, 1, 0), 
(1, 1, 1)]
>>> i
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'i' is not defined
>>> [(i, j, k) for i in range(2) for j in range(2) for k in range(2) if True or 
>>> (i:=10)]
[(0, 0, 0), (0, 0, 1), (0, 1, 0), (0, 1, 1), (1, 0, 0), (1, 0, 1), (1, 1, 0), 
(1, 1, 1)]
>>> i
1

I'm at the PyCon AU sprints today, and will be working on a PR to make these 
cases raise TargetScopeError as specified in the PEP.

--
assignee: ncoghlan
messages: 349012
nosy: Damien George, emilyemorehouse, ncoghlan
priority: deferred blocker
severity: normal
stage: needs patch
status: open
title: TargetScopeError not raised for comprehension scope conflict
type: behavior
versions: Python 3.8, Python 3.9

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