From 3c708029f652b6d4e3e34e4b79236f106418fd34 Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Fri, 11 Oct 2024 11:04:42 +0200
Subject: [PATCH 01/16] Fix #2017

---
 pvlib/modelchain.py | 76 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 58 insertions(+), 18 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 8456aac114..e0361ae7b6 100644
--- a/pvlib/modelchain.py
+++ b/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)

From 6121759c6eab2b4ee89d00bb4f7132ae9cf19f9c Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Fri, 11 Oct 2024 11:12:43 +0200
Subject: [PATCH 02/16] rm TODOs

---
 pvlib/modelchain.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index e0361ae7b6..14d0adbed1 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -876,7 +876,7 @@ def spectral_model(self, model):
         else:
             self._spectral_model = partial(model, self)
 
-    def infer_spectral_model(self):  # TODO: support other spectral models?
+    def infer_spectral_model(self):
         """Infer spectral model from system attributes."""
         module_parameters = tuple(
             array.module_parameters for array in self.system.arrays)
@@ -913,7 +913,6 @@ def infer_spectral_model(self):  # TODO: support other spectral models?
                 )
                 return self.no_spectral_loss
         else:
-            # TODO: list valid models in message down below
             raise ValueError(
                 "could not infer spectral model from "
                 "system.arrays[i].module_parameters. Check that "

From 08bfe62e12d43bb74e312c1cab24d2d6cdaa0655 Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Tue, 15 Oct 2024 17:13:31 +0200
Subject: [PATCH 03/16] cwhanse review

Co-Authored-By: Cliff Hansen <5393711+cwhanse@users.noreply.github.com>
---
 pvlib/modelchain.py | 71 ++++++++++++++++++++-------------------------
 1 file changed, 32 insertions(+), 39 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 14d0adbed1..4ceebff18a 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -330,10 +330,8 @@ 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
-        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:
+    spectral_model : str or function, default ``'no_loss'``
+        Valid strings are:
 
         - ``'sapm'``
         - ``'first_solar'``
@@ -342,6 +340,9 @@ class ModelChain:
         The ModelChain instance will be passed as the first argument to
         a user-defined function.
 
+        See :py:func:`~pvlib.modelchain.ModelChain.infer_spectral_model` to
+        infer the spectral model from system and weather information.
+
     temperature_model : str or function, optional
         Valid strings are: 'sapm', 'pvsyst', 'faiman', 'fuentes', 'noct_sam'.
         The ModelChain instance will be passed as the first argument to a
@@ -870,14 +871,30 @@ 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):
-        """Infer spectral model from system attributes."""
+    def infer_spectral_model(self, weather=None):
+        """
+        Infer spectral model from system attributes, and optionally from
+        the input weather dataframe, and set it to the ``ModelChain`` instance.
+
+        Parameters
+        ----------
+        weather : pd.DataFrame or collection of str, optional
+            An object with columns of available input data to help infer
+            the spectral model. If ``None``, the spectral model will be
+            inferred from the system attributes only.
+
+        Returns
+        -------
+        Inferred spectral correction model : function
+
+        Examples
+        --------
+        >>> mc = ModelChain(system, location)
+        >>> mc.spectral_model = mc.infer_spectral_model(weather=weather)
+        """
         module_parameters = tuple(
             array.module_parameters for array in self.system.arrays)
         params = _common_keys(module_parameters)
@@ -892,37 +909,13 @@ def infer_spectral_model(self):
             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
+            # type can be used. However, some models 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:
+            if weather is not None:  # weather is available
+                if "precipitable_water" in 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 "
-                "valid spectral coefficients, a valid "
-                "Material or Technology value, or a valid model "
-                "string; explicitly set the model with the "
-                "'spectral_model' kwarg."
-            )
+
+        return self.no_spectral_loss
 
     def first_solar_spectral_loss(self):
         self.results.spectral_modifier = self.system.first_solar_spectral_loss(
@@ -1715,7 +1708,7 @@ def run_model(self, weather):
         # 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.infer_spectral_model(weather=weather)
         self.spectral_model()
 
         self.effective_irradiance_model()

From 55faf8ac629eff8d799a55e9d3ff0f656ad4602d Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Tue, 15 Oct 2024 17:14:09 +0200
Subject: [PATCH 04/16] typo

---
 pvlib/modelchain.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 4ceebff18a..38eafd85e7 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -1596,7 +1596,7 @@ def _prepare_temperature(self, data):
         ----------
         data : DataFrame
             May contain columns ``'cell_temperature'`` or
-            ``'module_temperaure'``.
+            ``'module_temperature'``.
 
