Thank you for your question, I am here to help you and who will see this question.
if you mean you want to open the file like when click on the file and open it
you can use this command
Start filename.py
but if you want to open the file inside the gitbash use this command
vim filename.py
and if you mean to run the file from gitbash you can use this command
python pythonFileName.py
Now if the above command did not work with you, and you are in the windows10 Pro platform you should go to
environment variable >> Then system variable >> then choose path >> then Edit >> and put the python path >> restart the terminal and run it again
Notice: All of the above I tried and used in windows10 pro.
Thanks,
Hope to help anyone,