Switch to reify() for computed Time attributes

skyfield/descriptorlib.py

@@ -0,0 +1,14 @@
+from functools import update_wrapper
+
+class reify(object):
+    """Adapted from Pyramid's `reify()` memoizing decorator."""
+    def __init__(self, method):
+        self.method = method
+        update_wrapper(self, method)
+
+    def __get__(self, instance, objtype=None):
+        if instance is None:
+            return self
+        value = self.method(instance)
+        instance.__dict__[self.__name__] = value
+        return value

skyfield/timelib.py

@@ -3,6 +3,7 @@ from numpy import (array, concatenate, einsum, float_, interp, isnan, nan,
                    rollaxis, searchsorted, sin, where, zeros_like)
 from time import strftime
 from .constants import B1950, DAY_S, T0
+from .descriptorlib import reify
 from .earthlib import sidereal_time
 from .framelib import ICRS_to_J2000 as B
 from .functions import load_bundled_npy
@@ -470,69 +471,77 @@ class Time(object):
         """Return TT as a tuple (year, month, day, hour, minute, second)."""
         return calendar_tuple(self.tt)
 
-    def __getattr__(self, name):
+    # Convenient caching of several expensive functions of time.
 
-        # Cache of several expensive functions of time.
+    @reify
+    def P(self):
+        self.P = P = compute_precession(self.tdb)
+        return P
 
-        if name == 'P':
-            self.P = P = compute_precession(self.tdb)
-            return P
+    @reify
+    def PT(self):
+        self.PT = PT = rollaxis(self.P, 1)
+        return PT
 
-        if name == 'PT':
-            self.PT = PT = rollaxis(self.P, 1)
-            return PT
+    @reify
+    def N(self):
+        self.N = N = compute_nutation(self)
+        return N
 
-        if name == 'N':
-            self.N = N = compute_nutation(self)
-            return N
+    @reify
+    def NT(self):
+        self.NT = NT = rollaxis(self.N, 1)
+        return NT
 
-        if name == 'NT':
-            self.NT = NT = rollaxis(self.N, 1)
-            return NT
+    @reify
+    def M(self):
+        self.M = M = einsum('ij...,jk...,kl...->il...', self.N, self.P, B)
+        return M
 
-        if name == 'M':
-            self.M = M = einsum('ij...,jk...,kl...->il...', self.N, self.P, B)
-            return M
+    @reify
+    def MT(self):
+        self.MT = MT = rollaxis(self.M, 1)
+        return MT
 
-        if name == 'MT':
-            self.MT = MT = rollaxis(self.M, 1)
-            return MT
+    # Conversion between timescales.
 
-        # Conversion between timescales.
+    @reify
+    def tai(self):
+        self.tai = tai = self.tt - tt_minus_tai
+        return tai
 
-        if name == 'tai':
-            self.tai = tai = self.tt - tt_minus_tai
-            return tai
+    @reify
+    def utc(self):
+        utc = self._utc_tuple()
+        utc = array(utc) if self.shape else utc
+        self.utc = utc = utc
+        return utc
 
-        if name == 'utc':
-            utc = self._utc_tuple()
-            utc = array(utc) if self.shape else utc
-            self.utc = utc = utc
-            return utc
+    @reify
+    def tdb(self):
+        tt = self.tt
+        return tt + tdb_minus_tt(tt) / DAY_S
 
-        if name == 'tdb':
-            tt = self.tt
-            self.tdb = tdb = tt + tdb_minus_tt(tt) / DAY_S
-            return tdb
+    @reify
+    def ut1(self):
+        self.ut1 = ut1 = self.tt - self.delta_t / DAY_S
+        return ut1
 
-        if name == 'ut1':
-            self.ut1 = ut1 = self.tt - self.delta_t / DAY_S
-            return ut1
+    @reify
+    def delta_t(self):
+        table = self.ts.delta_t_table
+        self.delta_t = delta_t = interpolate_delta_t(table, self.tt)
+        return delta_t
 
-        if name == 'delta_t':
-            table = self.ts.delta_t_table
-            self.delta_t = delta_t = interpolate_delta_t(table, self.tt)
-            return delta_t
+    @reify
+    def gmst(self):
+        self.gmst = gmst = sidereal_time(self)
+        return gmst
 
-        if name == 'gmst':
-            self.gmst = gmst = sidereal_time(self)
-            return gmst
-
-        if name == 'gast':
-            self.gast = gast = self.gmst + earth_tilt(self)[2] / 3600.0
-            return gast
-
-        raise AttributeError('no such attribute %r' % name)
+    @reify
+    def gast(self):
+        self.gast = gast = self.gmst + earth_tilt(self)[2] / 3600.0
+        return gast
 
     def __eq__(self, other_time):
         if not isinstance(other_time, Time):