Repository navigation
Clarify the required behaviour of locals() #62160
Description
| BPO | 17960 |
|---|---|
| Nosy | @gvanrossum, @ncoghlan, @benjaminp, @ezio-melotti, @njsmith, @florentx, @xgid |
| PRs |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
Show more details
GitHub fields:
assignee = 'https://github.com/ncoghlan'
closed_at = None
created_at = <Date 2013-05-12.07:46:47.489>
labels = ['type-bug', '3.7', 'docs']
title = 'Clarify the required behaviour of locals()'
updated_at = <Date 2017-10-11.02:16:40.391>
user = 'https://github.com/ncoghlan'bugs.python.org fields:
activity = <Date 2017-10-11.02:16:40.391>
actor = 'ncoghlan'
assignee = 'ncoghlan'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2013-05-12.07:46:47.489>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 17960
keywords = ['patch']
message_count = 9.0
messages = ['188986', '188987', '296880', '296909', '297019', '297328', '301742', '302423', '304100']
nosy_count = 7.0
nosy_names = ['gvanrossum', 'ncoghlan', 'benjamin.peterson', 'ezio.melotti', 'njs', 'flox', 'xgdomingo']
pr_nums = ['3640']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue17960'
versions = ['Python 3.7']
Activity
ncoghlan commentedon May 12, 2013
As proposed at [1], I would like to tighten up the definition of locals so that defining enum members programmatically is officially supported behaviour.
I'll come up with a patch soonish.
[1] http://mail.python.org/pipermail/python-dev/2013-May/125917.html
ezio-melotti commentedon May 12, 2013
See also bpo-17546 and bpo-7083.
ncoghlan commentedon Jun 26, 2017
Since "soonish" turned out to be "4 years and counting", copying in the specifics of the proposal in from the old python-dev thread:
While nominally undefined, in practice lots of Python programs depend on the locals() builtin behaving exactly how it behaves in CPython.
PyPy at least has replicated that behaviour faithfully (to the extent of replicating our weird trace function related misbehaviour, as recently pointed out in issue bpo-30744)
For module scopes and class scopes (and the corresponding forms of exec and eval), the expected behaviour is relatively straightforward to both define and implement:
at module scope, as well as when using exec() or eval() with a
single namespace, locals() must return the same thing as globals(),
which must be the actual execution namespace. Subsequent execution may
change the contents of the returned mapping, and changes to the
returned mapping must change the execution environment.
at class scope, as well as when using exec() or eval() with separate
global and local namespaces, locals() must return the specified local
namespace (which may be supplied by the metaclass __prepare__ method
in the case of classes). Subsequent execution may change the contents
of the returned mapping, and changes to the returned mapping must
change the execution environment. For classes, this mapping will not
be used as the actual class namespace underlying the defined class
(the class creation process will copy the contents to a fresh
dictionary that is only accessible by going through the class
machinery).
It would be a lot simpler to document the expected behaviour at function scope if locals() were to be updated to return a true snapshot (i.e. a copy of f_locals, rather than a direct reference), with direct access to the shared locals reference requiring going through the frame attribute. That way, trace functions could still modify local variables (since they use
frame.f_locals), but setting a trace function wouldn't suddenly have the side effect of making modifications to locals() take effect at function scope.gvanrossum commentedon Jun 26, 2017
I've tried thinking though a few scenarios, and I think I'm +1 (or at least
+0 or +0.5) on the proposed change to locals(), and of course I'm happy
that we're going to specify its behavior better.
ncoghlan commentedon Jun 27, 2017
Guido: perhaps I should run this update through the PEP process?
Even though the actual proposed change is only to a pretty obscure edge case (having multiple concurrent live references to the result of locals() for a function namespace), the extra visibility should help developers of alternative implementations be clear on what is happening.
gvanrossum commentedon Jun 29, 2017
Yeah, I think a short PEP would be helpful here.
ncoghlan commentedon Sep 8, 2017
Posted as PEP-558:
ncoghlan commentedon Sep 18, 2017
Status update: I've posted an initial PR to bpo-30744 that relies on the trace hook semantic change proposed in the PEP to resolve the trace hook/cell reference incompatibility reported there.
That provides confidence that it really is only the semantics of *trace hooks* that we need to change, rather than anything about locals() or frame.f_locals in general.
So the next steps will be to do a final editing pass on the current PEP to account for the reference implementation, and then send it to python-dev for official review and pronouncement.
ncoghlan commentedon Oct 11, 2017
Nathaniel raised a valid concern about the draft PEP over in https://bugs.python.org/issue30744#msg302475, so I've been considering whether or not it would be possible to make the write-through proxy idea work without introducing other problems.
I think I have a workable design concept for that approach now: https://bugs.python.org/issue30744#msg304099
So the next step will be to see if that actually does work as well as I think it will, and if so, update the PEP accordingly.
ncoghlan commentedon May 6, 2024
With PEP 667 implemented for 3.13b1 (#115153) and the
locals()docs updated accordingly (#118265), this can finally be marked as resolved!gvanrossum commentedon May 6, 2024
Hm, there's more docs work to be done though. The new C APIs mentioned in the PEP haven't been added there and the deprecated ones haven't been marked as deprecated. The descriptions of the deprecated ones may also have to be updated to explain what they actually do (not much, in most cases).Never mind, see gh-74929.ncoghlan commentedon May 10, 2024
Yeah, this part is done (since it's just about the Python level API of
locals()), but there are other related open issues that can't be considered complete yet.