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gh-96859: Optimize argparse actions 'append', 'append_const' and 'extend' #124909

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gh-96859: Optimize argparse actions 'append', 'append_const' and 'extend' #124909

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gh-124745: Optimize argparse actions 'append', 'append_const' and 'ex…
serhiy-storchaka Oct 2, 2024
d536199
Update the NEWS entry.
serhiy-storchaka Oct 3, 2024
5e03f2c
Update Misc/NEWS.d/next/Library/2024-10-02-21-34-08.gh-issue-96859.ef…
serhiy-storchaka Oct 11, 2024
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9 changes: 6 additions & 3 deletions Lib/argparse.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1016,7 +1016,8 @@ def __init__(self,

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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@alexeypa alexeypa Oct 3, 2024

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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']

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@serhiy-storchaka serhiy-storchaka Oct 3, 2024

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Good point. There are no tests for this (and some other corner cases).

items = _copy_items(items)
items.append(values)
setattr(namespace, self.dest, items)

Expand Down Expand Up @@ -1045,7 +1046,8 @@ def __init__(self,

def __call__(self, parser, namespace, values, option_string=None):
items = getattr(namespace, self.dest, None)
items = _copy_items(items)
if items is self.default:
items = _copy_items(items)
items.append(self.const)
setattr(namespace, self.dest, items)

Expand Down Expand Up @@ -1238,7 +1240,8 @@ def __call__(self, parser, namespace, values, option_string=None):
class _ExtendAction(_AppendAction):
def __call__(self, parser, namespace, values, option_string=None):
items = getattr(namespace, self.dest, None)
items = _copy_items(items)
if items is self.default:
items = _copy_items(items)
items.extend(values)
setattr(namespace, self.dest, items)

Expand Down
4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2024-10-02-21-34-08.gh-issue-96859.efyaxu.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Optimize :mod:`argparse` actions 'append', 'append_const' and 'extend' for
the case when the option is specified many times.
The target list is no longer copied every time the action is taken, only
the default value is copied the first time the action is taken.
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