Fix: comets were always giving 'str' as their name

Author: brandon-rhodes

CHANGELOG.rst

@@ -32,6 +32,10 @@ Next version
   Another advantage is that the new name is self-documenting: it holds a
   3-vector with Cartesian components.
 
+* Fix: the position vectors for Kepler orbit bodies, like comets and
+  asteroids, now have a useful ``.target_name`` like ``'Ceres'`` or
+  ``'1P/Halley'`` instead of the less informative value ``'str'``.
+
 v1.50 — 2024 February 15
 ------------------------
 

skyfield/keplerlib.py

@@ -25,7 +25,7 @@ def __init__(self,
                  epoch,
                  mu_au3_d2,
                  center=None,
-                 target=None,
+                 target_name=None,
         ):
         """ Calculates the position of an object using 2 body propagation
 
@@ -50,10 +50,14 @@ def __init__(self,
         self.epoch = epoch
         self.mu_au3_d2 = mu_au3_d2
         self.center = center
-        self.target = target
+        self.target_name = target_name
 
         self._rotation = None  # TODO: make argument?
 
+    @property
+    def target(self):
+        return self  # this instance itself represents the target object
+
     @classmethod
     def _from_periapsis(
             cls,
@@ -65,7 +69,7 @@ def _from_periapsis(
             t_periapsis,
             gm_km3_s2,
             center=None,
-            target=None,
+            target_name=None,
         ):
         """Build a `KeplerOrbit` given its parameters and date of periapsis."""
         gm_au3_d2 = gm_km3_s2 * _CONVERT_GM
@@ -84,7 +88,7 @@ def _from_periapsis(
             t_periapsis,
             gm_au3_d2,
             center,
-            target,
+            target_name,
         )
 
     @classmethod
@@ -93,7 +97,7 @@ def _from_true_anomaly(cls, p, e, i, Om, w, v,
                           mu_km_s=None,
                           mu_au3_d2=None,
                           center=None,
-                          target=None,
+                          target_name=None,
         ):
         """ Creates a `KeplerOrbit` object from elements using true anomaly
 
@@ -142,7 +146,7 @@ def _from_true_anomaly(cls, p, e, i, Om, w, v,
                    epoch,
                    mu_km_s,
                    center=center,
-                   target=target,
+                   target_name=target_name,
         )
 
 
@@ -158,7 +162,7 @@ def _from_mean_anomaly(
             epoch,
             gm_km3_s2,
             center=None,
-            target=None,
+            target_name=None,
         ):
         """ Creates a `KeplerOrbit` object from elements using mean anomaly
 
@@ -214,7 +218,7 @@ def _from_mean_anomaly(
             epoch,
             gm_au3_d2,
             center,
-            target,
+            target_name,
         )
 
     def _at(self, time):

skyfield/tests/test_keplerlib.py

@@ -40,7 +40,7 @@ def test_against_horizons():
         epoch=t,
         gm_km3_s2=GM_SUN,
         center=None,
-        target=None,
+        target_name=None,
     )
     r, v = k._at(t)[:2]
     sun_au = [
@@ -70,7 +70,8 @@ def test_minor_planet_with_positive_M():
     ceres = mpc.mpcorb_orbit(row, ts, GM_SUN)
     ra, dec, distance = eph['earth'].at(t).observe(eph['sun'] + ceres).radec()
 
-    assert ceres.target == '(1) Ceres'
+    assert ceres.target is ceres
+    assert ceres.target_name == '(1) Ceres'
     assert abs(ra.hours - 23.1437) < 0.00005
     assert abs(dec.degrees - -17.323) < 0.0005
 
@@ -94,7 +95,8 @@ def test_minor_planet_with_negative_M():
 
     # We can't expect close agreement, since the HORIZONS orbital
     # elements are different than MPC's.
-    assert ceres.target == '(2) Pallas'
+    assert ceres.target is ceres
+    assert ceres.target_name == '(2) Pallas'
     assert abs(ra.degrees - 92.750) < 0.006
     assert abs(dec.degrees - -10.561) < 0.002
 
@@ -125,7 +127,8 @@ def test_comet():
         assert abs(dec_want.arcseconds() - dec.arcseconds()) < 0.2
         assert abs(distance.au - 43.266) < 0.0005
 
-        assert k.target == 'C/1995 O1 (Hale-Bopp)'
+        assert k.target is k
+        assert k.target_name == 'C/1995 O1 (Hale-Bopp)'
 
 def test_comet_with_eccentricity_of_exactly_one():
     ts = load.timescale()