Using every file in a directory [2.7] [Win7]

I have a dialog in Tkinter that asks for a Windows 7 directory name, and then they click "Okay" I want to use all the files of a certain file type in the directory.

dirname is the variable that holds the directory ex. C:/Users/Mochizuki/Documents/FilesIWantToUse

...
    foo = bar("%s*.avi", tag=tag) % dirname
    #  the idea is that it would take C:/Directory/wildcard.avi in a for loop and use all the files
    ...

I know that looks like an odd way of going about it but I'm coming from Linux and what I did on there was take a command line argument for the directory, and I'm not super familiar with Windows.