How to write xlwings code that works both with a local Python installation and via remote interpreter

xlwings quickstart myproject --fastapi
This will create a folder myproject in your current working directory with a few files:
myproject
├─ app.py
├─ main.py
├─ myproject.xlsm
└─ requirements.txt
Open the file main.py in your favorite editor or IDE:
# main.py
from fastapi import Body
import xlwings as xw
from app import app
@app.post("/hello")
def hello(data: dict = Body):
    # Instantiate a Book object with the deserialized request body
    book = xw.Book(json=data)
    # Use xlwings as usual
    sheet = book.sheets[0]
    if sheet["A1"].value == "Hello xlwings!":
        sheet["A1"].value = "Bye xlwings!"
    else:
        sheet["A1"].value = "Hello xlwings!"
    # Pass the following back as the response
    return book.json()
if __name__ == "__main__":
    import uvicorn
    uvicorn.run("main:app", host="127.0.0.1", port=8000, reload=True)
To make our hello function usable via the classic RunPython function in VBA as well as the new RunRemotePython function (or the runPython functions in Excel on the web or Google Sheets), let’s do the following changes:
  
Rename the current hello function into hello_remote to handle the RunRemotePython call
  
Factor out the core functionality into the new hello function
  
Add a function hello_local to handle the classic xlwings RunPython call
With all the changes done, main.py looks like this:
from fastapi import Body
import xlwings as xw
from app import app
def hello(book):
    sheet = book.sheets[0]
    if sheet["A1"].value == "Hello xlwings!":
        sheet["A1"].value = "Bye xlwings!"
    else:
        sheet["A1"].value = "Hello xlwings!"
def hello_local():
    book = xw.Book.caller()
    hello(book)
@app.post("/hello")
def hello_remote(data: dict = Body):
    book = xw.Book(json=data)
    hello(book)
    return book.json()
if __name__ == "__main__":
    import uvicorn
    uvicorn.run("main:app", host="127.0.0.1", port=8000, reload=True)
Now open myproject.xlsm and go to the VBA editor by hitting Alt+F11. In Module1, you’ll want to change the SampleCall to read as follows:
Sub SampleCall()
    RunPython "import main;main.hello_local()"
End Sub
Sub SampleRemoteCall()
    RunRemotePython "http://127.0.0.1:8000/hello", apiKey:="DEVELOPMENT"
End Sub
That’s it! You can now run SampleCall (e.g., by clicking into it and hitting F5), which will use a local installation of Python. If you want to use the remote interpreter, you’ll first have to start the Python server in a Terminal/Command prompt (note that this expects you to have the dependencies installed from requirements.txt):
python main.py
Instead of running the server from the Terminal, you could also run the script via the respective functionality of your editor or IDE. Once the server is running, you can call SampleRemoteCall from the VBA editor (or assign a button to it on one of your worksheets).
While the quickstart command has set up SampleRemoteCall to work with a Python web service on your localhost, the idea here is to eventually deploy it to some sort of a server (cloud function, etc.) so that the users of your spreadsheet tools don’t have to install Python on their system. Providing them with a Docker image would be another option.
Further information
For more information about the remote interpreter, see the docs.