| msg321226 - (view) |
Author: Serhiy Storchaka (serhiy.storchaka) *  |
Date: 2018-07-07 17:33 |
| There is a flaw in the example in the documentation for contextlib.nullcontext. def process_file(file_or_path): if isinstance(file_or_path, str): # If string, open file cm = open(file_or_path) else: # Caller is responsible for closing file cm = nullcontext(file_or_path) with cm as file: # Perform processing on the file 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 ). 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(). |
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| msg321254 - (view) |
Author: Alyssa Coghlan (ncoghlan) * |
Date: 2018-07-08 04:14 |
| 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: from functools import partial from os import fspath def process_file(file_or_path): try: as_fspath = os.fspath(file_or_path) except TypeError: # If it's a file, caller is responsible for closing it make_cm = partial(nullcontext, file_or_path) else: # If it's a path, open file when the context is entered make_cm = partial(open, as_fspath) with make_cm() as file: # Perform processing on the file 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: def update_resource(resource, updates, resource_lock=None): if resource_lock is None: resource_lock = nullcontext() with resource_lock: resource.apply_updates(updates) (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) |
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| msg321319 - (view) |
Author: Alyssa Coghlan (ncoghlan) * |
Date: 2018-07-09 13:49 |
| New changeset c287545d62edf1a1ee65727d3c57befa8c99c13a by Nick Coghlan (Daniel Porteous) in branch 'master': bpo-34067: Include a more easily understood example for nullcontext (GH-8158) https://github.com/python/cpython/commit/c287545d62edf1a1ee65727d3c57befa8c99c13a |
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| msg321322 - (view) |
Author: miss-islington (miss-islington) |
Date: 2018-07-09 14:11 |
| New changeset 1b6e21730417c56748d500a96cafebbaa1bd0120 by Miss Islington (bot) in branch '3.7': bpo-34067: Include a more easily understood example for nullcontext (GH-8158) https://github.com/python/cpython/commit/1b6e21730417c56748d500a96cafebbaa1bd0120 |
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| msg321461 - (view) |
Author: STINNER Victor (vstinner) * |
Date: 2018-07-11 14:37 |
| Can we close this issue? |
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| msg321463 - (view) |
Author: Serhiy Storchaka (serhiy.storchaka) * |
Date: 2018-07-11 14:39 |
| No, the merged PR is not directly related to the original issue. |
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| msg321472 - (view) |
Author: Alyssa Coghlan (ncoghlan) * |
Date: 2018-07-11 15:03 |
| 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. |
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| msg378332 - (view) |
Author: Irit Katriel (iritkatriel) * |
Date: 2020-10-09 15:02 |
| For the original problem, how about this? def process_file(file_or_path): if isinstance(file_or_path, str): # If string, open file cm = open else: # Caller is responsible for closing file cm = nullcontext with cm(file_or_path) as file: # Perform processing on the file |
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