How to prevent "Open With" dialog window when opening file from command window?

I associated Python's interpreter with Python scripts using environment variable:

C:\Users\Piotr>ftype Python.File
Python.File="%PYTHON_HOME%\python.exe" "%1" %*

Sometimes this environment variable is unset or set to non-existing path. When it's the case and I try to run Python script like this

C:\Users\Piotr>some_script.py

a dialog window titled Open With pops up.

I'd rather get some error info in the command line window instead of this dialog. How can I prevent this dialog from popping up in this situation?