My setup is to use vim (specifically MacVim in graphical mode) to edit source files and then I do (mercurial) repository diffs using vimdiff (in graphical mode again). Sometimes, when I do a diff, vim gives me the error:

> 
> Swap file "~/nn/src/imm/model/injector\_node/.values\_table.py.swp" already exists!
> 
> 
> 

How do I fix this? Can I command vim to stop making swp files, or the vimdiff to ignore them, or write its swp files to a different place?