Unit roundtrips do not always work

Description

Not sure whether this is an issue, however it would probably nice to have it fixed:

Expected behavior

I would expect that any roundtrip for unicode format works. i.e. for all units unit:

format='unicode'
assert u.Unit(unit.to_string(format=format), format=format) == unit

should work. I would expect the same for the other formatters; at least for generic and fits.

Actual behavior

For Angstrom, it works when not giving unicode as format in the u.Unit() constructor, but not when this is given:

>>> u.Unit(u.Angstrom.to_string(format='unicode')) == u.Angstrom
True
>>> u.Unit(u.Angstrom.to_string(format='unicode'), format='unicode') == u.Angstrom
[…]
ValueError: 'Å' did not parse as unicode unit: Can not parse Unicode If this […]

The second exception comes for any unit; unicode seems not be supported as parsing format.

For the solar mass, the first one does not work as well:

>>>  u.Unit(u.solMass.to_string(format='unicode')) == u.solMass
[…]
ValueError: 'M⊙' did not parse as unit: At col 0, M is not a valid unit. Did you […]

I would assume that

• the “unicode” parse format is identical to the “generic” format (as we usually use unicode in strings)
• Unicode output is also parseable, as the generic input

System Details

The tests were done with Astropy 4.2

[olebole]

Finding them is simple:

import astropy.units as u
import inspect

format='unicode'

print(f'|  Type        |  Generic     |  Unicode     | ')
print(f'| ------------ | ------------ | ------------ | ')

units = set(unit for name, unit in inspect.getmembers(u)
            if isinstance(unit, u.Unit)
            and not isinstance(unit, u.core.PrefixUnit))

for unit in units:
    formatted_name = unit.to_string(format=format)
    try:
        res = u.Unit(formatted_name) == unit
    except ValueError:
        res = False
    if not res:
        print(f'| {unit.physical_type:12s} | {unit.to_string():12s} | {formatted_name:12s} | ')

This shows all unique but the prefixed units that raise the error:

Type	Generic	Unicode
mass	M_e	Mₑ
power	solLum	L⊙
length	jupiterRad	R♃
mass	M_p	Mₚ
length	earthRad	R⊕
mass	earthMass	M⊕
angle	arcsec	″
volume	l	ℓ
mass	solMass	M⊙
mass	jupiterMass	M♃
angle	hourangle	ʰ
energy	Ry	R∞
angle	arcmin	′
length	solRad	R⊙

Looks not overwhelming to me. Maybe I will have a look myself, if this is agreed to be useful…

[mhvk]

OK, I made a fix to the parsing issue, see #11827.

For the parsing with ‘unicode’, it needs a better error message indeed, which I think is easier to deal with as a separate issue, so I raised #11828.

@maxnoe - indeed, you are right, we are now free to try to implement the module __getattr__ idea!