Create xlwings udf/macros In xlam Addin - xlwings

Question

I can run the xlwings udf example on my computer perfectly fine. However, my current project requires me to:
Create an xlam addin.
Use the xlam addin to create buttons.
Link these button to xlwings udf/macros where python functions are called. In other words, after the user clicks these buttons, some VBA codes will run and some of them will call python udf/macros which are imported by xlwings.
Intuitively, I think xlam addins are just special kind of workbooks, and thus I saved them along with appropriately named python scripts in the same folder. However, once I run a test macro calling xlwings udf's I get error message like this one (I am running "test_udf").
Running test_udf
Here is the full error message:
"ModuleNotFoundError: No module named 'udf'
File ""<frozen importlib._bootstrap>"", line 953, in _find_and_load_unlocked
File ""<frozen importlib._bootstrap>"", line 971, in _find_and_load
File ""<frozen importlib._bootstrap>"", line 994, in _gcd_import
return _bootstrap._gcd_import(name[level:], package, level)
File ""C:\ProgramData\Anaconda3\lib\importlib\__init__.py"", line 126, in import_module
module = import_module(module_name)
File ""C:\ProgramData\Anaconda3\lib\site-packages\xlwings\udfs.py"", line 208, in get_udf_module
module = get_udf_module(module_name)
File ""C:\ProgramData\Anaconda3\lib\site-packages\xlwings\udfs.py"", line 221, in call_udf
res = call_udf(script, fname, args, this_workbook, FromVariant(caller))
File ""C:\ProgramData\Anaconda3\lib\site-packages\xlwings\server.py"", line 190, in CallUDF
return func(*args)
File ""C:\ProgramData\Anaconda3\lib\site-packages\win32com\server\policy.py"", line 586, in _invokeex_
return S_OK, -1, self._invokeex_(dispid, lcid, wFlags, args, None, None)
File ""C:\ProgramData\Anaconda3\lib\site-packages\win32com\server\policy.py"", line 283, in _invoke_
return self._invoke_(dispid, lcid, wFlags, args)
File ""C:\ProgramData\Anaconda3\lib\site-packages\win32com\server\policy.py"", line 278, in _Invoke_"
Essentially, the important lines of my sub is just
Sub test_udf()
Debug.Print double_sum(1.1, 2.2)
End Sub
where double_sum lives in the same module. Do you know where things got wrong? Is it because they live inside of a xlam addin? How to fix this problem? Many thanks!