pola-rs/polars · error · RuntimeError

Cannot locate

Error message

Cannot locate {launch_file}

What it means

After checking argv, launch_debugging computes the repo's .vscode/launch.json path (two directories above launch.py) and raises this RuntimeError if the file does not exist, since it needs to rewrite the pid there.

Solutions

  1. Create .vscode/launch.json with the 'Rust LLDB' and 'Python: Debug Rust' configurations per https://docs.pola.rs/development/contributing/ide/#debugging
  2. Run the script from a proper polars source checkout where py-polars/debug/launch.py sits two levels under the repo root
  3. Verify the file exists: ls <repo-root>/.vscode/launch.json

Example fix

# before: no .vscode/launch.json in repo root
# after: create .vscode/launch.json containing the 'Rust LLDB' configuration with a "pid" field, per contributing guide
Defensive patterns

Strategy: validation

Validate before calling

from pathlib import Path
launch_file = Path('py-polars/debug/launch.py').parents[2] / '.vscode/launch.json'
assert launch_file.exists(), f'Missing {launch_file}; create it per the contributing guide'

Try / catch

try:
    run_debug_flow()
except RuntimeError as e:
    if str(e).startswith('Cannot locate'):
        print('Create .vscode/launch.json per https://docs.pola.rs/development/contributing/ide/#debugging')

Prevention

When it happens

Trigger: Running the debug flow from a checkout or worktree where .vscode/launch.json is missing, deleted, or was never created; running launch.py from an installed/copied location rather than the repo root layout.

Common situations: Fresh clones without the debug launch configuration set up; monorepo layouts where launch.py's parents[2] is not the repo root.

Understand the failure class

Background: "Config file not found": what it means and how to fix it in docker-sync, Maven, Vagrant, Turborepo and other tools — this error's family across 60 libraries.

Related errors

AI-assisted analysis of pola-rs/polars@fe841f959e (2026-09-18). Data as JSON: /api/errors/e01ef4991e11a661. Report an issue: GitHub.

Appendix: source

Thrown at py-polars/debug/launch.py:39

    and execute the originally-requested script.
    """
    if len(sys.argv) == 1:
        msg = (
            "launch.py is not meant to be executed directly; please use the `Python: "
            "Debug Rust` debugging configuration to run a python script that uses the "
            "polars library."
        )
        raise RuntimeError(msg)

    # Get the current process ID.
    pID = os.getpid()

    # Print to the debug console to allow VSCode to pick up on the signal and start the
    # Rust LLDB configuration automatically.
    launch_file = Path(__file__).parents[2] / ".vscode/launch.json"
    if not launch_file.exists():
        msg = f"Cannot locate {launch_file}"
        raise RuntimeError(msg)
    with launch_file.open("r") as f:
        launch_info = f.read()

    # Overwrite the pid found in launch.json with the pid for the current process.
    # Match the initial "Rust LLDB" definition with the pid defined immediately after.
    pattern = re.compile('("Rust LLDB",\\s*"pid":\\s*")\\d+(")')
    found = pattern.search(launch_info)
    if not found:
        msg = (
            "Cannot locate pid definition in launch.json for Rust LLDB configuration. "
            "Please follow the instructions in the debugging section of the "
            "contributing guide (https://docs.pola.rs/development/contributing/ide/#debugging) "
            "for creating the launch configuration."
        )
        raise RuntimeError(msg)

    launch_info_with_new_pid = pattern.sub(rf"\g<1>{pID}\g<2>", launch_info)
    with launch_file.open("w") as f:

View on GitHub (pinned to fe841f959e)