[Python-ideas] Proposal: Use mypy syntax for function annotations

[Dennis Brakhane]

Am 13.08.2014 23:46, schrieb Guido van Rossum:
>
>
> Mypy has a cast() operator that you can use to shut it up when you
> (think you) know the conversion is safe.
>
Does Mypy provide a way to "fix/monkeypatch" incorrect type declarations
in function signatures? For example, by modifying __annotations__?

My pet peeve of static languages is that programmers are often too
fixated on their particular problem that they don't think about alternate
uses for their code and make the type declarations uncessarily complex.

For example, in Java nearly every method in Java that deals with
character strings uses "String" as parameter type, while they should
have used "CharSequence". Having to read an entire input stream and
storing it in a String just to be able to use a method is not fun
(String is final in Java)

I'm worried that in Python we will have utility functions that declare
they require a List[int], when in fact they actually only require a
Sequence[int] or
Sequence[Number].

While Mypy's cast is nice in that I won't have to wrap my Integer Tuple
in list like object, having to cast it every time I use a particular
broken utility method
feels very ugly to me; and defining a wrapper function with the correct
type information feels like unnecessary run time overhead for no gain.

Don't get me wrong, I'm not entirely against some kind of type checking,
but I fear that there must exist possible workarounds for badly written
code.