From 3aa38d411bf16d56bd7fdcf89a9a1bac34760564 Mon Sep 17 00:00:00 2001
From: "Marten H. van Kerkwijk" <mhvk@astro.utoronto.ca>
Date: Wed, 23 Oct 2024 12:30:38 -0400
Subject: [PATCH 1/2] TST: verify np.nan* functions give back correct class

---
 astropy/coordinates/tests/test_angles.py | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/astropy/coordinates/tests/test_angles.py b/astropy/coordinates/tests/test_angles.py
index 0ee290877bc9..0c5f17290df7 100644
--- a/astropy/coordinates/tests/test_angles.py
+++ b/astropy/coordinates/tests/test_angles.py
@@ -216,6 +216,28 @@ def test_angle_methods():
     assert a_min == 0.0 * u.degree
 
 
+def test_angle_nan_functions():
+    # Most numpy functions tested as part of the Quantity tests.
+    # But check that we drop to Quantity when appropriate; see
+    # https://github.com/astropy/astropy/pull/17221#discussion_r1813060768
+    a = Angle([0.0, 2.0, np.nan], "deg")
+    a_mean = np.nanmean(a)
+    assert type(a_mean) is Angle
+    assert a_mean == 1.0 * u.degree
+    a_std = np.nanstd(a)
+    assert type(a_std) is Angle
+    assert a_std == 1.0 * u.degree
+    a_var = np.nanvar(a)
+    assert type(a_var) is u.Quantity
+    assert a_var == 1.0 * u.degree**2
+    a_max = np.nanmax(a)
+    assert type(a_max) is Angle
+    assert a_max == 2.0 * u.degree
+    a_min = np.nanmin(a)
+    assert type(a_min) is Angle
+    assert a_min == 0.0 * u.degree
+
+
 def test_angle_convert():
     """
     Test unit conversion of Angle objects

From f3bcf5f60940e4a1d9b29c04c33a5cf67ba1413f Mon Sep 17 00:00:00 2001
From: "Marten H. van Kerkwijk" <mhvk@astro.utoronto.ca>
Date: Wed, 23 Oct 2024 12:32:01 -0400
Subject: [PATCH 2/2] BUG: ensure nanvar(angle) drops to quantity.

---
 astropy/units/quantity_helper/function_helpers.py | 15 +++++++++++++--
 docs/changes/coordinates/17239.bugfix.rst         |  2 ++
 2 files changed, 15 insertions(+), 2 deletions(-)
 create mode 100644 docs/changes/coordinates/17239.bugfix.rst

diff --git a/astropy/units/quantity_helper/function_helpers.py b/astropy/units/quantity_helper/function_helpers.py
index c515d28fc951..be464c573b07 100644
--- a/astropy/units/quantity_helper/function_helpers.py
+++ b/astropy/units/quantity_helper/function_helpers.py
@@ -97,8 +97,7 @@
     np.isposinf, np.isneginf, np.isreal, np.iscomplex,
     np.average, np.mean, np.std, np.var, np.trace,
     np.nanmax, np.nanmin, np.nanargmin, np.nanargmax, np.nanmean,
-    np.nansum, np.nancumsum, np.nanstd, np.nanvar,
-    np.nanprod, np.nancumprod,
+    np.nansum, np.nancumsum, np.nanprod, np.nancumprod,
     np.einsum_path, np.linspace,
     np.sort, np.partition, np.meshgrid,
     np.common_type, np.result_type, np.can_cast, np.min_scalar_type,
@@ -219,6 +218,7 @@ def __call__(self, f=None, helps=None, module=np):
         np.fft.fft2, np.fft.ifft2, np.fft.rfft2, np.fft.irfft2,
         np.fft.fftn, np.fft.ifftn, np.fft.rfftn, np.fft.irfftn,
         np.fft.hfft, np.fft.ihfft,
+        np.nanstd,  # See comment on nanvar helper.
         np.linalg.eigvals, np.linalg.eigvalsh,
     } | ({np.asfarray} if NUMPY_LT_2_0 else set())  # noqa: NPY201
 )  # fmt: skip
@@ -252,6 +252,17 @@ def like_helper(a, *args, **kwargs):
     return (a.view(np.ndarray),) + args, kwargs, unit, None
 
 
+# nanvar is safe for Quantity and was previously in SUBCLASS_FUNCTIONS, but it
+# is not safe for Angle, since the resulting unit is inconsistent with being
+# an Angle. By using FUNCTION_HELPERS, the unit gets passed through
+# _result_as_quantity, which will correctly drop to Quantity.
+# A side effect would be that np.nanstd then also produces Quantity; this
+# is avoided by it being helped by invariant_a_helpers above.
+@function_helper
+def nanvar(a, *args, **kwargs):
+    return (a.view(np.ndarray),) + args, kwargs, a.unit**2, None
+
+
 @function_helper
 def sinc(x):
     from astropy.units.si import radian
diff --git a/docs/changes/coordinates/17239.bugfix.rst b/docs/changes/coordinates/17239.bugfix.rst
new file mode 100644
index 000000000000..34f071f1f925
--- /dev/null
+++ b/docs/changes/coordinates/17239.bugfix.rst
@@ -0,0 +1,2 @@
+``np.nanvar(angle)`` now produces a ``Quantity`` with the correct
+unit, rather than raising an exception.