User scoped python.defaultInterpreterPath is constantly overwritten

[ajstair]

Type: Bug

Behaviour

When attempting to set:
"python.defaultInterpreterPath": "${workspaceFolder}/backend/.venv/bin/python3"
in the user scope settings, reloading the extension causes replacement of that variable with:
"python.defaultInterpreterPath": "/usr/bin/python3",

Note: I see the file update live around when the last two lines in the Python log display:

[info] Starting Pylance language server.
[info] Discover tests for workspace name: undefined - uri: /Users/username/Library/Application Support/Code/User/settings.json

There are also some errors in the logs below.

Steps to reproduce:

1. Set "python.defaultInterpreterPath": "${workspaceFolder}/backend/.venv/bin/python3" in user settings.json
2. Reload window/quit
3. New value is "python.defaultInterpreterPath": "/usr/bin/python3"

Diagnostic data

Output for Python in the Output panel (View→Output, change the drop-down the upper-right of the Output panel to Python)

2025-12-04 16:20:11.529 [info] Telemetry level is off
2025-12-04 16:20:11.529 [info] Experiments are disabled, only manually opted experiments are active.
2025-12-04 16:20:11.793 [info] > pyenv which python
2025-12-04 16:20:11.793 [info] cwd: .
2025-12-04 16:20:11.835 [info] > conda info --json
2025-12-04 16:20:11.837 [info] > ~/.anaconda/bin/conda info --json
2025-12-04 16:20:11.888 [info] Active interpreter [/Users/username/dev/workspace-name]:  /usr/bin/python3
2025-12-04 16:20:11.971 [info] Active interpreter [/Users/username/dev/workspace-name]:  /Users/username/dev/workspace-name/backend/.venv/bin/python
2025-12-04 16:20:11.972 [info] Discover tests for workspace name: workspace-name - uri: /Users/username/dev/workspace-name
2025-12-04 16:20:11.980 [info] > . ./backend/.venv/bin/activate && echo 'e8b39361-0157-4923-80e1-22d70d46dee6' && python ~/.vscode/extensions/ms-python.python-2025.18.0-darwin-arm64/python_files/printEnvVariables.py
2025-12-04 16:20:11.980 [info] shell: bash
2025-12-04 16:20:11.981 [info] > . ./backend/.venv/bin/activate && echo 'e8b39361-0157-4923-80e1-22d70d46dee6' && python ~/.vscode/extensions/ms-python.python-2025.18.0-darwin-arm64/python_files/printEnvVariables.py
2025-12-04 16:20:11.981 [info] shell: bash
2025-12-04 16:20:11.986 [error] getActivatedEnvironmentVariables Error: Command failed: . /Users/username/dev/workspace-name/backend/.venv/bin/activate && echo 'e8b39361-0157-4923-80e1-22d70d46dee6' && python /Users/username/.vscode/extensions/ms-python.python-2025.18.0-darwin-arm64/python_files/printEnvVariables.py
bash: line 1: 55171 Killed: 9               python /Users/username/.vscode/extensions/ms-python.python-2025.18.0-darwin-arm64/python_files/printEnvVariables.py

    at genericNodeError (node:internal/errors:983:15)
    at wrappedFn (node:internal/errors:537:14)
    at ChildProcess.exithandler (node:child_process:418:12)
    at ChildProcess.emit (node:events:531:35)
    at maybeClose (node:internal/child_process:1101:16)
    at Socket.<anonymous> (node:internal/child_process:456:11)
    at Socket.emit (node:events:519:28)
    at Pipe.<anonymous> (node:net:346:12) {
  code: 137,
  killed: false,
  signal: null,
  cmd: ". /Users/username/dev/workspace-name/backend/.venv/bin/activate && echo 'e8b39361-0157-4923-80e1-22d70d46dee6' && python /Users/username/.vscode/extensions/ms-python.python-2025.18.0-darwin-arm64/python_files/printEnvVariables.py"
}
2025-12-04 16:20:11.989 [error] getActivatedEnvironmentVariables Error: Command failed: . /Users/username/dev/workspace-name/backend/.venv/bin/activate && echo 'e8b39361-0157-4923-80e1-22d70d46dee6' && python /Users/username/.vscode/extensions/ms-python.python-2025.18.0-darwin-arm64/python_files/printEnvVariables.py
bash: line 1: 55172 Killed: 9               python /Users/username/.vscode/extensions/ms-python.python-2025.18.0-darwin-arm64/python_files/printEnvVariables.py

