Associate file extension to python script, so that I can open the file by double click, in windows

This isn't really a programming question, but what you need to do is to figure out how to get the Python executable into the registry key that opens your data file.

For example, I created a little Python script called opener.py that looks like this:

import sys
print(sys.argv)
input()

Then I created a testfile.foo and used the "change" button in that file's property dialog to choose opener.py. (You can do this if you click Browse and change the Open With dialog's file filter to "All Files".)

Of course this didn't work (as you noticed). So I opened regedit and searched for opener.py and found it at the following registry key:

HKEY_CURRENT_USER\Software\Classes\Applications\opener.py\shell\open\command

The default value of this key was "C:\opener.py" %1. I changed it to python "C:\opener.py" %1. It worked!

Long story short, to do this properly you need to custom-edit the registry. Actually setting up the file association is more complex than just editing that one key (you have to also indicate that .foo is associated with opener.py).

An alternative approach would be to turn your Python script into a standalone executable using one of the several tools available for that purpose, or write a small executable in C that launches the script.

1. press the windows key
2. type cmd
3. right click the result and choose "run as administrator"
4. assoc .foo=foofile
5. ftype foofile="C:\Users\<user>\AppData\Local\Programs\Python\PYTHON~1\python.exe" "C:\<whatever>\fooOpener.py" "%1" %*

Use pythonw.exe if it's a .pyw file (to prevent a cmd window from spawning).

If you want to use an existing file type, you can find its alias by not assigning anything. For example, assoc .txt returns .txt=txtfile.