The causes could be:

1. The name of the file - fastapi.py, if you name it in this could way you would get import errors. (Avoid filenames similar to package names)
2. Installing on other environments. Most of the time it happens when you use both Conda and PiP on you system.
3. If you run the code from VS Code, check if the same interpreter is selected.

Verify the installation:

$ pip list | grep fastapi

Suggestions:

OPT Virtual Environments to avoid these troubles.

Thank you!

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.

If you installed FastAPI, this is probably because you're not using the right Python environment in VS Code. Check this tutorial by Microsoft, Section "Locally in VS Code", to set up FastAPI in VS Code. Make sure to follow all steps exactly, especially those related to the environment!

1. Open the Command Palette (Ctrl+Shift+P), then select the Python: Select Interpreter. From the list, select the virtual environment in your project folder that starts with .env.

2. Run Terminal: Create New Integrated Terminal (Ctrl+Shift+` or from the Command Palette), which creates a terminal and automatically activates the virtual environment by running its activation script.

3. Install sqlalchemy and mongoengine with command pip install. Once installing them successfully, there will intellisense when you import them and no warnings shown.

Besides, the folder .vscode is to store Workspace settings as well as debugging and task configurations.