bpo-45235: Fix argparse overrides namespace with subparser defaults #28420

ALSchwalm · 2021-09-17T15:17:59Z

argparse will now preserve existing values in a provided namespace
even if a subparser has a default for a value in the namespace. For
example:

import argparse
parser = argparse.ArgumentParser()
sub = parser.add_subparsers()
example_subparser = sub.add_parser("example")
example_subparser.add_argument("--flag", default=10)
print(parser.parse_args(["example"], argparse.Namespace(flag=20)))

This snippet now returns 'Namespace(flag=20)' instead of
'Namespace(flag=10)'.

https://bugs.python.org/issue45235

argparse will now preserve existing values in a provided namespace even if a subparser has a default for a value in the namespace. For example: import argparse parser = argparse.ArgumentParser() sub = parser.add_subparsers() example_subparser = sub.add_parser("example") example_subparser.add_argument("--flag", default=10) print(parser.parse_args(["example"], argparse.Namespace(flag=20))) This snippet now returns 'Namespace(flag=20)' instead of 'Namespace(flag=10)'.

the-knights-who-say-ni · 2021-09-17T15:18:02Z

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rhettinger · 2021-09-17T15:58:46Z

This looks good.

Please add a Misc/NEWS entry with blurb.

ALSchwalm · 2021-09-17T16:57:03Z

Done, I have also signed the CLA, it's just not showing up yet.

miss-islington · 2021-09-18T04:20:34Z

Thanks @ALSchwalm for the PR, and @rhettinger for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

bedevere-bot · 2021-09-18T04:20:43Z

GH-28442 is a backport of this pull request to the 3.10 branch.

bedevere-bot · 2021-09-18T04:20:47Z

GH-28443 is a backport of this pull request to the 3.9 branch.

miss-islington · 2021-09-18T04:24:41Z

Thanks @ALSchwalm for the PR, and @rhettinger for merging it 🌮🎉.. I'm working now to backport this PR to: 3.10.
🐍🍒⛏🤖

miss-islington · 2021-09-18T04:24:41Z

Thanks @ALSchwalm for the PR, and @rhettinger for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖

…ythonGH-28420) (cherry picked from commit a6e8db5) Co-authored-by: Adam Schwalm <adamschwalm@gmail.com>

…H-28420) (GH-28442)

…H-28420) (GH-28443)

…ythonGH-28420)

tacaswell · 2021-09-27T21:43:00Z

Lib/argparse.py

@@ -1843,11 +1844,6 @@ def parse_known_args(self, args=None, namespace=None):
                    if action.default is not SUPPRESS:
                        setattr(namespace, action.dest, action.default)

-        # add any parser defaults that aren't present


Does this block need to be moved below? This change is breaking traitlets (and IPython) who rely on there being a default value set before the Action logic fires in self._parse_known_args.

I had to move setting the defaults to after the actions because otherwise subparser defaults will be hidden by the parent's defaults (e.g., test_set_defaults_on_parent_and_subparser will fail). It's not obvious to me how that could be avoided. Perhaps the defaults could be exposed in some other way for libraries that depend on that?

I see.

Maybe all that is needed is documentation that relying on the defaults to be there before the Actions run is broken. I am pretty sure that the old behavior was not intentional and that you used to have access to mutable default values in the Actions was a weird implementation detail.

This change on when default value is set also breaks azure-cli: Azure/azure-cli#20269 (comment)

In python/cpython#28420 the order of setting the default values on the Namespace object and processing the Actions. This means that in the `__call__` method of `_FlagAction` the `_flags` attribute has not yet been put on the namespace. This change makes `_FlagAction.__call__` forgiving if the `_flags` attribute does not exist (by creating it!).

In python/cpython#28420 the order of setting the default values on the Namespace object and processing the Actions was reversed. This means that in the `__call__` method of `_FlagAction` the `_flags` attribute has not yet been put on the namespace. This change makes `_FlagAction.__call__` forgiving if the `_flags` attribute does not exist (by creating it!).

jiasli · 2021-11-10T07:12:14Z

Unfortunately, this causes a breaking change and backward compatibility issue as detailed in

…faults (pythonGH-28420) (pythonGH-28443)" This reverts commit a18d522.