         Returns
         -------

From 831698a2e920ee0b8bffce68ac4cf79df2dd72a5 Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Tue, 15 Oct 2024 17:14:21 +0200
Subject: [PATCH 05/16] pep8 compliance

---
 pvlib/modelchain.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 38eafd85e7..f5a309478a 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -253,7 +253,7 @@ def __repr__(self):
         def _head(obj):
             try:
                 return obj[:3]
-            except:
+            except Exception:
                 return obj
 
         if type(self.dc) is tuple:
@@ -269,7 +269,7 @@ def _head(obj):
                  '\n')
         lines = []
         for attr in mc_attrs:
-            if not (attr.startswith('_') or attr=='times'):
+            if not (attr.startswith('_') or attr == 'times'):
                 lines.append(f' {attr}: ' + _mcr_repr(getattr(self, attr)))
         desc4 = '\n'.join(lines)
         return (desc1 + desc2 + desc3 + desc4)
@@ -392,7 +392,6 @@ def __init__(self, system, location,
 
         self.results = ModelChainResult()
 
-
     @classmethod
     def with_pvwatts(cls, system, location,
                      clearsky_model='ineichen',

From 49acea8ab2e4df698ff2ed5118bcb4b0e3da4c2b Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Tue, 15 Oct 2024 17:21:44 +0200
Subject: [PATCH 06/16] ups

---
 pvlib/modelchain.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index f5a309478a..13b10d4b67 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -367,7 +367,7 @@ def __init__(self, system, location,
                  solar_position_method='nrel_numpy',
                  airmass_model='kastenyoung1989',
                  dc_model=None, ac_model=None, aoi_model=None,
-                 spectral_model=None, temperature_model=None,
+                 spectral_model='no_loss', temperature_model=None,
                  dc_ohmic_model='no_loss',
                  losses_model='no_loss', name=None):
 

From f06bbf49ad1ee1109e400f0c9fad284d223c4ad9 Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Tue, 15 Oct 2024 17:26:03 +0200
Subject: [PATCH 07/16] Update modelchain.py

---
 pvlib/modelchain.py | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 13b10d4b67..f86162d941 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -330,7 +330,7 @@ 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, default ``'no_loss'``
+    spectral_model : str or function, optional
         Valid strings are:
 
         - ``'sapm'``
@@ -340,6 +340,8 @@ class ModelChain:
         The ModelChain instance will be passed as the first argument to
         a user-defined function.
 
+        By default, it will be inferred from the system attributes only.
+
         See :py:func:`~pvlib.modelchain.ModelChain.infer_spectral_model` to
         infer the spectral model from system and weather information.
 
@@ -367,7 +369,7 @@ def __init__(self, system, location,
                  solar_position_method='nrel_numpy',
                  airmass_model='kastenyoung1989',
                  dc_model=None, ac_model=None, aoi_model=None,
-                 spectral_model='no_loss', temperature_model=None,
+                 spectral_model=None, temperature_model=None,
                  dc_ohmic_model='no_loss',
                  losses_model='no_loss', name=None):
 
@@ -870,6 +872,8 @@ 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:
+            self._spectral_model = self.infer_spectral_model(weather=None)
         else:
             self._spectral_model = partial(model, self)
 

From 774edfee66304ca1cb26e69ef911a43c9da3d4d2 Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Sun, 20 Oct 2024 20:37:45 +0100
Subject: [PATCH 08/16] Infer a string, key for setter

---
 pvlib/modelchain.py            | 20 +++++++++++---------
 pvlib/tests/test_modelchain.py | 22 ++++++++++++++++++++++
 2 files changed, 33 insertions(+), 9 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index f86162d941..cb5e58dbed 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -873,8 +873,9 @@ def spectral_model(self, model):
             else:
                 raise ValueError(model + ' is not a valid spectral loss model')
         elif model is None:
-            self._spectral_model = self.infer_spectral_model(weather=None)
-        else:
+            # uses recursive setter to infer model, which returns a string
+            self.spectral_model = self.infer_spectral_model(weather=None)
+        else:  # assume model is a callable
             self._spectral_model = partial(model, self)
 
     def infer_spectral_model(self, weather=None):
@@ -891,7 +892,7 @@ def infer_spectral_model(self, weather=None):
 
         Returns
         -------
-        Inferred spectral correction model : function
+        Inferred spectral correction model : string key for model setter
 
