[issue46754] Improve Python Language Reference based on [Köhl 2020]

[Jelle Zijlstra]

Jelle Zijlstra  added the comment:

Some specific points from the document:

"While the PLR explicitly states that “x < y calls x.__lt__(y)” [20, 3.3.1.] 
this is actually false." They had to read the implementation to actually figure 
out how this works.

"If no expression is provided, the PLR states that “the last exception that was 
active in the current scope” should be re-raised. Unfortunately, the PLR stays 
unspecific on what it means for an exception to be “the last exception that was 
active in the current scope.”"

"The description of this name binding and resolution process in the PLR [20, 
4.2.] is unfortunately not particularly clear"

"While generator objects seem not to be specified as part of the PLR, the 
documentation of the inspect module5 describes them."

(I didn't read the whole thing, but searched for "PLR".)

Here's a few similar things I noticed while reading through the reference 
recently:

- 
https://docs.python.org/3.10/reference/datamodel.html#the-standard-type-hierarchy
 is supposed to be a list of primitive types but has a few problems:
  - Lists "numbers.Number" as a standard type, when it's an ABC that's really 
not part of the core language
  - Lists "I/O objects" as a kind of standard object, but there are many kinds 
of file-like objects, and I'd argue they're not part of the core language, but 
of the standard library.
  - On the other hand, some builtin types (range, map, filter, zip) are not 
listed.
- https://docs.python.org/3.10/reference/expressions.html#subscriptions doesn't 
actually give the rules about when __class_getitem__ is called vs. __getitem__. 
That's in 
https://docs.python.org/3.10/reference/datamodel.html#class-getitem-versus-getitem.
- The text at https://docs.python.org/3.10/reference/expressions.html#calls 
doesn't cover positional-only parameters.

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nosy: +Jelle Zijlstra

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[issue46754] Improve Python Language Reference based on [Köhl 2020]

[Guido van Rossum]

Guido van Rossum  added the comment:

Another:

- The description of this name binding and resolution process
  in the PLR [20, §4.2.] is unfortunately not particularly clear.

(I found this to be the case too, and wrote up what I found:
https://gvanrossum.github.io/formal/scopesblog.html
Hopefully it matches what Kohl derived.)

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[issue46754] Improve Python Language Reference based on [Köhl 2020]

[Guido van Rossum]

Guido van Rossum  added the comment:

A few examples of issues brought up by Kohl:

- While the PLR explicitly states that “x < y calls x.__lt__(y)”
  [20, §3.3.1.] this is actually false.
  There are cases where x < y does not call x.__lt__(y)
  and there are other cases where x.__lt__(y) is called
  but more than that happens.

- If no expression is provided, the PLR states that
  “the last exception that was active in the current scope”
  should be re-raised. Unfortunately, the PLR stays
  unspecific on what it means for an exception to be
  “the last exception that was active in the current scope.
  [...]
  Instead, raise re-raises the exception that *is active*
  in the respective execution context

(Perhaps unrelated, but indicative of how out of date the PLR is: in 
executionmodel.rst there's still a mention of and even an index entry for 
restricted execution, a feature that was removed in some early Python 2 
release.)

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[issue46754] Improve Python Language Reference based on [Köhl 2020]

[Brandt Bucher]

Change by Brandt Bucher :


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nosy: +brandtbucher

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[issue46754] Improve Python Language Reference based on [Köhl 2020]

[Dong-hee Na]

Change by Dong-hee Na :


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nosy: +corona10

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[issue46754] Improve Python Language Reference based on [Köhl 2020]

[Alex Waygood]

Change by Alex Waygood :


--
keywords:  -patch
nosy:  -AlexWaygood
stage: patch review -> 

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[issue46754] Improve Python Language Reference based on [Köhl 2020]

[Alex Waygood]

Change by Alex Waygood :


--
pull_requests:  -29560

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[issue46754] Improve Python Language Reference based on [Köhl 2020]

[Alex Waygood]

Change by Alex Waygood :


--
keywords: +patch
nosy: +AlexWaygood
nosy_count: 1.0 -> 2.0
pull_requests: +29560
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/29479

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[issue46754] Improve Python Language Reference based on [Köhl 2020]

[Guido van Rossum]

New submission from Guido van Rossum :

In https://arxiv.org/pdf/2109.03139.pdf ("M Köhl, An Executable Structural 
Operational Formal Semantics for Python, Master Thesis 2020 Saarland 
University) there are some observations on cases where the Language Reference 
(referred to as PLR) is ambiguous or incorrect.

Somebody should go over the thesis, collect the issues, and then we can update 
the language reference.

See also 
https://github.com/faster-cpython/ideas/issues/208#issuecomment-1039612432

--
messages: 413275
nosy: gvanrossum
priority: normal
severity: normal
status: open
title: Improve Python Language Reference based on [Köhl 2020]

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