gh-143636: fix a crash when calling __replace__ on invalid SimpleNamespace instances

[picnixz]

• Issue: Type confusion in SimpleNamespace.__replace__ via re-entrant __new__ #143636

[serhiy-storchaka]

There are many ways to clone an object, you should make a decision whether call __init__ and/or __new__bor just create an instance of the base class. This is done differently for other types.

The following code follows the way of __reduce__() (which is also used in copy.copy()). It is equivalent to:

new_ns = type(ns)()
new_dict = dict(ns.__dict__)
for k, v in kwargs.items():
    new_dict[k] = v
for k, v in new_dict.items():
    new_ns.__dict__[k] = v

but more efficient.

This is what copy.copy() will execute.

We can implement this for the case when the result of type(ns)() is not a SimpleNamespace, but this will add an amount of code, and all this for the case which perhaps never occurs in real code. So, I am fine with simply raising an error.

[picnixz]

I didn't want to change the existing behavior. Maybe we could decide in a follow-up PR how to change this?

[picnixz]

@serhiy-storchaka Do you want me to address your concerns or are you fine with the current approach I took? I didn't want to change the way we construct the instances so that I can backport this fix and I suggest that we address the construction alternatives in a follow-up PR. WDYT?

[vstinner]

LGTM. Adding _PyNamespace_Check() test sounds like the right fix for this problem. I don't have a strong opinion on the error message.

[miss-islington-app]

Thanks @picnixz for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @picnixz, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 97968564b61965f2a65a9be8af731cee6913eb7a 3.13

[bedevere-app]

GH-145938 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-145940 is a backport of this pull request to the 3.13 branch.