[Python-ideas] Add an option that allow check Type Hints in runtime

[Chris Angelico]

On Fri, Jul 8, 2016 at 6:50 PM, Paul Moore <p.f.moore at gmail.com> wrote:
> Just to clarify for the OP - the reason you're getting pushback on
> making this a built in interpreter function is twofold - first, an
> external facility could be used *now*, and would not need to be
> limited to Python 3.6+, and second, adding checks to the function call
> protocol is not free, and would therefore affect not just the minority
> of people who want to add checks for debugging, but also all other
> users (which include people for whom Python's performance is a key
> issue). The Python interpreter's function calling process is known to
> be slower than (some) people would like, and making it slower - even
> marginally - isn't something we would want to do lightly.

Exactly. The main reason IMO is the first one - you can add this
checker to any Python 3 program, rather than wait for it to get coded,
debugged, and shipped.

Oh, and you can easily have a flag that disables checking for performance:

if no_type_checks:
    def enforce_types(func):
        return func

Now your decorator does nothing, and there is literally ZERO run-time
cost (the only cost is a single cheap check when the function is
defined, which usually means on module import), closures aside. That
wouldn't be possible with a core interpreter change, unless it's made
a compile-time option *for the interpreter* - something like
"./configure --with-annotations-enforced" - and then you run python3.6
for the fast one, or python3.6tc for the type-checking one. That would
be a pain.

ChrisA