classmethod to validate mix-and-match kwargs

orbit_with_multi_dispatch.py

"""
https://chat.stackoverflow.com/transcript/message/47865711#47865711

Well it's mainly "utility functions", ie I need to define a (keplerian) orbit. Which is normally done by giving
semi-major axis, eccentricity, inclination, argument periapsis and longitude ascending node. However there are lots
of "variations": ie for parabolic orbits the semi major axis is replaced by semi latus rectum. Or you can just use
state variables at a point (position + velocity in 3 dimensions). Or you could replace the semi major axis +
eccentricity by peri + apo apsis.
"""

class Orbit:
    def __init__(self,
                 _validated=False,
                 semi_major_axis=None,
                 eccentrity=None,
                 inclination=None,
                 periapsis=None,
                 longitude_ascending_node=None,
                 semi_latus_rectum=None,
                 init_position=None,
                 init_velocity=None,
                 peri_apsis=None,
                 apo_apsis=None,
                 ):
        if not _validated:
            raise TypeError("do not use default constructor for Orbit, use Orbit.make_orbit")

    @classmethod
    def make_orbit(cls, **kwargs):
        valid_kwarg_combs = [
            'semi_major_axis eccentricity inclination periapsis longitude_ascending_node'.split(),
            'semi_latus_rectum eccentricity inclination periapsis longitude_ascending_node'.split(),
            'init_position init_velocity'.split(),
            'peri_apsis apo_apsis inclination periapsis longitude_ascending_node'.split(),
        ]
        for kwargs_comb in valid_kwarg_combs:
            if set(kwargs_comb) == set(kwargs):
                return cls(_validated=True, **kwargs)

        raise TypeError("improper kwargs combination")

# a legit construction
o = Orbit.make_orbit(init_position=[1,2,3], init_velocity=[4,5,6])

# raises a TypeError
o = Orbit.make_orbit(init_position=[1,2,3])

# also raises a TypeError - don't call the constructor directly
o = Orbit(init_position=[1,2,3], init_velocity=[4,5,6])