Given near-consensus on the "Return ..." form, the following discussion is of academic interest only.

On Mon, Sep 23, 2013 at 9:31 PM, Steven D'Aprano <steve@pearwood.info> wrote:

> I would not mind
>
> def foo():
> """returns bar"""
>
> which I would read as "Function foo() returns bar," but in this case
> "returns" should be in lower case.

I certainly don't like that. Sentences, even eroded sentences, start
with capital letters in English. Unless you are the poet e.e. cummings,
capital letters are non-negotiable.

I still believe "Returns bar." is a non-sentence even if "Return bar!" is. While absence of subject noun in itself may not be an issue, but combined with a singular third-person verb form, it makes "Returns bar." grammatically incorrect.