Fix doubled comma in CRTF output when range/corr is the only metadata

CHANGES.rst

@@ -10,6 +10,9 @@ New Features
 Bug Fixes
 ---------
 
+- Fixed a doubled comma in the serialized CRTF output when ``range`` or
+  ``corr`` is the only metadata of a region. [#700]
+
 API Changes
 -----------
 

regions/io/crtf/io_core.py

@@ -183,7 +183,6 @@ def to_crtf(self, coordsys='fk5', fmt='.6f', radunit='deg'):
             for key, val in shape.meta.items():
                 if key not in keylist:
                     meta_pairs.append(f'{key}={val}')
-            meta_str = ', '.join(meta_pairs)
 
             # The first item should be the coordinates, since CASA
             # cannot recognize a region without an inline coordinate
@@ -192,22 +191,23 @@ def to_crtf(self, coordsys='fk5', fmt='.6f', radunit='deg'):
             shape_coordsys = shape.coordsys
             if shape_coordsys.lower() != coordsys.lower():
                 coord = coordsys_mapping['CRTF'][coordsys.lower()]
-                if meta_str.strip():
-                    meta_str = f'coord={coord}, ' + meta_str
-                else:
-                    # if there is no metadata at all (above), the
-                    # trailing comma is incorrect
-                    meta_str = f'coord={coord}'
+                meta_pairs.insert(0, f'coord={coord}')
 
             if 'comment' in shape.meta:
-                meta_str += ', ' + shape.meta['comment']
+                meta_pairs.append(shape.meta['comment'])
 
             if 'range' in shape.meta:
                 shape.meta['range'] = [str(str(x).replace(' ', '')) for x in
                                        shape.meta['range']]
-                meta_str += f", range={shape.meta['range']}".replace("'", '')
+                meta_pairs.append(
+                    f"range={shape.meta['range']}".replace("'", ''))
             if 'corr' in shape.meta:
-                meta_str += f", corr={shape.meta['corr']}".replace("'", '')
+                meta_pairs.append(
+                    f"corr={shape.meta['corr']}".replace("'", ''))
+
+            # join only once so that an empty item cannot leave a
+            # doubled comma in the output (issue #322)
+            meta_str = ', '.join(meta_pairs)
 
             coord = []
             if coordsys not in ['image', 'physical']:

regions/io/crtf/tests/test_crtf.py

@@ -11,7 +11,7 @@
 from astropy.tests.helper import assert_quantity_allclose
 from astropy.utils.data import get_pkg_data_filename
 
-from regions.core import Regions
+from regions.core import RegionMeta, Regions
 from regions.io.crtf.core import CRTFRegionParserError
 from regions.io.crtf.read import _CRTFParser
 from regions.shapes.circle import CircleSkyRegion
@@ -124,6 +124,25 @@ def test_issue_312_regression():
     assert crtfstr.strip()[-1] != ','
 
 
+@pytest.mark.parametrize(('meta', 'expected'),
+                         [({'corr': ['XX']}, 'deg], corr=[XX]'),
+                          ({'range': ['1000MHz', '2000MHz']},
+                           'deg], range=[1000MHz, 2000MHz]')])
+def test_issue_322_regression(meta, expected):
+    """
+    Make sure there is no doubled comma when writing a CRTF string
+    where range or corr is the only metadata.
+    """
+    reg = EllipseSkyRegion(center=SkyCoord(279.486483 * u.deg,
+                                           -20.683327 * u.deg, frame='fk5'),
+                           width=0.009218 * u.deg, height=0.005954 * u.deg,
+                           angle=100.410911 * u.deg, meta=RegionMeta(meta))
+    crtfstr = reg.serialize(format='crtf', coordsys='fk5', fmt='.6f',
+                            radunit='deg')
+    assert ', ,' not in crtfstr
+    assert expected in crtfstr
+
+
 @pytest.mark.parametrize(('filename', 'outname', 'coordsys', 'fmt'),
                          [('data/CRTFgeneral.crtf',
                            'data/CRTFgeneraloutput.crtf',