Repository navigation
Problem with contextlib.nullcontext #78248
Description
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 = None
closed_at = None
created_at = <Date 2018-07-07.17:33:25.936>
labels = ['3.8', '3.9', '3.10', 'performance']
title = 'Problem with contextlib.nullcontext'
updated_at = <Date 2020-11-02.14:13:49.172>
user = 'https://github.com/serhiy-storchaka'bugs.python.org fields:
activity = <Date 2020-11-02.14:13:49.172>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2018-07-07.17:33:25.936>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 34067
keywords = ['patch']
message_count = 8.0
messages = ['321226', '321254', '321319', '321322', '321461', '321463', '321472', '378332']
nosy_count = 5.0
nosy_names = ['ncoghlan', 'eric.smith', 'serhiy.storchaka', 'miss-islington', 'iritkatriel']
pr_nums = ['8158', '8199']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue34067'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']
Activity
serhiy-storchaka commentedon Jul 7, 2018
There is a flaw in the example in the documentation for contextlib.nullcontext.
The execution can be interrupted by Ctrl-C between opening a file and entering a 'with' block. There is the same problem with the simple "with open(path) as file:", but it can be easily solved (see bpo-34066). It is harder to do for this example, because there are more opcodes between calling open() and entering 'with'. I afraid that this problem can't be solved in general case. Maybe contextlib.nullcontext has a flaw similar to the flaw in contextlib.nested().
ncoghlan commentedon Jul 8, 2018
That's certainly similar to the problems with contextlib.nested, but I don't think it's as inherently flawed as nested was. What I'd suggest we change the existing example to is this:
Optionally, we could also present a cleaner example where a pre-created context manager is passed in and we're just coping with the fact it may be None:
(I'll also note that ExitStack is *far* from being immune to the Ctrl-C problem, as it's implemented in Python itself, which allows its __exit__ method to be interrupted, as well as for interrupt to occur between a resource being created or acquired, and it being registered with the stack)
ncoghlan commentedon Jul 9, 2018
New changeset c287545 by Nick Coghlan (Daniel Porteous) in branch 'master':
bpo-34067: Include a more easily understood example for nullcontext (GH-8158)
c287545
miss-islington commentedon Jul 9, 2018
New changeset 1b6e217 by Miss Islington (bot) in branch '3.7':
bpo-34067: Include a more easily understood example for nullcontext (GH-8158)
1b6e217
vstinner commentedon Jul 11, 2018
Can we close this issue?
serhiy-storchaka commentedon Jul 11, 2018
No, the merged PR is not directly related to the original issue.
ncoghlan commentedon Jul 11, 2018
Right, the merged PR adds a simpler example that handles "ignore exceptions or not" by selecting between suppress() and nullcontext().
The documentation still needs updating to improve or remove the file-based example, and point out that for resources that get acquired on construction, you either need to delay construction to the start of the with statement, or else use ExitStack instead of nullcontext.
gpshead commentedon Apr 24, 2023
triage: removed the docs and easy tags as the remaining questions weren't right to call clear or "easy" for a new person in a sprint. :)