Feature request: method for human readable units

(from discussion with @adrn at PyData2017 NYC)

Would like to automatically determine the units for a quantity so it’s more human readable. For example, Googling “100000000000 bytes” (https://www.google.com/search?q=100000000000 bytes) returns 100 GB automatically.

Currently, this functionality only exists if the desired units are specified, i.e., (100000000000 * u.m).to(u.Gm), but it would be convenient to have a function that automatically converted (100000000000 m).human_readable() to 100 Gm.

[eerovaher]

What to do with array-valued quantities.

This code in the Distance constructor uses the mean of the array: https://github.com/astropy/astropy/blob/da206faba923db66e53aac29450dd57cea211cc0/astropy/coordinates/distances.py#L132-L143

[adonath]

1. What to do with composite units like km/s

I agree this is tricky. I think it would require to manually assign priorities / weights to prefixes, effectively making it an optimization problem. But this might be highly specific on the domain as well. In any case supporting this for PrefixUnit only would already be useful.

1. What to do with array-valued quantities

If all numbers are in the same range the mean should work fine. If the numbers vary by multiple orders of magnitudes it might work as well, but not necessarily. Alternatively one could do a “democratic decision” and take the preferred unit that works for the “majority” of entries in the array.