From e0ff4ddd89bde97deeedb461783d0479e4da9ce9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cl=C3=A9ment=20Robert?= <cr52@protonmail.com>
Date: Sun, 6 Aug 2023 20:05:23 +0200
Subject: [PATCH] BUG: fix converting a dimensionless array with
 unyt_array.to_astropy

---
 unyt/array.py                 |  6 +++++-
 unyt/tests/test_unyt_array.py | 11 +++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/unyt/array.py b/unyt/array.py
index ef06fb41..913850bf 100644
--- a/unyt/array.py
+++ b/unyt/array.py
@@ -1313,7 +1313,11 @@ def to_astropy(self, **kwargs):
         >>> data.to_astropy()
         <Quantity [3., 4., 5.] g / cm3>
         """
-        return self.value * _astropy.units.Unit(str(self.units), **kwargs)
+        if self.units.is_dimensionless:
+            s_units = ""
+        else:
+            s_units = str(self.units)
+        return self.value * _astropy.units.Unit(s_units, **kwargs)
 
     @classmethod
     def from_pint(cls, arr, unit_registry=None):
diff --git a/unyt/tests/test_unyt_array.py b/unyt/tests/test_unyt_array.py
index e61db46d..8ab0af9a 100644
--- a/unyt/tests/test_unyt_array.py
+++ b/unyt/tests/test_unyt_array.py
@@ -1402,6 +1402,17 @@ def test_astropy():
     assert_equal(yt_quan, unyt_quantity.from_astropy(yt_quan.to_astropy()))
 
 
+def test_astropy_dimensionless():
+    # see https://github.com/yt-project/unyt/issues/436
+    pytest.importorskip("astropy")
+
+    arr = unyt_array([1, 2, 3], "")
+    ap_arr = np.array([1, 2, 3]) * _astropy.units.Unit("")
+
+    assert_array_equal(ap_arr, arr.to_astropy())
+    assert_array_equal(arr, unyt_array.from_astropy(ap_arr))
+
+
 def test_pint():
     pytest.importorskip("pint")