If something:
do_this()
do_that()
If the pointer is at the start of the if statement, and the user issues
python-nav-forward-sentence he must be taken after the do_that() statement.
That sounds like sexp-based navigation, then.
python-nav-{backward,forward}-sentence that would navigate blocks of code.
I don't think "sentence" is a good word choice since there is no
accepted meaning of a "sentence" in the context of python code (or code
in general), AFAIK.
If you want to bind these commands to the same keys as
for/backward-sentence, that's fine, but better give them names related
to what they do, rather than to to this key-mapping choice.
Stefan