it's no longer possible to delete items from f_locals (FrameLocalsProxy) in 3.13+

Bug report

Bug description:

           try:
                driver.switch_to.alert.accept()
                driver.switch_to.alert.dismiss()
            except:
                pass

In the example script above, if the code falls into the except: pass block due to an alert not being displayed, the script will ultimately fail (even if no coding errors are encountered) with FAILED (errors=1) .
Also, the following messages will be displayed in the output:

This code works fine in Python 3.12. The script succeeds with message OK and none of the above error messages. We can't move to 3.13 version until this is fixed.

CPython versions tested on:

3.13

Operating systems tested on:

Windows

Linked PRs

@gaogaotiantian

This sounds like an issue in the version of pydevd bundled by the Microsoft Python extension.

I assume it's related to the FrameLocalsProxy changes from PEP-667; cc @gaogaotiantian.

Maybe FrameLocalsProxy should just implement pop? (Or better yet, the rest of the dict interface.)

Maybe FrameLocalsProxy should just implement pop? (Or better yet, the rest of the dict interface.)

problem is you cannot remove items from FrameLocalsProxy:

cpython/Objects/frameobject.c

Line 162 in d83fcf8

    
           PyErr_SetString(PyExc_TypeError, "cannot remove variables from FrameLocalsProxy");

I think pydevd has to do

try:
    del frame.f_locals["__exception__"]
except KeyError:
    pass
except TypeError:
    frame.f_locals["__exception__"] = None

@markshannon

The reason why FrameLocalsProxy did not support pop and del in the first place, is because it does not make sense to "remove" a local variable from the frame. However, in this specific case, the debugger is trying to remove an extra variable, which is artificially added. I think it might be fine to check if it's an actual local variable and only report error when it is.

@markshannon and @gvanrossum as this was the discussion result from PEP 667 implementation.

I'm not particularly familiar with how PEP 667 was implemented. From my understanding, deleting from f_locals was previously possible--is it that it can't be implemented right now, or is it that it's dangerous to expose to users?

Popping and deleting keys in the "extras" dict should be fine.
It gets complicated when the key is a "fast" local.

In the style of the PEP, this would give us:

    def __delitem__(self, name):
        del self._extra_locals[name]

    def pop(self, name, default):
        return self._extra_locals.pop(name, default)

The semantics of f_locals has changed. f_locals used to be a pure dict which was written back to the real locals on certain occasions (the most common one is returning from trace functions). The old behavior, when you remove a key from f_locals, if it was written back, was to set the local variable to None and generate a warning - a very inconsistent and confusing behavior.

Popping and deleting keys in the "extras" dict should be fine. It gets complicated when the key is a "fast" local.

Let's allow popping and deleting keys in extras, and generate an exception (we are generating one now on del) if the user tries to remove a real local variable. Do we consider this a bug so we need to backport it to 3.13?

The semantics of f_locals has changed. f_locals used to be a pure dict which was written back to the real locals on certain occasions (the most common one is returning from trace functions). The old behavior, when you remove a key from f_locals, if it was written back, was to set the local variable to None and generate a warning - a very inconsistent and confusing behavior.

how do you generate the warning?

I tried this on 3.12 and didn't see a warning, and the local didn't get set to None:

import sys

class A:
    pass


def foo():
    a = A()
    f_locals = sys._getframe().f_locals
    del f_locals['a']
    print(a)
    print(f_locals)


foo()

 graingert@conscientious  ~/projects  python3.12 -Werror demo.py
<__main__.A object at 0x7ae782f40230>
{}
 graingert@conscientious  ~/projects  python3.13 -Werror demo.py
Traceback (most recent call last):
  File "/home/graingert/projects/demo.py", line 15, in <module>
    foo()
    ~~~^^
  File "/home/graingert/projects/demo.py", line 10, in foo
    del f_locals['a']
        ~~~~~~~~^^^^^
TypeError: cannot remove variables from FrameLocalsProxy

I tried this on 3.12 and didn't see a warning, and the local didn't get set to None:

This is the case where the f_locals is not written back. Like I said, it was only written back on certain occasions, for example, when it returns from a trace function. And that's why we need to change it.

Hello. Just checking back on this bug. Is there a plan to fix it and update the current Python 3.13 version with the fix? If so, when is that expected so I can reinstall Python 3.13? Thanks.

The fix is in #125590 and it should be merged soon. We don't update the current version of 3.13, so 3.13.0 will be always like this without the fix. However, 3.13.1 should come up with the fix. It's estimated to be released two months after 3.13.0 so probably December.

gh-125590: Allow FrameLocalsProxy to delete and pop keys from extra l…

pythongh-125590: Allow FrameLocalsProxy to delete and pop keys from e…

[3.13] gh-125590: Allow FrameLocalsProxy to delete and pop keys from …

The fix is in #125590 and it should be merged soon. We don't update the current version of 3.13, so 3.13.0 will be always like this without the fix. However, 3.13.1 should come up with the fix. It's estimated to be released two months after 3.13.0 so probably December.

Has version 3.13.1 been released yet?

Expected on the 12th https://peps.python.org/pep-0719/#bugfix-releases

I believe the release is currently in progress, actually. The announcements should come shortly.

https://discuss.python.org/t/python-3-13-1-3-12-8-3-11-11-3-10-16-and-3-9-21-are-now-available/73214

It's out

pythongh-125590: Allow FrameLocalsProxy to delete and pop keys from e…

Build with python 3.13 in parallel (#18611)

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it's no longer possible to delete items from f_locals (FrameLocalsProxy) in 3.13+ #125590

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it's no longer possible to delete items from f_locals (FrameLocalsProxy) in 3.13+ #125590

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

Activity

JelleZijlstra commented on Oct 16, 2024

ZeroIntensity commented on Oct 16, 2024

graingert commented on Oct 16, 2024

gaogaotiantian commented on Oct 16, 2024

ZeroIntensity commented on Oct 16, 2024

markshannon commented on Oct 16, 2024

gaogaotiantian commented on Oct 16, 2024

gaogaotiantian commented on Oct 16, 2024

graingert commented on Oct 16, 2024

gaogaotiantian commented on Oct 16, 2024

113908 commented on Oct 18, 2024

gaogaotiantian commented on Oct 18, 2024

113908 commented on Dec 3, 2024

graingert commented on Dec 3, 2024

JelleZijlstra commented on Dec 3, 2024

graingert commented on Dec 4, 2024

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