[Python-ideas] Units in type hints

[Koos Zevenhoven]

Hi all,

How about extending the type annotations for int, float and complex to 
optionally include also a unit?

For instance,

     def sleep(duration : Float['s']):
         ...

Now the type checker could catch the error of trying to pass the sleep 
duration in milliseconds, Float['ms']. This would also be useful for 
documentation, avoiding the 'need' for having names like duration_s. At 
least the notation with square brackets would resemble the way units are 
often written in science.

Another example:

     def calculate_travel_time(distance: Float['km']) -> Float['h']:
         speed = get_current_speed()  # type: Float['km/h']
         return distance / speed

Now, if you try to pass the distance in miles, or Float['mi'], the type 
checker would catch the error. Note that the type checker would also 
understand that 'km' divided by 'km/h' becomes 'h'. Or should these be 
something like units.km / units.h?

But if you do have your distance in miles, you do

     calculate_travel_time(units.convert(distance_mi, 'mi', 'km'))

and the type checker and programmer get what they want.

Anyone interested?


-- Koos