When I did not install the module "flask" in the Python environment currently used in VSCode:

Please use the command "pip --version" to check the source of the module installation tool "pip", the module is installed at this location:

Then, we can use the command "pip show flask" to check the installation location of the module "flask": (It checks whether the installation location of the module is consistent with the Python environment displayed in the lower left corner of VSCode.)

If the "reportMissingModuleSource" message is still displayed here, please reload VS Code.

(F1, Developer: Reload Window)

Pylance requires you to set the Python PATH:

If you're in Mac/Linux, use:

which python3

And in Windows (Command Prompt cmd.exe):

where python

So that the path in which your Python is installed is returned.

Copy that path.

Go to your vscode and open the settings.json file (CTRL + SHIFT + P, and type "settings.json" at search bar)

Add the following key to the json file:

"python.defaultInterpreterPath": "<PATH RETURNED ABOVE>"

The path may look like "C:\\Users\\YOURUSERNAME\\anaconda3\\bin\\python.exe on Windows or "/usr/local/bin/python3" on Mac/Linux.

Note: backslashes in settings.json must be escaped with an additional backslash, so C:\Users becomes "C:\\Users".

The following documentation provides more information about how to configure Python for Visual Studio Code: https://code.visualstudio.com/docs/python/settings-reference

I finally figured out what was wrong. There seems to be an open issue with LSP Notebooks experiment on vscode that affects Jupyter Notebooks in the editor. The solution seems to be to set the "python.pylanceLspNotebooksEnabled" flag to False which by default is set to True and it fixes the problem of this error in Jupyter Notebooks in vscode.

This problem only seems to affect the docstring but doesn't affect autocomplete. Thanks to others who tried to help with this question and hope it helps those who might come across the same problem.

When I did not install the module "flask" in the Python environment currently used in VSCode:

Please use the command "pip --version" to check the source of the module installation tool "pip", the module is installed at this location:

Then, we can use the command "pip show flask" to check the installation location of the module "flask": (It checks whether the installation location of the module is consistent with the Python environment displayed in the lower left corner of VSCode.)

If the "reportMissingModuleSource" message is still displayed here, please reload VS Code.

(F1, Developer: Reload Window)

If you want to install a package into your specific virtual environment, you need to first "activate" that environment. Likewise, if you want to run your script in that environment, you need to first "activate" it. You can do this manually or preferably let VSCode handle it for you.

In order to tell VSCode(especially the language server which is pylance) to use that environment:

1. Open up your Command Palette(press ctrl+shift+P or f1) and type : "python: select interpreter".
2. Browse/Select your newly created python interpreter's path inside your venv.
3. Add "python.terminal.activateEnvironment": true to your setting.json file(It's also possible via GUI setting of course by searching for python: activate environment). This will automatically detect and activate your venv whenever you open your integrated terminal. Obviously If the path should points to a venv interpreter. (Note: you have to have a Python file opened in your editor).

You can also set your Python's interpreter path manually:

1. Create a folder called .vscode in your workspace.
2. Create a file called settings.json inside it.
3. Add these to settings.json:

{
    "python.defaultInterpreterPath": "PATH_TO_VENV_INTERPRETER",
    "python.terminal.activateEnvironment": true
}

Note: What I normally do is, I insert a "python.defaultInterpreterPath" key to my User settings.json which points to my global interpreter. Then I create Workspace settings.json for each of my projects and add the same key which points to my venv's interpreter. Remember, workspace settings.json will overwrite user's settings.json.

This way whenever you open VSCode in a project folder it automatically knows it should activate your venv's interpreter(I told it to do so with "python.terminal.activateEnvironment") and if you open VSCode in a normal folder it correctly use your global's interpreter.

Difference between User and Workspace settings.json.