[ENH] Add cache/memoization to PVcell properties (#121)

pvmismatch/pvmismatch_lib/pvcell.py

@@ -11,6 +11,8 @@
 import numpy as np
 from matplotlib import pyplot as plt
 from scipy.optimize import newton
+import functools
+import copy
 
 # Defaults
 RS = 0.004267236774264931  # [ohm] series resistance
@@ -27,6 +29,27 @@
 ALPHA_ISC = 0.0003551  # [1/K] short circuit current temperature coefficient
 EPS = np.finfo(np.float64).eps
 
+
+def cached(f):
+    """
+    Memoize an object's method using the _cache dictionary on the object.
+    """
+    @functools.wraps(f)
+    def wrapper(self):
+        # note:  we use self.__wrapped__ instead of just using f directly
+        # so that we can spy on the original function in the test suite.
+        key = wrapper.__wrapped__.__name__
+        if key in self._cache:
+            return self._cache[key]
+        value = wrapper.__wrapped__(self)
+        self._cache[key] = value
+        return value
+    # store the original function to be accessible by the test suite.
+    # functools.wraps already sets this in python 3.2+, but for older versions:
+    wrapper.__wrapped__ = f
+    return wrapper
+
+
 class PVcell(object):
     """
     Class for PV cells.
@@ -54,6 +77,8 @@ def __init__(self, Rs=RS, Rsh=RSH, Isat1_T0=ISAT1_T0, Isat2_T0=ISAT2_T0,
                  Isc0_T0=ISC0_T0, aRBD=ARBD, bRBD=BRBD, VRBD=VRBD_,
                  nRBD=NRBD, Eg=EG, alpha_Isc=ALPHA_ISC,
                  Tcell=TCELL, Ee=1., pvconst=PVconstants()):
+        # set up property cache
+        self._cache = {}
         # user inputs
         self.Rs = Rs  #: [ohm] series resistance
         self.Rsh = Rsh  #: [ohm] shunt resistance
@@ -74,6 +99,7 @@ def __init__(self, Rs=RS, Rsh=RSH, Isat1_T0=ISAT1_T0, Isat2_T0=ISAT2_T0,
         self.Pcell = None  #: cell power on IV curve [W]
         self.VocSTC = self._VocSTC()  #: estimated Voc at STC [V]
         # set calculation flag
+        super(PVcell, self).__setattr__('_calc_now', True)
         self._calc_now = True  # overwrites the class attribute
 
     def __str__(self):
@@ -91,10 +117,19 @@ def __setattr__(self, key, value):
             pass  # fail silently if not float, eg: pvconst or _calc_now
         super(PVcell, self).__setattr__(key, value)
         # recalculate IV curve
+        self._cache.clear()
         if self._calc_now:
             Icell, Vcell, Pcell = self.calcCell()
             self.__dict__.update(Icell=Icell, Vcell=Vcell, Pcell=Pcell)
 
+    def clone(self):
+        """
+        Return a copy of this object with the same pvconst.
+        """
+        cloned = copy.copy(self)
+        super(PVcell, cloned).__setattr__('_cache', self._cache.copy())
+        return cloned
+
     def update(self, **kwargs):
         """
         Update user-defined constants.
@@ -108,17 +143,20 @@ def update(self, **kwargs):
         self._calc_now = True  # recalculate
 
     @property
+    @cached
     def Vt(self):
         """
         Thermal voltage in volts.
         """
         return self.pvconst.k * self.Tcell / self.pvconst.q
 
     @property
+    @cached
     def Isc(self):
         return self.Ee * self.Isc0
 
     @property
+    @cached
     def Aph(self):
         """
         Photogenerated current coefficient, non-dimensional.
@@ -134,6 +172,7 @@ def Aph(self):
         return 1. + (Idiode1_sc + Idiode2_sc + Ishunt_sc) / self.Isc
 
     @property
+    @cached
     def Isat1(self):
         """
         Diode one saturation current at Tcell in amps.
@@ -146,6 +185,7 @@ def Isat1(self):
         return self.Isat1_T0 * _Tstar * _expTstar  # [A] Isat1(Tcell)
 
     @property
+    @cached
     def Isat2(self):
         """
         Diode two saturation current at Tcell in amps.
@@ -158,6 +198,7 @@ def Isat2(self):
         return self.Isat2_T0 * _Tstar * _expTstar  # [A] Isat2(Tcell)
 
     @property
+    @cached
     def Isc0(self):
         """
         Short circuit current at Tcell in amps.
@@ -166,6 +207,7 @@ def Isc0(self):
         return self.Isc0_T0 * (1. + self.alpha_Isc * _delta_T)  # [A] Isc0
 
     @property
+    @cached
     def Voc(self):
         """
         Estimate open circuit voltage of cells.
@@ -196,6 +238,7 @@ def _VocSTC(self):
         )
 
     @property
+    @cached
     def Igen(self):
         """
         Photovoltaic generated light current (AKA IL or Iph)

