VS Code + Flask: ValueError: sourcecode string cannot contain null bytes

Question

I've just started using VS Code with the Python plugin. I've set up a venv, launched code within that venv, installed all my necessary modules and updated my launch.json to launch flask apps as follows:

        {
        "name": "Python: Flask (0.11.x or later)",
        "type": "python",
        "request": "launch",
        "stopOnEntry": false,
        "pythonPath": "${config:python.pythonPath}",
        "program": "${workspaceFolder}/env/Scripts/flask.exe",
        "cwd": "${workspaceFolder}",
        "env": {
            "FLASK_APP": "${workspaceFolder}/main.py"
        },
        "args": [
            "run",
            "--no-debugger",
            "--no-reload"
        ],
        "envFile": "${workspaceFolder}/.env",
        "debugOptions": [
            "RedirectOutput"
        ]
    },

However when I launch the debugger for Flask in VS Code the following appears in my debug console:

ValueError: source code string cannot contain null bytes

However, if I just launch the app from the commandline doing:

set FLASK_APP=main.py
python -m flask

it works just fine. I'm sure it's something stupid I've done but I can't figure out what that is.

Note also that pythonPath points to my venv python installation.

I don't know what flask is, and I don't have time to look into it right now, but: "program": "${workspaceFolder}/env/Scripts/flask.exe" Is that right, or should it be a Python script or something? — David Knipe
– David Knipe, Commented Dec 22, 2017 at 9:08
According to this link: code.visualstudio.com/docs/python/debugging it should point at the flask executable. — FreeMemory
– FreeMemory, Commented Dec 22, 2017 at 9:29
Hmmm... And is there any way to run flask through the usual Python interpreter, instead of flask.exe? For example, do you have a file called flask.py? It does seem slightly unlikely to me that launch.json would have a field which can be either a binary or a script. — David Knipe
– David Knipe, Commented Dec 22, 2017 at 10:20
Yes, you can do python -m flask In either case, though, your app's entry point is defined as an env var, FLASK_APP. — FreeMemory
– FreeMemory, Commented Dec 22, 2017 at 11:34
You might call flask itself the entry point, I suppose it depends on your point of view. But my point is: try setting "program" to the Python script for flask, if you can find it. It must exist somewhere, if python -m flask works. — David Knipe
– David Knipe, Commented Dec 22, 2017 at 12:01

Kristoffer Bakkejord · Accepted Answer · 2017-12-26 22:26:43Z

I am seeing the ValueError as well, and haven't found a proper solution for this, but have a workaround, using the "module": "flask":

{
    "name": "Python: Flask (0.11.x or later)",
    "type": "python",
    "request": "launch",
    "stopOnEntry": false,
    "pythonPath": "${config:python.pythonPath}",
    //"program": "${workspaceFolder}/env/Scripts/flask.exe",
    "module": "flask",
    "cwd": "${workspaceFolder}",
    "env": {
        "FLASK_APP": "${workspaceFolder}/main.py"
    },
    "args": [
        "run",
        "--no-debugger",
        "--no-reload"
    ],
    "envFile": "${workspaceFolder}/.env",
    "debugOptions": [
        "RedirectOutput"
    ]
}

It does, however, take some time to start the Flask app each time I start debugging, not sure if this is due to Flask being started via Python. Invoking flask.exe and python.exe -m flask from the command line seems to be evenly fast - notably faster than starting debugging in VS Code.

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