    at genericNodeError (node:internal/errors:983:15)
    at wrappedFn (node:internal/errors:537:14)
    at ChildProcess.exithandler (node:child_process:418:12)
    at ChildProcess.emit (node:events:531:35)
    at maybeClose (node:internal/child_process:1101:16)
    at Socket.<anonymous> (node:internal/child_process:456:11)
    at Socket.emit (node:events:519:28)
    at Pipe.<anonymous> (node:net:346:12) {
  code: 137,
  killed: false,
  signal: null,
  cmd: ". /Users/username/dev/workspace-name/backend/.venv/bin/activate && echo 'e8b39361-0157-4923-80e1-22d70d46dee6' && python /Users/username/.vscode/extensions/ms-python.python-2025.18.0-darwin-arm64/python_files/printEnvVariables.py"
}
2025-12-04 16:20:22.426 [info] Starting Pylance language server.
2025-12-04 16:20:22.703 [info] Discover tests for workspace name: undefined - uri: /Users/username/Library/Application Support/Code/User/settings.json

Extension version: 2025.18.0
VS Code version: Code 1.106.3 (Universal) (bf9252a2fb45be6893dd8870c0bf37e2e1766d61, 2025-11-25T22:28:18.024Z)
OS version: Darwin arm64 24.4.0
Modes:

• Python version (& distribution if applicable, e.g. Anaconda): 3.9.6.final.0
• Type of virtual environment used (e.g. conda, venv, virtualenv, etc.): System
• Value of the python.languageServer setting: Pylance

User Settings

languageServer: "Pylance"

Installed Extensions

Extension Name	Extension Id	Version
Bookmarks	ale	13.5.0
chatgpt	ope	0.4.49
claude-code	Ant	2.0.58
code-runner	for	0.12.2
continue	Con	1.2.11
copilot	Git	1.388.0
copilot-chat	Git	0.33.4
datawrangler	ms-	1.24.0
debugpy	ms-	2025.16.0
gitlens	eam	17.7.1
js-debug	ms-	1.105.0
js-debug-companion	ms-	1.1.3
jupyter	ms-	2025.9.1
jupyter-keymap	ms-	1.1.2
jupyter-renderers	ms-	1.3.0
live-server	ms-	0.4.16
pylint	ms-	2025.2.0
python	ms-	2025.18.0
rainbow-csv	mec	3.23.0
remote-containers	ms-	0.431.1
remote-explorer	ms-	0.5.0
remote-server	ms-	1.5.3
remote-ssh	ms-	0.120.0
remote-ssh-edit	ms-	0.87.0
remote-wsl	ms-	0.104.3
sql-developer	Ora	25.3.2
vscode-js-profile-table	ms-	1.0.10
vscode-jupyter-cell-tags	ms-	0.1.9
vscode-jupyter-slideshow	ms-	0.1.6
vscode-pylance	ms-	2025.10.3
vscode-python-envs	ms-	1.12.0
vscode-remote-extensionpack	ms-	0.26.0
vscode-tailwindcss	bra	0.14.29
vscode-yaml	red	1.19.1

System Info

Item	Value
CPUs	Apple M3 Max (16 x 2400)
GPU Status	2d_canvas: enabled direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: enabled_on trees_in_viz: disabled_off video_decode: enabled video_encode: enabled webgl: enabled webgl2: enabled webgpu: enabled webnn: disabled_off
Load (avg)	4, 4, 4
Memory (System)	64.00GB (1.45GB free)
Process Argv
Screen Reader	no
VM	0%

[ajstair]

I think I found the root cause. The python executable was somehow corrupted; the extension silently was falling back to the global python installation. The status bar interpreter version stayed on the corrupted version (3.11.5) but VSCode effectively started using the global installation (3.9). I confirmed that the executable (which was managed by UV) was corrupted by attempting to call it manually.

This was a nightmare to debug - nowhere in the vscode or extension logs could I find an indication that the python executable was problematic. The interpreter selection status was misleading as well, making it seem like interpreter selection was successful (while silently using another version under the hood!). The python environment management plugin also was displaying inconsistent information in this case.

I would recommend any of:
a) displaying an error notification to the user if the python interpreter selection fails, rather than it silently falling back to global
b) adding python/python environment extension logging for issues with the selected python executable