Launching module with --nodebug breaks imports

[ghost]

My project structure is

demo/
  foo/
    __main__.py
    bar.py

bar.py is an empty file. __main__.py contains the single statement import bar. My launch.json configuration is

{
    "name": "Demo",
    "type": "pythonExperimental",
    "request": "launch",
    "program": "${workspaceFolder}/foo",
    "console": "integratedTerminal"
}

Running in debug mode causes the program to immediately terminate successfully, as expected. With --nodebug the program crashes with a ModuleNotFoundError exception complaining that there is no module named bar.

I am running ptvsd in VSCode with Python3.6 on Ubuntu 18.04.

[karthiknadig]

Can you try this configuration?

        {
            "name": "Module: foo",
            "type": "pythonExperimental",
            "request": "launch",
            "console": "integratedTerminal",
            "module": "foo",
            "cwd": "${workspaceFolder}",
            "env": {"PYTHONPATH": "${workspaceFolder}"}
        },

[ghost]

Fails with the same error.

[karthiknadig]

Can you try it with an empty __init__.py file? You currently have __main__.py, and bar.py. Add a __init__.py.

[ghost]

I did, same issue.

[karthiknadig]

Can you tell me the version of ptvsd? I am not seeing this in master. But I want to make sure we have addressed this.

[ghost]

I'm using 4.1.1a10, which ships with the current version of vscode.

Actually I'd like to clarify that the configuration you gave above (with module set instead of program) actually fails with and without --nodebug. Presumably this happens since it's trying to run something like python -m ptvsd -m foo which should fail since bar needs to be a relative import. But with the original configurations vscode runs something like python -m ptvsd /path/to/foo which works fine, but python -m ptvsd --nodebug /path/to/foo fails with the error mention in the OP.

To add some motivation for this bug, I found it when working on a Django app, which internally uses some wonky imports that break when running ptvsd in nodebug mode.

[karthiknadig]

With the configuration i gave it should work if you have __init__.py. However, there seems to be a issue with namespace packages, in that case too. I will look into this.

[fabioz]

This is not a bug -- things are working as expected.

i.e.: if you have a structure such as the one below where demo is the folder in the PYTHONPATH, you should write an import on __main__.py as from foo import bar or from . import bar (a single import bar expects the bar module to be on the root -- which is not the case).

demo/
  foo/
    __main__.py
    bar.py

As a note, I suggest you don't use relative imports on this case, only full imports (because if it's a relative import it won't work unless python is executed with the -m flag -- when executed passing the __main__.py file itself it'd fail).

[ghost]

Regardless of how you feel imports should be done, the fact remains that python foo will execute the code without throwing an exception, as will python -m ptvsd foo. Passing the --nodebug option should break this behaviour.

Incidentally, I agree that import bar is a poor way to perform the import. However, projects like Django will perform imports this way e.g. when setting DJANGO_SETTINGS_MODULE, so this bug will break programs that ought to work.

[fabioz]

Note that it'll fail in the command line if python -m foo is used in plain Python, so, ideally you wouldn't write imports that way, but I agree with your point that ptvsd should replicate what Python does even if it means shooting yourself in the foot ;)

-- Python itself appears to be executing foo/__main__.py directly (so, it'll find bar as a root module -- and if you do in another place an from foo import bar you'll have another version of the same module).