Better specify number of nested scopes

[RoscoeRHiggins]

BPO	26205
Nosy	@terryjreedy, @ezio-melotti, @eryksun, @vedgar, @Mariatta, @csabella, @miss-islington, @nanjekyejoannah
PRs	bpo-26205: Specify the number of nested scopes #21324 [3.9] bpo-26205: Specify the number of nested scopes (GH-21324) #21341 [3.8] bpo-26205: Specify the number of nested scopes (GH-21324) #21342

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/Mariatta'
closed_at = None
created_at = <Date 2016-01-26.02:16:27.767>
labels = ['type-feature', '3.8', '3.9', '3.10', 'docs']
title = 'Better specify number of nested scopes'
updated_at = <Date 2020-11-04.21:36:36.791>
user = 'https://bugs.python.org/RoscoeRHiggins'

bugs.python.org fields:

activity = <Date 2020-11-04.21:36:36.791>
actor = 'brett.cannon'
assignee = 'Mariatta'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2016-01-26.02:16:27.767>
creator = 'Roscoe R. Higgins'
dependencies = []
files = []
hgrepos = []
issue_num = 26205
keywords = ['patch']
message_count = 20.0
messages = ['258941', '258964', '258978', '258980', '259228', '340000', '373022', '373048', '373049', '373055', '373056', '373058', '373059', '373061', '373062', '373063', '373068', '373085', '373087', '373125']
nosy_count = 11.0
nosy_names = ['terry.reedy', 'ezio.melotti', 'docs@python', 'eryksun', 'veky', 'Hibou57', 'Roscoe R. Higgins', 'Mariatta', 'cheryl.sabella', 'miss-islington', 'nanjekyejoannah']
pr_nums = ['21324', '21341', '21342']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue26205'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

• PR: gh-70393: Clarify mention of "middle" scope #98839

• PR: [3.11] gh-70393: Clarify mention of "middle" scope (GH-98839) #100186

• PR: [3.10] gh-70393: Clarify mention of "middle" scope (GH-98839) #100187

[RoscoeRHiggins]

In chapter 9. Classes of the Python3.5 documentation it says:

"At any time during execution, there are at least three nested scopes whose namespaces are directly accessible:",

followed by a list containing 4 items.
Further down a middle scope is mentioned (although mentioned by name). This was confusing for a while.

[brettcannon]

https://docs.python.org/3/tutorial/classes.html for the docs that Roscoe is talking about.

So the sentence is technically correct, it just takes careful reading to grasp what's being said. There are "at least three nested scopes", but there can be *up to* four scopes. Since "the scopes of any enclosing functions" is not necessarily true for all code scopes, you end up with at least three, but possibly four scopes.

Obviously the wording could be clearer, so if you want to sign the CLA, Roscoe, and propose a rewording that would be appreciated!

[Hibou57]

Better at least two, if I'm not wrong: the innermost scope may be the module's scope. So there is always at least the module scope and the built‑ins scope, at least two up to four.

(if I have not overlooked something)

[brettcannon]

It depends on how you want to view things as to whether you can claim there are two or three scopes for module-level code. While it's true that the local scope operates just like global scope, to Python it's more like local == global rather than the local scope simply doesn't exist (hence why locals() never throws an exception saying the scope doesn't exist but instead is the same as globals()).

The "three scope" phrasing also predates nested scopes from back when Python had its LGB scoping rules: Local-Global-Builtin. Back then we just said Python had three scopes and left it at that since it was basically always true and didn't confuse anyone.

At this point I'm fine with just removing the number from the sentence and saying something like "At any time during execution, there are various nested scopes whose namespaces are directly accessible:".

[terryjreedy]

Would 'three or more' be any clearer than 'at least three'? They mean the same, but the first seems better to me in this context.

The real problem with this section are a) the use of Guido's first person 'I' and b) statements that were not changed when nested scope were added, but should have been.

"If a name is declared global, then all references and assignments go directly to the middle scope containing the module’s global names."

The global scope is no longer the middle scope. Roscoe pointed at this. With that removed, the sentence says that if a name is declared global, assignments go to global scope. This would be more meaningful if prefixed by a revised version of the following, which is several paragraphs down.

"A special quirk of Python is that – if no global statement is in effect – assignments to names always go into the innermost scope."

The special quirk part should go; 'global' would now have to be 'global or nonlocal', but I now think the following, preceeding the revised 'global' sentence above, would be better.

"By default, assignments to names always go into the innermost, local, namespace."

In other words, I think the sentence Roscoe flagged is the least of the problems with this section.

[csabella]

Assigning to @Mariatta for the sprints.