         Examples
         --------
@@ -899,13 +900,14 @@ def infer_spectral_model(self, weather=None):
         >>> mc.spectral_model = mc.infer_spectral_model(weather=weather)
         """
         module_parameters = tuple(
-            array.module_parameters for array in self.system.arrays)
+            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
+        if {"A4", "A3", "A2", "A1", "A0"} <= params:
+            return "sapm"
         elif "first_solar_spectral_coefficients" in params:
             # user explicitly sets spectral coefficients
-            return self.first_solar_spectral_loss
+            return "first_solar"
         elif (
             # cell type is known or can be inferred
             ("Technology" in params or "Material" in params)
@@ -916,9 +918,9 @@ def infer_spectral_model(self, weather=None):
             # parameters, so we need to check if they are available.
             if weather is not None:  # weather is available
                 if "precipitable_water" in weather:
-                    return self.first_solar_spectral_loss
+                    return "first_solar"
 
-        return self.no_spectral_loss
+        return "no_loss"
 
     def first_solar_spectral_loss(self):
         self.results.spectral_modifier = self.system.first_solar_spectral_loss(
diff --git a/pvlib/tests/test_modelchain.py b/pvlib/tests/test_modelchain.py
index dcbd820f16..95c201214b 100644
--- a/pvlib/tests/test_modelchain.py
+++ b/pvlib/tests/test_modelchain.py
@@ -1271,6 +1271,28 @@ def test_infer_spectral_model(location, sapm_dc_snl_ac_system,
     assert isinstance(mc, ModelChain)
 
 
+def test_infer_spectral_model_with_weather(location, sapm_dc_snl_ac_system,
+                                           cec_dc_snl_ac_system, weather):
+    # instantiate example ModelChain to get the default spectral model
+    # inferred without weather available by default
+    # - should resolve to sapm
+    mc = ModelChain(sapm_dc_snl_ac_system, location, aoi_model='physical')
+    assert mc.spectral_model == mc.sapm_spectral_loss
+    # - should resolve to no loss
+    mc = ModelChain(cec_dc_snl_ac_system, location, aoi_model='physical')
+    assert mc.spectral_model == mc.no_spectral_loss
+
+    # infer spectral model from weather
+    # - without precipitable water in it, should resolve to no loss
+    mc.spectral_model = mc.infer_spectral_model(weather=weather)
+    assert mc.spectral_model == mc.no_spectral_loss
+    # - with precipitable water in it, should resolve to first solar
+    weather['precipitable_water'] = 1.42
+    mc.spectral_model = mc.infer_spectral_model(weather=weather)
+    assert mc.spectral_model == mc.first_solar_spectral_loss
+    assert isinstance(mc, ModelChain)
+
+
 @pytest.mark.parametrize('temp_model', [
     'sapm_temp', 'faiman_temp', 'pvsyst_temp', 'fuentes_temp',
     'noct_sam_temp'])

From 240f47968e276214b46b3d787a7f9a97473b08ae Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Sun, 20 Oct 2024 20:37:56 +0100
Subject: [PATCH 09/16] pep8 linting

---
 pvlib/tests/test_modelchain.py | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/pvlib/tests/test_modelchain.py b/pvlib/tests/test_modelchain.py
index 95c201214b..d4b302a7b3 100644
--- a/pvlib/tests/test_modelchain.py
+++ b/pvlib/tests/test_modelchain.py
@@ -7,13 +7,10 @@
 from pvlib.modelchain import ModelChain
 from pvlib.pvsystem import PVSystem
 from pvlib.location import Location
-from pvlib._deprecation import pvlibDeprecationWarning
 
 from .conftest import assert_series_equal, assert_frame_equal
 import pytest
 
-from .conftest import fail_on_pvlib_version
-
 
 @pytest.fixture(scope='function')
 def sapm_dc_snl_ac_system(sapm_module_params, cec_inverter_parameters,

From 2680b4e9bfc65748c87445a1ccbac24623074e5c Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Sun, 20 Oct 2024 20:44:51 +0100
Subject: [PATCH 10/16] Remove delayed-inference code - codecov warning

---
 pvlib/modelchain.py | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index cb5e58dbed..7188f29521 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -1709,11 +1709,6 @@ 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(weather=weather)
         self.spectral_model()
 
         self.effective_irradiance_model()

From feb99649b46674991185cb520573d717f3be55bc Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Mon, 6 Jan 2025 14:42:22 +0000
Subject: [PATCH 11/16] default to no_loss

---
 pvlib/modelchain.py            | 14 +++++-------
 pvlib/tests/test_modelchain.py | 42 ++++++++++++++++++++++------------
 2 files changed, 33 insertions(+), 23 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 7188f29521..05d40fcc54 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -29,7 +29,7 @@
 
 # Optional keys to communicate temperature data. If provided,
 # 'cell_temperature' overrides ModelChain.temperature_model and sets
-# ModelChain.cell_temperature to the data. If 'module_temperature' is provdied,
+# ModelChain.cell_temperature to the data. If 'module_temperature' is provided,
 # overrides ModelChain.temperature_model with
 # pvlib.temperature.sapm_celL_from_module
 TEMPERATURE_KEYS = ('module_temperature', 'cell_temperature')
@@ -330,7 +330,7 @@ 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, default ``'no_loss'``
         Valid strings are:
 
         - ``'sapm'``
@@ -340,8 +340,6 @@ class ModelChain:
         The ModelChain instance will be passed as the first argument to
         a user-defined function.
 
-        By default, it will be inferred from the system attributes only.
-
         See :py:func:`~pvlib.modelchain.ModelChain.infer_spectral_model` to
         infer the spectral model from system and weather information.
 
@@ -369,7 +367,7 @@ def __init__(self, system, location,
                  solar_position_method='nrel_numpy',
                  airmass_model='kastenyoung1989',
                  dc_model=None, ac_model=None, aoi_model=None,
-                 spectral_model=None, temperature_model=None,
+                 spectral_model='no_loss', temperature_model=None,
                  dc_ohmic_model='no_loss',
                  losses_model='no_loss', name=None):
 
@@ -873,9 +871,9 @@ def spectral_model(self, model):
             else:
                 raise ValueError(model + ' is not a valid spectral loss model')
         elif model is None:
-            # uses recursive setter to infer model, which returns a string
-            self.spectral_model = self.infer_spectral_model(weather=None)
-        else:  # assume model is a callable
+            # not setting a model is equivalent to setting no_loss
+            self._spectral_model = self.no_spectral_loss
+        else:  # assume model is callable with 1st argument = the MC instance
             self._spectral_model = partial(model, self)
 
     def infer_spectral_model(self, weather=None):
diff --git a/pvlib/tests/test_modelchain.py b/pvlib/tests/test_modelchain.py
index d4b302a7b3..1caf7720d2 100644
--- a/pvlib/tests/test_modelchain.py
+++ b/pvlib/tests/test_modelchain.py
@@ -346,7 +346,7 @@ def test_with_pvwatts(pvwatts_dc_pvwatts_ac_system, location, weather):
 
 
 def test_run_model_with_irradiance(sapm_dc_snl_ac_system, location):
-    mc = ModelChain(sapm_dc_snl_ac_system, location)
+    mc = ModelChain(sapm_dc_snl_ac_system, location, spectral_model='sapm')
     times = pd.date_range('20160101 1200-0700', periods=2, freq='6h')
     irradiance = pd.DataFrame({'dni': 900, 'ghi': 600, 'dhi': 150},
                               index=times)
@@ -629,9 +629,13 @@ def test_run_model_arrays_weather(sapm_dc_snl_ac_system_same_arrays,
 
 
 def test_run_model_perez(sapm_dc_snl_ac_system, location):
-    mc = ModelChain(sapm_dc_snl_ac_system, location,
-                    transposition_model='perez')
-    times = pd.date_range('20160101 1200-0700', periods=2, freq='6h')
+    mc = ModelChain(
+        sapm_dc_snl_ac_system,
+        location,
+        transposition_model="perez",
+        spectral_model="sapm",
+    )
+    times = pd.date_range("20160101 1200-0700", periods=2, freq="6h")
     irradiance = pd.DataFrame({'dni': 900, 'ghi': 600, 'dhi': 150},
                               index=times)
     ac = mc.run_model(irradiance).results.ac
@@ -642,10 +646,14 @@ def test_run_model_perez(sapm_dc_snl_ac_system, location):
 
 
 def test_run_model_gueymard_perez(sapm_dc_snl_ac_system, location):
-    mc = ModelChain(sapm_dc_snl_ac_system, location,
-                    airmass_model='gueymard1993',
-                    transposition_model='perez')
-    times = pd.date_range('20160101 1200-0700', periods=2, freq='6h')
+    mc = ModelChain(
+        sapm_dc_snl_ac_system,
+        location,
+        airmass_model="gueymard1993",
+        transposition_model="perez",
+        spectral_model="sapm",
+    )
+    times = pd.date_range("20160101 1200-0700", periods=2, freq="6h")
     irradiance = pd.DataFrame({'dni': 900, 'ghi': 600, 'dhi': 150},
                               index=times)
     ac = mc.run_model(irradiance).results.ac
@@ -1271,12 +1279,16 @@ def test_infer_spectral_model(location, sapm_dc_snl_ac_system,
 def test_infer_spectral_model_with_weather(location, sapm_dc_snl_ac_system,
                                            cec_dc_snl_ac_system, weather):
     # instantiate example ModelChain to get the default spectral model
-    # inferred without weather available by default
-    # - should resolve to sapm
+    # default should resolve to no loss
     mc = ModelChain(sapm_dc_snl_ac_system, location, aoi_model='physical')
     assert mc.spectral_model == mc.sapm_spectral_loss
-    # - should resolve to no loss
-    mc = ModelChain(cec_dc_snl_ac_system, location, aoi_model='physical')
+    # - next inference should resolve to no loss
+    mc = ModelChain(
+        cec_dc_snl_ac_system,
+        location,
+        aoi_model="physical",
+        spectral_model=None,
+    )
     assert mc.spectral_model == mc.no_spectral_loss
 
     # infer spectral model from weather
@@ -2008,9 +2020,9 @@ def test__irrad_for_celltemp():
 
 
 def test_ModelChain___repr__(sapm_dc_snl_ac_system, location):
-
-    mc = ModelChain(sapm_dc_snl_ac_system, location,
-                    name='my mc')
+    mc = ModelChain(
+        sapm_dc_snl_ac_system, location, name="my mc", spectral_model="sapm"
+    )
 
     expected = '\n'.join([
         'ModelChain: ',

From 09c5bdae93d3a287ff65545a154ef015d4198974 Mon Sep 17 00:00:00 2001
From: echedey-ls <80125792+echedey-ls@users.noreply.github.com>
Date: Mon, 6 Jan 2025 14:42:52 +0000
Subject: [PATCH 12/16] infer_spectral_model tweak -> return list if possible

---
 pvlib/modelchain.py            | 65 +++++++++++++++++++++++-----------
 pvlib/tests/test_modelchain.py |  4 +++
 2 files changed, 48 insertions(+), 21 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 05d40fcc54..d675ef5ac3 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -341,7 +341,7 @@ class ModelChain:
         a user-defined function.
 
         See :py:func:`~pvlib.modelchain.ModelChain.infer_spectral_model` to
-        infer the spectral model from system and weather information.
+        get a list of available spectral models for the current system.
 
     temperature_model : str or function, optional
         Valid strings are: 'sapm', 'pvsyst', 'faiman', 'fuentes', 'noct_sam'.
@@ -878,8 +878,10 @@ def spectral_model(self, model):
 
     def infer_spectral_model(self, weather=None):
         """
-        Infer spectral model from system attributes, and optionally from
-        the input weather dataframe, and set it to the ``ModelChain`` instance.
+        Return a spectral model key or a list of possible spectral models based
+        on the system attributes and weather data. If the weather data is not
+        provided, the spectral model will be inferred from the system
+        attributes only.
 
         Parameters
         ----------
@@ -890,35 +892,56 @@ def infer_spectral_model(self, weather=None):
 
         Returns
         -------
-        Inferred spectral correction model : string key for model setter
+        Inferred spectral correction model : string key or list of string keys
+            If no spectral model is inferred, 'no_loss' is returned.
+            If only one spectral model is inferred, a string key is returned.
+            If multiple spectral models are inferred, a list of string keys is
+            returned.
+
+            The following spectral models keys may be returned:
+
+            - ``'sapm'`` for
+              :py:func:`~pvlib.spectrum.spectral_factor_sapm`
+            - ``'first_solar'`` for
+              :py:func:`~pvlib.spectrum.spectral_factor_first_solar`.
+              Requires ``'precipitable_water'`` in the weather dataframe.
+
+            If no spectral model is inferred, 'no_loss' is returned.
 
         Examples
         --------
         >>> mc = ModelChain(system, location)
         >>> mc.spectral_model = mc.infer_spectral_model(weather=weather)
         """
+        possible_models_result = []  # list of possible spectral models
         module_parameters = tuple(
             array.module_parameters for array in self.system.arrays
         )
         params = _common_keys(module_parameters)
+
+        # infer all the possible spectral models and append them to the list
         if {"A4", "A3", "A2", "A1", "A0"} <= params:
-            return "sapm"
-        elif "first_solar_spectral_coefficients" in params:
-            # user explicitly sets spectral coefficients
-            return "first_solar"
-        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 models depend on other weather
-            # parameters, so we need to check if they are available.
-            if weather is not None:  # weather is available
-                if "precipitable_water" in weather:
-                    return "first_solar"
-
-        return "no_loss"
+            possible_models_result.append("sapm")
+        if weather is not None:  # models that depend on weather data
+            if "precipitable_water" in weather:
+                if "first_solar_spectral_coefficients" in params:
+                    # user explicitly sets first solar spectral coefficients
+                    possible_models_result.append("first_solar")
+                # cell type is known or can be inferred
+                if ("Technology" in params or "Material" in params) and (
+                    self.system._infer_cell_type() is not None
+                ):
+                    possible_models_result.append("first_solar")
+
+        # result resolution based on the number of inferred spectral models
+        if (result_len := len(possible_models_result)) == 0:
+            # if no spectral model is inferred, return no_loss
+            return "no_loss"
+        elif result_len == 1:
+            # when only one spectral model is inferred, return it
+            return possible_models_result[0]
+        else:  # multiple spectral models are inferred (avoiding duplicates)
+            return list(set(possible_models_result))
 
     def first_solar_spectral_loss(self):
         self.results.spectral_modifier = self.system.first_solar_spectral_loss(
diff --git a/pvlib/tests/test_modelchain.py b/pvlib/tests/test_modelchain.py
index 1caf7720d2..384f5b8e1a 100644
--- a/pvlib/tests/test_modelchain.py
+++ b/pvlib/tests/test_modelchain.py
@@ -1281,6 +1281,9 @@ def test_infer_spectral_model_with_weather(location, sapm_dc_snl_ac_system,
     # instantiate example ModelChain to get the default spectral model
     # default should resolve to no loss
     mc = ModelChain(sapm_dc_snl_ac_system, location, aoi_model='physical')
+    assert mc.spectral_model == mc.no_spectral_loss
+    # - inference should resolve to sapm
+    mc.spectral_model = mc.infer_spectral_model(weather=weather)
     assert mc.spectral_model == mc.sapm_spectral_loss
     # - next inference should resolve to no loss
     mc = ModelChain(
@@ -1289,6 +1292,7 @@ def test_infer_spectral_model_with_weather(location, sapm_dc_snl_ac_system,
         aoi_model="physical",
         spectral_model=None,
     )
+    mc.spectral_model = mc.infer_spectral_model(weather=weather)
     assert mc.spectral_model == mc.no_spectral_loss
 
     # infer spectral model from weather

From e67080049b9851237bbfb12b66339e257b017fab Mon Sep 17 00:00:00 2001
From: Cliff Hansen <cwhanse@sandia.gov>
Date: Thu, 13 Mar 2025 13:52:19 -0600
Subject: [PATCH 13/16] remove infer_spectral_model

---
 docs/sphinx/source/whatsnew/v0.11.3.rst | 14 +++--
 pvlib/modelchain.py                     | 69 -------------------------
 tests/test_modelchain.py                | 42 ---------------
 3 files changed, 9 insertions(+), 116 deletions(-)

diff --git a/docs/sphinx/source/whatsnew/v0.11.3.rst b/docs/sphinx/source/whatsnew/v0.11.3.rst
index b431cda364..d3c8932728 100644
--- a/docs/sphinx/source/whatsnew/v0.11.3.rst
+++ b/docs/sphinx/source/whatsnew/v0.11.3.rst
@@ -4,6 +4,15 @@
 v0.11.3 (Anticipated March, 2025)
 ---------------------------------
 
+Breaking Changes
+~~~~~~~~~~~~~~~~
+* The pvlib.location.Location.pytz attribute is now read only. The
+  pytz attribute is now set internally to be consistent with the
+  pvlib.location.Location.tz attribute. (:issue:`2340`, :pull:`2341`)
+* Users must now provide ModelChain.spectral_model, or the 'no_loss' spectral
+  model is assumed. pvlib.modelchain.ModelChain no longer attempts to infer
+  the spectral from PVSystem attributes. (:issue:`2017`, :pull:`2253`)
+
 Bug fixes
 ~~~~~~~~~
 * Fix a bug in :py:func:`pvlib.bifacial.get_irradiance_poa` which may have yielded non-zero
@@ -62,11 +71,6 @@ Maintenance
 * asv 0.4.2 upgraded to asv 0.6.4 to fix CI failure due to pinned older conda.
   (:pull:`2352`)
 
-Breaking Changes
-~~~~~~~~~~~~~~~~
-* The pvlib.location.Location.pytz attribute is now read only. The
-  pytz attribute is now set internally to be consistent with the
-  pvlib.location.Location.tz attribute. (:issue:`2340`, :pull:`2341`)
 
 Contributors
 ~~~~~~~~~~~~
diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index d675ef5ac3..36b10f980e 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -340,9 +340,6 @@ class ModelChain:
         The ModelChain instance will be passed as the first argument to
         a user-defined function.
 
-        See :py:func:`~pvlib.modelchain.ModelChain.infer_spectral_model` to
-        get a list of available spectral models for the current system.
-
     temperature_model : str or function, optional
         Valid strings are: 'sapm', 'pvsyst', 'faiman', 'fuentes', 'noct_sam'.
         The ModelChain instance will be passed as the first argument to a
@@ -876,72 +873,6 @@ def spectral_model(self, model):
         else:  # assume model is callable with 1st argument = the MC instance
             self._spectral_model = partial(model, self)
 
-    def infer_spectral_model(self, weather=None):
-        """
-        Return a spectral model key or a list of possible spectral models based
-        on the system attributes and weather data. If the weather data is not
-        provided, the spectral model will be inferred from the system
-        attributes only.
-
-        Parameters
-        ----------
-        weather : pd.DataFrame or collection of str, optional
-            An object with columns of available input data to help infer
-            the spectral model. If ``None``, the spectral model will be
-            inferred from the system attributes only.
-
-        Returns
-        -------
-        Inferred spectral correction model : string key or list of string keys
-            If no spectral model is inferred, 'no_loss' is returned.
-            If only one spectral model is inferred, a string key is returned.
-            If multiple spectral models are inferred, a list of string keys is
-            returned.
-
-            The following spectral models keys may be returned:
-
-            - ``'sapm'`` for
-              :py:func:`~pvlib.spectrum.spectral_factor_sapm`
-            - ``'first_solar'`` for
-              :py:func:`~pvlib.spectrum.spectral_factor_first_solar`.
-              Requires ``'precipitable_water'`` in the weather dataframe.
-
-            If no spectral model is inferred, 'no_loss' is returned.
-
-        Examples
-        --------
-        >>> mc = ModelChain(system, location)
-        >>> mc.spectral_model = mc.infer_spectral_model(weather=weather)
-        """
-        possible_models_result = []  # list of possible spectral models
-        module_parameters = tuple(
-            array.module_parameters for array in self.system.arrays
-        )
-        params = _common_keys(module_parameters)
-
-        # infer all the possible spectral models and append them to the list
-        if {"A4", "A3", "A2", "A1", "A0"} <= params:
-            possible_models_result.append("sapm")
-        if weather is not None:  # models that depend on weather data
-            if "precipitable_water" in weather:
-                if "first_solar_spectral_coefficients" in params:
-                    # user explicitly sets first solar spectral coefficients
-                    possible_models_result.append("first_solar")
-                # cell type is known or can be inferred
-                if ("Technology" in params or "Material" in params) and (
-                    self.system._infer_cell_type() is not None
-                ):
-                    possible_models_result.append("first_solar")
-
-        # result resolution based on the number of inferred spectral models
-        if (result_len := len(possible_models_result)) == 0:
-            # if no spectral model is inferred, return no_loss
-            return "no_loss"
-        elif result_len == 1:
-            # when only one spectral model is inferred, return it
-            return possible_models_result[0]
-        else:  # multiple spectral models are inferred (avoiding duplicates)
-            return list(set(possible_models_result))
 
     def first_solar_spectral_loss(self):
         self.results.spectral_modifier = self.system.first_solar_spectral_loss(
diff --git a/tests/test_modelchain.py b/tests/test_modelchain.py
index 38fe78bf92..1bdcb87f90 100644
--- a/tests/test_modelchain.py
+++ b/tests/test_modelchain.py
@@ -1280,48 +1280,6 @@ def test_singlediode_dc_arrays(location, dc_model,
         assert isinstance(dc, (pd.Series, pd.DataFrame))
 
 
-@pytest.mark.parametrize('dc_model', ['sapm', 'cec', 'cec_native'])
-def test_infer_spectral_model(location, sapm_dc_snl_ac_system,
-                              cec_dc_snl_ac_system,
-                              cec_dc_native_snl_ac_system, dc_model):
-    dc_systems = {'sapm': sapm_dc_snl_ac_system,
-                  'cec': cec_dc_snl_ac_system,
-                  'cec_native': cec_dc_native_snl_ac_system}
-    system = dc_systems[dc_model]
-    mc = ModelChain(system, location, aoi_model='physical')
-    assert isinstance(mc, ModelChain)
-
-
-def test_infer_spectral_model_with_weather(location, sapm_dc_snl_ac_system,
-                                           cec_dc_snl_ac_system, weather):
-    # instantiate example ModelChain to get the default spectral model
-    # default should resolve to no loss
-    mc = ModelChain(sapm_dc_snl_ac_system, location, aoi_model='physical')
-    assert mc.spectral_model == mc.no_spectral_loss
-    # - inference should resolve to sapm
-    mc.spectral_model = mc.infer_spectral_model(weather=weather)
-    assert mc.spectral_model == mc.sapm_spectral_loss
-    # - next inference should resolve to no loss
-    mc = ModelChain(
-        cec_dc_snl_ac_system,
-        location,
-        aoi_model="physical",
-        spectral_model=None,
-    )
-    mc.spectral_model = mc.infer_spectral_model(weather=weather)
-    assert mc.spectral_model == mc.no_spectral_loss
-
-    # infer spectral model from weather
-    # - without precipitable water in it, should resolve to no loss
-    mc.spectral_model = mc.infer_spectral_model(weather=weather)
-    assert mc.spectral_model == mc.no_spectral_loss
-    # - with precipitable water in it, should resolve to first solar
-    weather['precipitable_water'] = 1.42
-    mc.spectral_model = mc.infer_spectral_model(weather=weather)
-    assert mc.spectral_model == mc.first_solar_spectral_loss
-    assert isinstance(mc, ModelChain)
-
-
 @pytest.mark.parametrize('temp_model', [
     'sapm_temp', 'faiman_temp', 'pvsyst_temp', 'fuentes_temp',
     'noct_sam_temp'])

