`del` doesn't delete object and `x = None` doesn't create `None` object

[rcasero]

Before creating a new issue, please check the FAQ to see if your question is answered there.

Environment data

• debugpy version: 1.6.3 (run import debugpy; print(debugpy.__version__) if uncertain)
• OS and version: MacOS X Ventura 13.0.1
• Python version (& distribution if applicable, e.g. Anaconda): 3.8.13, miniconda 3
• Using VS Code or Visual Studio: VS Code Version: 1.73.1 (Universal)

Actual behavior

In Debug Console

• del x is ignored.
• x = None doesn't create an object.

Expected behavior

• del x should delete the object.
• x = None should create an object with None value.

Steps to reproduce:

1. Create dummy script with just

   import sys

   and place a breakpoint on that line
2. Start debugging session using remote -SSH extension on server running Linux CentOS Stream 8, with Microsoft Python extension and following launch.json. The script

   {
       "version": "0.2.0",
       "configurations": [
           {
               "name":       "Python: Current File",
               "type":       "python",
               "request":    "launch",
               "cwd":        "${workspaceFolder}/bin",
               "program":    "${file}",
               "console":    "integratedTerminal",
               "redirectOutput": true,
               "justMyCode": true,
               "env": {"PYDEVD_WARN_SLOW_RESOLVE_TIMEOUT": "2"}
           }
       ]
   }

3. Go to Debug Console
4. Create variable x=5
5. Print variable print(x). This shows 5
6. Delete variable del x.
7. Print deleted variable print(x). This still shows 5. Even after import gc; gc.collect(), x still exists.
8. Create new variable with None value. y = None
9. Print new variable print(y). This produces the error

   Traceback (most recent call last):
     File "<string>", line 1, in <module>
   NameError: name 'y' is not defined
   

10. However, if a previously defined variable is assigned None, e.g. x = None, then the variable is still defined.

[int19h]

The part about assigning to None not working is because it happens to hit this check:

debugpy/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_save_locals.py

Lines 86 to 87 in 79b7c56

	for key, val in updated_globals.items():
	if initial_globals.get(key) is not val:

del should probably also be handled in this code, by doing the comparison in the other direction (loop over initial vars, check presence in updated).

[fabioz]

I've just provided a PR for this.

So, it works in the case given, but still, it has caveats in that it's not possible to del a var from a variable depending on how it's set (it works for variables you create in the frame during evaluation but may not work for variables already defined in the frame -- this is a CPython limitation).

The example below shows what I mean:

import ctypes
import sys

def method():
    A = 10
    return sys._getframe()

frame = method()

del frame.f_locals['A']
ctypes.pythonapi.PyFrame_LocalsToFast(ctypes.py_object(frame), ctypes.c_int(0))

# "A" is still there even though we deleted it...
print(frame.f_locals)