gh-96859: Optimize argparse actions 'append', 'append_const' and 'extend' #124909
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…nd 'extend' Previously, the target list was copied every time the action was taken. Now only the default value is copied the first time the action is taken.
This was referenced Oct 2, 2024
eendebakpt
approved these changes
Oct 2, 2024
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The title of the PR should be changed to gh-96859: Optimize ... to link to a github issue. (gh-124745 is the number of an alternative PR)
| @@ -0,0 +1,4 @@ | |||
| Optimize :mod:`argparse` actions 'append', 'append_const' and 'extend' for | |||
| the case when the option is specified many times. | |||
| The targed list no longer copied every time the action is taken, only | |||
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Suggested change
| The targed list no longer copied every time the action is taken, only | |
| The target list is no longer copied every time the action is taken, only |
alexeypa
reviewed
Oct 3, 2024
| def __call__(self, parser, namespace, values, option_string=None): | ||
| items = getattr(namespace, self.dest, None) | ||
| items = _copy_items(items) | ||
| if items is self.default: |
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As far as I can tell this changes the behavior in some corner cases. For example, when parse_known_args() is called with a not empty namespace. The new code will only clone lists when default is set. The old code would have cloned any existing list. This matters if the caller has another reference to the same list.
Something like this
some_existing_list = []
ns = argparse.Namespace()
ns.setattr("foo", some_existing_list)
parser = argparse.ArgumentParser()
parser.add_argument('--foo', action='append')
...
... = parser.parse_known_args(args=['--foo=xxx'], namespace=ns)
# At this point some_existing_list is ['xxx']There was a problem hiding this comment.
Good point. There are no tests for this (and some other corner cases).
serhiy-storchaka
changed the title
hugovk
reviewed
Oct 3, 2024
Misc/NEWS.d/next/Library/2024-10-02-21-34-08.gh-issue-96859.efyaxu.rst
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…yaxu.rst Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
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Previously, the target list was copied every time the action was taken. Now only the default value is copied the first time the action is taken.