Fix #2017

pvlib/modelchain.py

@@ -330,10 +330,15 @@ class ModelChain:
  'interp' and 'no_loss'. The ModelChain instance will be passed as the
  first argument to a user-defined function.
 
- spectral_model : str, or function, optional
+ spectral_model : str or function, optional
  If not specified, the model will be inferred from the parameters that
- are common to all of system.arrays[i].module_parameters.
- Valid strings are 'sapm', 'first_solar', 'no_loss'.
+ are common to all of system.arrays[i].module_parameters. Valid strings
+ are:
+
+ - ``'sapm'``
+ - ``'first_solar'``
+ - ``'no_loss'``
+
  The ModelChain instance will be passed as the first argument to
  a user-defined function.
 
@@ -855,9 +860,7 @@ def spectral_model(self):
 
  @spectral_model.setter
  def spectral_model(self, model):
- if model is None:
- self._spectral_model = self.infer_spectral_model()
- elif isinstance(model, str):
+ if isinstance(model, str):
  model = model.lower()
  if model == 'first_solar':
  self._spectral_model = self.first_solar_spectral_loss
@@ -867,29 +870,60 @@ def spectral_model(self, model):
  self._spectral_model = self.no_spectral_loss
  else:
  raise ValueError(model + ' is not a valid spectral loss model')
+ elif model is None:
+ # None is a valid value, model inference will be done later
+ self._spectral_model = None
  else:
  self._spectral_model = partial(model, self)
 
- def infer_spectral_model(self):
+ def infer_spectral_model(self): # TODO: support other spectral models?
  """Infer spectral model from system attributes."""
  module_parameters = tuple(
  array.module_parameters for array in self.system.arrays)
  params = _common_keys(module_parameters)
  if {'A4', 'A3', 'A2', 'A1', 'A0'} <= params:
  return self.sapm_spectral_loss
- elif ((('Technology' in params or
- 'Material' in params) and
- (self.system._infer_cell_type() is not None)) or
- 'first_solar_spectral_coefficients' in params):
+ elif "first_solar_spectral_coefficients" in params:
+ # user explicitly sets spectral coefficients
  return self.first_solar_spectral_loss
+ elif (
+ # cell type is known or can be inferred
+ ("Technology" in params or "Material" in params)
+ and (self.system._infer_cell_type() is not None)
+ ):
+ # This suggests models that provide default parameters per cell
+ # type can be used. However, some of them depend on other weather
+ # parameters, so we need to check if they are available.
+ # At this point, weather may not be available, so let's take that
+ # into account.
+ if self.results is not None and self.results.weather is not None:
+ # weather is available
+ if "precipitable_water" in self.results.weather:
+ return self.first_solar_spectral_loss
+ else:
+ return self.no_spectral_loss
+ else:
+ # weather is not available, so it's unknown what model to use.
+ # Warn user about this and default to no_spectral_loss.
+ warnings.warn(
+ "Weather data is not available to infer spectral model. "
+ "Defaulting to 'no_loss'. Explicitly set the "
+ "spectral model with the 'spectral_model' kwarg to "
+ "suppress this warning."
+ )
+ return self.no_spectral_loss
  else:
- raise ValueError('could not infer spectral model from '
- 'system.arrays[i].module_parameters. Check that '
- 'the module_parameters for all Arrays in '
- 'system.arrays contain valid '
- 'first_solar_spectral_coefficients, a valid '
- 'Material or Technology value, or set '
- 'spectral_model="no_loss".')
+ # TODO: list valid models in message down below
+ raise ValueError(
+ "could not infer spectral model from "
+ "system.arrays[i].module_parameters. Check that "
+ "the module_parameters for all Arrays in "
+ "system.arrays contain valid "
+ "valid spectral coefficients, a valid "
+ "Material or Technology value, or a valid model "
+ "string; explicitly set the model with the "
+ "'spectral_model' kwarg."
+ )
 
  def first_solar_spectral_loss(self):
  self.results.spectral_modifier = self.system.first_solar_spectral_loss(
@@ -1678,7 +1712,13 @@ def run_model(self, weather):
  weather = _to_tuple(weather)
  self.prepare_inputs(weather)
  self.aoi_model()
+
+ # spectral model is optional, and weather may impact model inference
+ # check if spectral model inference is requested by means of "None"
+ if self._spectral_model is None:
+ self._spectral_model = self.infer_spectral_model()
  self.spectral_model()
+
  self.effective_irradiance_model()
 
  self._run_from_effective_irrad(weather)