pvmismatch/pvmismatch_lib/pvmodule.py

@@ -303,7 +303,7 @@ def setSuns(self, Ee, cells=None):
                 old_pvcells = dict.fromkeys(self.pvcells)  # same as set(pvcells)
                 for cell_id, pvcell in enumerate(self.pvcells):
                     if old_pvcells[pvcell] is None:
-                        new_pvcells[cell_id] = copy(pvcell)
+                        new_pvcells[cell_id] = pvcell.clone()
                         old_pvcells[pvcell] = new_pvcells[cell_id]
                     else:
                         new_pvcells[cell_id] = old_pvcells[pvcell]
@@ -314,7 +314,7 @@ def setSuns(self, Ee, cells=None):
             elif np.size(Ee) == self.numberCells:
                 self.pvcells = copy(self.pvcells)  # copy list first
                 for cell_idx, Ee_idx in enumerate(Ee):
-                    self.pvcells[cell_idx] = copy(self.pvcells[cell_idx])
+                    self.pvcells[cell_idx] = self.pvcells[cell_idx].clone()
                     self.pvcells[cell_idx].Ee = Ee_idx
             else:
                 raise Exception("Input irradiance value (Ee) for each cell!")
@@ -326,7 +326,7 @@ def setSuns(self, Ee, cells=None):
                 old_pvcells = dict.fromkeys(cells_to_update)
                 for cell_id, pvcell in zip(cells, cells_to_update):
                     if old_pvcells[pvcell] is None:
-                        self.pvcells[cell_id] = copy(pvcell)
+                        self.pvcells[cell_id] = pvcell.clone()
                         self.pvcells[cell_id].Ee = Ee
                         old_pvcells[pvcell] = self.pvcells[cell_id]
                     else:
@@ -344,7 +344,7 @@ def setSuns(self, Ee, cells=None):
                     old_pvcells = dict.fromkeys(cells_to_update)
                     for cell_id, pvcell in zip(cells_subset, cells_to_update):
                         if old_pvcells[pvcell] is None:
-                            self.pvcells[cell_id] = copy(pvcell)
+                            self.pvcells[cell_id] = pvcell.clone()
                             self.pvcells[cell_id].Ee = a_Ee
                             old_pvcells[pvcell] = self.pvcells[cell_id]
                         else:
@@ -375,7 +375,7 @@ def setTemps(self, Tc, cells=None):
                 old_pvcells = dict.fromkeys(self.pvcells)  # same as set(pvcells)
                 for cell_id, pvcell in enumerate(self.pvcells):
                     if old_pvcells[pvcell] is None:
-                        new_pvcells[cell_id] = copy(pvcell)
+                        new_pvcells[cell_id] = pvcell.clone()
                         old_pvcells[pvcell] = new_pvcells[cell_id]
                     else:
                         new_pvcells[cell_id] = old_pvcells[pvcell]
@@ -386,7 +386,7 @@ def setTemps(self, Tc, cells=None):
             elif np.size(Tc) == self.numberCells:
                 self.pvcells = copy(self.pvcells)  # copy list first
                 for cell_idx, Tc_idx in enumerate(Tc):
-                    self.pvcells[cell_idx] = copy(self.pvcells[cell_idx])
+                    self.pvcells[cell_idx] = self.pvcells[cell_idx].clone()
                     self.pvcells[cell_idx].Tcell = Tc_idx
             else:
                 raise Exception("Input temperature value (Tc) for each cell!")
@@ -398,7 +398,7 @@ def setTemps(self, Tc, cells=None):
                 old_pvcells = dict.fromkeys(cells_to_update)
                 for cell_id, pvcell in zip(cells, cells_to_update):
                     if old_pvcells[pvcell] is None:
-                        self.pvcells[cell_id] = copy(pvcell)
+                        self.pvcells[cell_id] = pvcell.clone()
                         self.pvcells[cell_id].Tcell = Tc
                         old_pvcells[pvcell] = self.pvcells[cell_id]
                     else:
@@ -416,7 +416,7 @@ def setTemps(self, Tc, cells=None):
                     old_pvcells = dict.fromkeys(cells_to_update)
                     for cell_id, pvcell in zip(cells_subset, cells_to_update):
                         if old_pvcells[pvcell] is None:
-                            self.pvcells[cell_id] = copy(pvcell)
+                            self.pvcells[cell_id] = pvcell.clone()
                             self.pvcells[cell_id].Tcell = a_Tc
                             old_pvcells[pvcell] = self.pvcells[cell_id]
                         else:

pvmismatch/tests/test_pvcell.py

@@ -107,6 +107,43 @@ def test_update():
     assert pvc._calc_now
 
 
+def test_cache(mocker):
+    """
+    Test that the cache mechanism actually works and doesn't calculate the same
+    thing more than once.
+    """
+    pvc = PVcell()
+    attrs = ['Vt', 'Isc', 'Aph', 'Isat1', 'Isat2', 'Isc0', 'Voc', 'Igen']
+    # it's a little tricky to get ahold of the underlying function for the
+    # properties -- by accessing the property through the class rather than
+    # the instance, we get access to the `fget` object, which is the wrapper
+    # function.  And the @cached decorator stores the underlying function
+    # in the __wrapped__ attribute on the wrapper.
+    spies = [
+        mocker.spy(getattr(PVcell, attr).fget, "__wrapped__")
+        for attr in attrs
+    ]
+    pvc.Ee = 0.5
+    for spy in spies:
+        spy.assert_called_once()
+
+
+def test_clone():
+    """
+    Test that the clone method returns an independent object.
+    """
+    # test independence
+    pvc1 = PVcell()
+    pvc2 = pvc1.clone()
+    pvc1.Ee = 0.5
+    assert pvc2.Ee == 1.0
+
+    # test returns identical
+    pvc3 = pvc1.clone()
+    assert np.allclose(pvc1.calcCell(), pvc3.calcCell())
+    assert pvc1.pvconst is pvc3.pvconst
+
+
 if __name__ == "__main__":
     i, v = test_calc_series()
     iv_calc = np.concatenate([[i], [v]], axis=0).T

setup.py

@@ -20,7 +20,8 @@
 ]
 
 TESTS_REQUIRES = [
-    'nose>=1.3.7', 'pytest>=3.2.1', 'sympy>=1.1.1', 'pvlib>=0.5.1'
+    'nose>=1.3.7', 'pytest>=3.2.1', 'sympy>=1.1.1', 'pvlib>=0.5.1',
+    'pytest-mock'
 ]
 
 CLASSIFIERS = [