[Python-ideas] Type hints for functions with side-effects and for functions raising exceptions

[Ben Rudiak-Gould]

On Wed, Feb 20, 2019 at 2:43 AM Chris Angelico <rosuav at gmail.com> wrote:
> That's because a generator function conceptually has three ways to
> provide data (yield, return, and raise), but mechanically, one of them
> is implemented over the other ("return" is "raise StopIteration with a
> value"). For other raised exceptions, this isn't a problem.

Other functions also conceptually have three ways of returning:
ordinary return with a value, a documented special return like
KeyError, and pass-through exceptions. If the pass-through exception
is KeyError, it gets conflated with the documented exceptional return,
but correct code should handle them differently. It doesn't matter
whether the syntax for the documented special return is "return x" or
"raise KeyError(x)". I've never been screwed by this as badly with
other exceptions as I was by StopIteration, but it's a general problem
with the design of exceptions.

I don't think exception specifications would solve that problem since
you probably couldn't describe the KeyError's origin in the spec
either. But that doesn't mean it isn't a problem.