error rather than warn in select for unevenly spaced axes

Also update to use the utils method for checking array spacing

src/pyuvdata/uvbeam/uvbeam.py

@@ -844,12 +844,12 @@ def check(
 
         # Check if the interpolation points are evenly-spaced
         if self.pixel_coordinate_system == "az_za":
-            for i, ax in enumerate((self.axis1_array, self.axis2_array)):
+            for i, ax_param in enumerate((self._axis1_array, self._axis2_array)):
+                ax = ax_param.value
                 if len(ax) < 3:
                     continue
 
-                diff = np.diff(ax)
-                if not np.allclose(diff, diff[0]):
+                if not utils.tools._test_array_constant_spacing(ax, tols=ax_param.tols):
                     raise ValueError(
                         f"axis{i+1}_array must be evenly spaced in az_za coordinates."
                     )
@@ -3159,18 +3159,17 @@ def select(
             axis1_inds = sorted(set(axis1_inds))
             if min(axis1_inds) < 0 or max(axis1_inds) > beam_object.Naxes1 - 1:
                 raise ValueError("axis1_inds must be > 0 and < Naxes1")
-            beam_object.Naxes1 = len(axis1_inds)
-            beam_object.axis1_array = beam_object.axis1_array[axis1_inds]
 
-            if beam_object.Naxes1 > 1 and not utils.tools._test_array_constant_spacing(
-                beam_object._axis1_array
+            if len(axis1_inds) > 1 and not utils.tools._test_array_constant_spacing(
+                beam_object.axis1_array[axis1_inds], tols=beam_object._axis1_array.tols
             ):
-                warnings.warn(
-                    "Selected values along first image axis are "
-                    "not evenly spaced. This is not supported by "
-                    "the regularly gridded beam fits format"
+                raise ValueError(
+                    "Selected values along first image axis must be evenly spaced."
                 )
 
+            beam_object.Naxes1 = len(axis1_inds)
+            beam_object.axis1_array = beam_object.axis1_array[axis1_inds]
+
             beam_object.data_array = beam_object.data_array[..., axis1_inds]
             if beam_object.beam_type == "efield":
                 beam_object.basis_vector_array = beam_object.basis_vector_array[
@@ -3192,18 +3191,17 @@ def select(
             axis2_inds = sorted(set(axis2_inds))
             if min(axis2_inds) < 0 or max(axis2_inds) > beam_object.Naxes2 - 1:
                 raise ValueError("axis2_inds must be > 0 and < Naxes2")
-            beam_object.Naxes2 = len(axis2_inds)
-            beam_object.axis2_array = beam_object.axis2_array[axis2_inds]
 
-            if beam_object.Naxes2 > 1 and not utils.tools._test_array_constant_spacing(
-                beam_object._axis2_array
+            if len(axis2_inds) > 1 and not utils.tools._test_array_constant_spacing(
+                beam_object.axis2_array[axis2_inds], tols=beam_object._axis2_array.tols
             ):
-                warnings.warn(
-                    "Selected values along second image axis are "
-                    "not evenly spaced. This is not supported by "
-                    "the regularly gridded beam fits format"
+                raise ValueError(
+                    "Selected values along second image axis must be evenly spaced."
                 )
 
+            beam_object.Naxes2 = len(axis2_inds)
+            beam_object.axis2_array = beam_object.axis2_array[axis2_inds]
+
             beam_object.data_array = beam_object.data_array[..., axis2_inds, :]
             if beam_object.beam_type == "efield":
                 beam_object.basis_vector_array = beam_object.basis_vector_array[

tests/uvbeam/test_uvbeam.py

@@ -1692,7 +1692,10 @@ def test_select_axis(cst_power_1freq, tmp_path):
 
     # check for warnings and errors associated with unevenly spaced image pixels
     power_beam2 = power_beam.copy()
-    with pytest.raises(ValueError, match="axis1_array must be evenly spaced"):
+    with pytest.raises(
+        ValueError,
+        match="Selected values along first image axis must be evenly spaced.",
+    ):
         power_beam2.select(axis1_inds=[0, 5, 6], inplace=False)
 
     # Test selecting on axis2
@@ -1726,7 +1729,10 @@ def test_select_axis(cst_power_1freq, tmp_path):
 
     # check for warnings and errors associated with unevenly spaced image pixels
     power_beam2 = power_beam.copy()
-    with pytest.raises(ValueError, match="axis2_array must be evenly spaced in az_za"):
+    with pytest.raises(
+        ValueError,
+        match="Selected values along second image axis must be evenly spaced.",
+    ):
         power_beam2.select(axis2_inds=[0, 5, 6])