Debugging

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You can connect the debugger in your editor, for example with Visual Studio Code or PyCharm.

Call uvicorn

In your FastAPI application, import and run uvicorn directly:

Python 3.8+

import uvicorn
from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def root():
    a = "a"
    b = "b" + a
    return {"hello world": b}


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)

About __name__ == "__main__"

The main purpose of the __name__ == "__main__" is to have some code that is executed when your file is called with:

$ python myapp.py

but is not called when another file imports it, like in:

from myapp import app

More details

Let's say your file is named myapp.py.

If you run it with:

$ python myapp.py

then the internal variable __name__ in your file, created automatically by Python, will have as value the string "__main__".

So, the section:

    uvicorn.run(app, host="0.0.0.0", port=8000)

will run.

---

This won't happen if you import that module (file).

So, if you have another file importer.py with:

from myapp import app

# Some more code

in that case, the automatically created variable inside of myapp.py will not have the variable __name__ with a value of "__main__".

So, the line:

    uvicorn.run(app, host="0.0.0.0", port=8000)

will not be executed.

Info

For more information, check the official Python docs.

Run your code with your debugger

Because you are running the Uvicorn server directly from your code, you can call your Python program (your FastAPI application) directly from the debugger.

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For example, in Visual Studio Code, you can:

• Go to the "Debug" panel.
• "Add configuration...".
• Select "Python"
• Run the debugger with the option "Python: Current File (Integrated Terminal)".

It will then start the server with your FastAPI code, stop at your breakpoints, etc.

Here's how it might look:

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If you use Pycharm, you can:

• Open the "Run" menu.
• Select the option "Debug...".
• Then a context menu shows up.
• Select the file to debug (in this case, main.py).

It will then start the server with your FastAPI code, stop at your breakpoints, etc.

Here's how it might look: