From e2add9442071d49e60de479e85268e05eba78075 Mon Sep 17 00:00:00 2001
From: Will Vining <wfvining@gmail.com>
Date: Mon, 10 May 2021 09:29:14 -0600
Subject: [PATCH 1/4] Raise ValueError if PVSystem constructed with 0 Arrays

Provides a useful error message directing users to the solution
they are most likely to want (construct a PVSystem with an
arbitrary default Array).
---
 pvlib/pvsystem.py            | 8 +++++++-
 pvlib/tests/test_pvsystem.py | 6 ++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 6894facc4a..c366b1f977 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -98,7 +98,8 @@ class PVSystem:
     arrays : iterable of Array, optional
         List of arrays that are part of the system. If not specified
         a single array is created from the other parameters (e.g.
-        `surface_tilt`, `surface_azimuth`). If `arrays` is specified
+        `surface_tilt`, `surface_azimuth`). Must contain at least one Array,
+        if empty a ValueError is raised. If `arrays` is specified
         the following parameters are ignored:
 
         - `surface_tilt`
@@ -210,6 +211,11 @@ def __init__(self,
                 racking_model,
                 array_losses_parameters,
             ),)
+        elif len(arrays) == 0:
+            raise ValueError("PVSystem must have at least one Array. "
+                             "If you want to create a PVSystem instance "
+                             "with a single arbitrary default Array pass "
+                             "`arrays=None`.")
         else:
             self.arrays = tuple(arrays)
 
diff --git a/pvlib/tests/test_pvsystem.py b/pvlib/tests/test_pvsystem.py
index 0a4125dd5b..2c2a64d672 100644
--- a/pvlib/tests/test_pvsystem.py
+++ b/pvlib/tests/test_pvsystem.py
@@ -2084,6 +2084,12 @@ def test_PVSystem_num_arrays():
     assert system_two.num_arrays == 2
 
 
+def test_PVSystem_at_least_one_array():
+    with pytest.raises(ValueError,
+                       match="PVSystem must have at least one Array"):
+        pvsystem.PVSystem(arrays=[])
+
+
 def test_combine_loss_factors():
     test_index = pd.date_range(start='1990/01/01T12:00', periods=365, freq='D')
     loss_1 = pd.Series(.10, index=test_index)

From 6fca2aefbd14b1b8c61226c90d12bb73b3c121b9 Mon Sep 17 00:00:00 2001
From: Will Vining <wfvining@gmail.com>
Date: Mon, 17 May 2021 07:03:37 -0600
Subject: [PATCH 2/4] Reword description of `arrays` param to PVSystem

---
 pvlib/pvsystem.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index c366b1f977..53c3479ba6 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -99,8 +99,8 @@ class PVSystem:
         List of arrays that are part of the system. If not specified
         a single array is created from the other parameters (e.g.
         `surface_tilt`, `surface_azimuth`). Must contain at least one Array,
-        if empty a ValueError is raised. If `arrays` is specified
-        the following parameters are ignored:
+        if length of arrays is 0 a ValueError is raised. If `arrays` is
+        specified the following parameters are ignored:
 
         - `surface_tilt`
         - `surface_azimuth`

From 9ffc3aa0266bef334be0d776c83ce4edf09ffd2d Mon Sep 17 00:00:00 2001
From: Will Vining <wfvining@gmail.com>
Date: Mon, 17 May 2021 07:05:47 -0600
Subject: [PATCH 3/4] Reword ValueError raised when arrays is empty

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

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 53c3479ba6..ac47a294fd 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -214,8 +214,9 @@ def __init__(self,
         elif len(arrays) == 0:
             raise ValueError("PVSystem must have at least one Array. "
                              "If you want to create a PVSystem instance "
-                             "with a single arbitrary default Array pass "
-                             "`arrays=None`.")
+                             "with a single Array pass `arrays=None` and pass "
+                             "values directly to PVSystem attributes, e.g., "
+                             "`surface_tilt=30`")
         else:
             self.arrays = tuple(arrays)
 

From 18458944cf5fef6c0972ab87d3341d34ff8ee7fc Mon Sep 17 00:00:00 2001
From: Will Vining <wfvining@gmail.com>
Date: Mon, 17 May 2021 07:23:00 -0600
Subject: [PATCH 4/4] Add Raises section to PVSystem docstring

---
 pvlib/pvsystem.py | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index ac47a294fd..edd1149244 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -174,6 +174,11 @@ class PVSystem:
         Arbitrary keyword arguments.
         Included for compatibility, but not used.
 
+    Raises
+    ------
+    ValueError
+        If `arrays` is not None and has length 0.
+
     See also
     --------
     pvlib.location.Location