snipMate snippets are like the built-in :abbreviate
on steroids, usually with parameter insertions, mirroring, and multiple stops inside them. It appears that you don't need all this advanced functionality here, so instead you would be better served with a plain insert-mode abbreviation, where you can indeed use the Vim key-notation:
:ia pdb import pdb; pdb.set_trace()<ESC>
With snipMate itself, leaving insert mode is not possible.