How to disable auto password filling (Keychain) while in private browsing mode in Safari? Keychain offers to auto complete forms including passwords even in private browsing mode. How can I disable this? 

I'm using Safari 8.0.5 on Yosemite 10.10.3.

Edit: The use case here is when other people need to borrow my computer. I'll open a private browsing window for them. I don't want my credentials auto-filling when they visit site.
There is no way currently offered by Apple to disable auto-fill based on the browsing mode. However, you could try to switch auto-fill on/off based browsing mode by making a trigger-effect script or macro with AppleScript or an application like Keyboard Maestro.