* Revert "bpo-45235: Fix argparse overrides namespace with subparser defaults (GH-28420) (GH-28443)" This reverts commit a18d522.

…GH-29525) * Revert "bpo-45235: Fix argparse overrides namespace with subparser defaults (pythonGH-28420) (pythonGH-28443)" This reverts commit a18d522. (cherry picked from commit 807f839) Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>

…GH-29525) * Revert "bpo-45235: Fix argparse overrides namespace with subparser defaults (pythonGH-28420) (pythonGH-28443)" This reverts commit a18d522.

the-knights-who-say-ni added the CLA not signed label Sep 17, 2021

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rhettinger added needs backport to 3.9 only security fixes needs backport to 3.10 only security fixes type-bug An unexpected behavior, bug, or error labels Sep 17, 2021

📜🤖 Added by blurb_it.

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rhettinger merged commit a6e8db5 into python:main Sep 18, 2021

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bedevere-bot removed the needs backport to 3.10 only security fixes label Sep 18, 2021

bedevere-bot removed the needs backport to 3.9 only security fixes label Sep 18, 2021

rhettinger added needs backport to 3.9 only security fixes needs backport to 3.10 only security fixes labels Sep 18, 2021

rhettinger pushed a commit that referenced this pull request Sep 18, 2021

bpo-45235: Fix argparse overrides namespace with subparser defaults (G…

6e4101a

…H-28420) (GH-28442)

rhettinger pushed a commit that referenced this pull request Sep 18, 2021

bpo-45235: Fix argparse overrides namespace with subparser defaults (G…

a18d522

…H-28420) (GH-28443)

niyas-sait pushed a commit to niyas-sait/cpython that referenced this pull request Sep 21, 2021

bpo-45235: Fix argparse overrides namespace with subparser defaults (p…

19ba77f

…ythonGH-28420)

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tacaswell mentioned this pull request Sep 27, 2021

FIX: account for changes in py310 ipython/traitlets#678

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jiasli mentioned this pull request Nov 10, 2021

Global Arguments stop working in Python 3.9.8 Azure/azure-cli#20269

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jiasli mentioned this pull request Nov 11, 2021

{CI} Use Python 3.10 as the default and disable Python 3.9 tests Azure/azure-cli#20287

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rhettinger added a commit to rhettinger/cpython that referenced this pull request Nov 11, 2021

Revert "bpo-45235: Fix argparse overrides namespace with subparser de…

c2278aa

…faults (pythonGH-28420) (pythonGH-28443)" This reverts commit a18d522.

rhettinger added a commit that referenced this pull request Nov 12, 2021

bpo-45235: Revert an argparse bugfix that caused a regression (GH-29525)

807f839

* Revert "bpo-45235: Fix argparse overrides namespace with subparser defaults (GH-28420) (GH-28443)" This reverts commit a18d522.

This was referenced Nov 12, 2021

[3.10] bpo-45235: Revert an argparse bugfix that caused a regression (GH-29525) #29530

Merged

[3.9] bpo-45235: Revert an argparse bugfix that caused a regression (GH-29525) #29531

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tuzi3040 mentioned this pull request Nov 13, 2021

azure-cli: migrate to python@3.10 Homebrew/homebrew-core#86758

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jiasli mentioned this pull request Dec 13, 2021

bpo-45235: Fix argparse namespace overridden by subparsers default #29574

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ALSchwalm mannequin mentioned this pull request Apr 10, 2022

argparse does not preserve namespace with subparser defaults #89398

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bpo-45235: Fix argparse overrides namespace with subparser defaults #28420

bpo-45235: Fix argparse overrides namespace with subparser defaults #28420

ALSchwalm commented Sep 17, 2021 •

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bpo-45235: Fix argparse overrides namespace with subparser defaults #28420

bpo-45235: Fix argparse overrides namespace with subparser defaults #28420

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