I ran into this issue by doing the following steps, though I did not try to reproduce:

1 - install Python 3.4
2 - have windows Always Open .py files with Python
3 - install Python 2.7

Then I encountered issues where all .py scripts were opened with Python 2.7. After some unsuccessful attempts I decided to uninstall Python 2.7

4 - remove Python 2.7

At this point, I had to update the file associations because Windows could not find Python 2.7 anymore

5 - update file associations

The issue is obviously that in at least a few places in the registry, the ..\shell\open\command has a value of ..\python.exe "%1"

In my case it was under all roots:

HKEY_CLASSES_ROOT\py_auto_file
HKEY_LOCAL_MACHINE
HKEY_USERS

I corrected only the one under HKEY_USERS by adding %* and it resolved the issue.