From f638d93b4556760a4ae6f18252dfe7ec5b98bf92 Mon Sep 17 00:00:00 2001
From: Cliff Hansen <cwhanse@sandia.gov>
Date: Thu, 13 Mar 2025 13:55:05 -0600
Subject: [PATCH 14/16] lint

---
 pvlib/modelchain.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 36b10f980e..f4c19507e3 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -873,7 +873,6 @@ def spectral_model(self, model):
         else:  # assume model is callable with 1st argument = the MC instance
             self._spectral_model = partial(model, self)
 
-
     def first_solar_spectral_loss(self):
         self.results.spectral_modifier = self.system.first_solar_spectral_loss(
             _tuple_from_dfs(self.results.weather, 'precipitable_water'),

From 0a62ed65ef094f31301f86902b98494aae28f900 Mon Sep 17 00:00:00 2001
From: Cliff Hansen <cwhanse@sandia.gov>
Date: Fri, 14 Mar 2025 07:54:25 -0600
Subject: [PATCH 15/16] review

---
 docs/sphinx/source/whatsnew/v0.11.3.rst | 2 +-
 pvlib/modelchain.py                     | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/docs/sphinx/source/whatsnew/v0.11.3.rst b/docs/sphinx/source/whatsnew/v0.11.3.rst
index d3c8932728..1cc524ee07 100644
--- a/docs/sphinx/source/whatsnew/v0.11.3.rst
+++ b/docs/sphinx/source/whatsnew/v0.11.3.rst
@@ -11,7 +11,7 @@ Breaking Changes
   pvlib.location.Location.tz attribute. (:issue:`2340`, :pull:`2341`)
 * Users must now provide ModelChain.spectral_model, or the 'no_loss' spectral
   model is assumed. pvlib.modelchain.ModelChain no longer attempts to infer
-  the spectral from PVSystem attributes. (:issue:`2017`, :pull:`2253`)
+  the spectral model from PVSystem attributes. (:issue:`2017`, :pull:`2253`)
 
 Bug fixes
 ~~~~~~~~~
diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index f4c19507e3..0f58da533a 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -330,7 +330,7 @@ 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, default ``'no_loss'``
+    spectral_model : str or function, optional
         Valid strings are:
 
         - ``'sapm'``
@@ -338,7 +338,7 @@ class ModelChain:
         - ``'no_loss'``
 
         The ModelChain instance will be passed as the first argument to
-        a user-defined function.
+        a user-defined function. If not specified, ``'no_loss'`` is assumed.
 
     temperature_model : str or function, optional
         Valid strings are: 'sapm', 'pvsyst', 'faiman', 'fuentes', 'noct_sam'.
@@ -364,7 +364,7 @@ def __init__(self, system, location,
                  solar_position_method='nrel_numpy',
                  airmass_model='kastenyoung1989',
                  dc_model=None, ac_model=None, aoi_model=None,
-                 spectral_model='no_loss', temperature_model=None,
+                 spectral_model=None, temperature_model=None,
                  dc_ohmic_model='no_loss',
                  losses_model='no_loss', name=None):
 

From d201f4a0c924cc60e6d4467af5b055934d7a9a23 Mon Sep 17 00:00:00 2001
From: Cliff Hansen <cwhanse@sandia.gov>
Date: Fri, 14 Mar 2025 08:41:11 -0600
Subject: [PATCH 16/16] remove reference to infer_spectral_model method

---
 docs/sphinx/source/reference/modelchain.rst | 1 -
 1 file changed, 1 deletion(-)

diff --git a/docs/sphinx/source/reference/modelchain.rst b/docs/sphinx/source/reference/modelchain.rst
index bcbb0006b9..89de825f0d 100644
--- a/docs/sphinx/source/reference/modelchain.rst
+++ b/docs/sphinx/source/reference/modelchain.rst
@@ -112,7 +112,6 @@ on the information in the associated :py:class:`~pvsystem.PVSystem` object.
    modelchain.ModelChain.infer_dc_model
    modelchain.ModelChain.infer_ac_model
    modelchain.ModelChain.infer_aoi_model
-   modelchain.ModelChain.infer_spectral_model
    modelchain.ModelChain.infer_temperature_model
    modelchain.ModelChain.infer_losses_model