Solving issue with wcs.wcs.cunit equality

CHANGES.rst

@@ -332,6 +332,11 @@ astropy.visualization
 astropy.wcs
 ^^^^^^^^^^^
 
+- Added a ``PyUnitListProxy_richcmp`` method in ``UnitListProxy`` class to enable 
+  ``WCS.wcs.cunit`` equality testing. It helps to check whether the two instances of 
+  ``WCS.wcs.cunit`` are equal or not by comparing the data members of 
+  ``UnitListProxy`` class [#8480]
+
 Other Changes and Additions
 ---------------------------
 

astropy/wcs/src/unit_list_proxy.c

@@ -13,14 +13,15 @@
  ***************************************************************************/
 
 #define MAXSIZE 68
+#define ARRAYSIZE 72
 
 static PyTypeObject PyUnitListProxyType;
 
 typedef struct {
   PyObject_HEAD
   /*@null@*/ /*@shared@*/ PyObject* pyobject;
   Py_ssize_t size;
-  char (*array)[72];
+  char (*array)[ARRAYSIZE];
   PyObject* unit_class;
 } PyUnitListProxy;
 
@@ -94,7 +95,7 @@ PyUnitListProxy_clear(
 PyUnitListProxy_New(
     /*@shared@*/ PyObject* owner,
     Py_ssize_t size,
-    char (*array)[72]) {
+    char (*array)[ARRAYSIZE]) {
 
   PyUnitListProxy* self = NULL;
   PyObject *units_module;
@@ -186,6 +187,36 @@ PyUnitListProxy_getitem(
   return result;
 }
 
+static PyObject*
+PyUnitListProxy_richcmp(
+	PyObject *a,
+	PyObject *b,
+	int op){
+  PyUnitListProxy *lhs, *rhs;
+  assert(a != NULL && b != NULL);
+  if (!PyObject_TypeCheck(a, &PyUnitListProxyType) ||
+      !PyObject_TypeCheck(b, &PyUnitListProxyType)) {
+    Py_RETURN_NOTIMPLEMENTED;
+  }
+  if (op != Py_EQ && op != Py_NE) {
+    Py_RETURN_NOTIMPLEMENTED;
+  }
+  lhs = (PyUnitListProxy *)a;
+  rhs = (PyUnitListProxy *)b;
+  int equal = PyObject_RichCompareBool(lhs->unit_class, rhs->unit_class, Py_EQ);
+  if (equal == -1) {
+    return NULL;  // Exception will be set because the rich-compare failed
+  }
+  equal = equal == 1 && !strncmp(lhs->array, rhs->array, ARRAYSIZE) && lhs->size == rhs->size;
+  if ((op == Py_EQ && equal == 1) ||
+      (op == Py_NE && equal == 0)) {
+    Py_RETURN_TRUE;
+  } 
+  else {
+    Py_RETURN_FALSE;
+  }
+}
+
 static int
 PyUnitListProxy_setitem(
     PyUnitListProxy* self,
@@ -274,7 +305,7 @@ static PyTypeObject PyUnitListProxyType = {
   0,                          /* tp_doc */
   (traverseproc)PyUnitListProxy_traverse, /* tp_traverse */
   (inquiry)PyUnitListProxy_clear, /* tp_clear */
-  0,                          /* tp_richcompare */
+  (richcmpfunc)PyUnitListProxy_richcmp, /* tp_richcompare */
   0,                          /* tp_weaklistoffset */
   0,                          /* tp_iter */
   0,                          /* tp_iternext */
@@ -298,7 +329,7 @@ set_unit_list(
     const char* propname,
     PyObject* value,
     Py_ssize_t len,
-    char (*dest)[72]) {
+    char (*dest)[ARRAYSIZE]) {
 
   PyObject*  unit  = NULL;
   PyObject*  proxy = NULL;
@@ -362,3 +393,4 @@ _setup_unit_list_proxy_type(
 
   return 0;
 }
+

astropy/wcs/tests/test_wcs.py

@@ -1183,3 +1183,28 @@ def test_footprint_contains():
 
     hasCoord = test_wcs.footprint_contains(SkyCoord(24,2,unit='deg'))
     assert hasCoord == False
+
+
+def test_cunit():
+    # Initializing WCS
+    w1 = wcs.WCS(naxis=2)
+    w2 = wcs.WCS(naxis=2)
+    w3 = wcs.WCS(naxis=2)
+    # Initializing the values of cunit
+    w1.wcs.cunit = ['deg', 'm/s']
+    w2.wcs.cunit = ['km/h', 'km/h']
+    w3.wcs.cunit = ['deg', 'm/s']
+
+    # Equality checking a cunit with itself
+    assert w1.wcs.cunit == w1.wcs.cunit
+    # Equality checking of two different cunit object having same values
+    assert w1.wcs.cunit == w3.wcs.cunit
+    # Inequality checking of two different cunit object having different values
+    assert not w1.wcs.cunit == w2.wcs.cunit
+    # Inequality checking of cunit with a list of literals
+    assert not w1.wcs.cunit == [1, 2, 3]
+    # Inequality checking with some characters
+    assert w1.wcs.cunit != ['a', 'b', 'c']
+    # Comparison is not implemented TypeError will raise
+    with pytest.raises(TypeError):
+        w1.wcs.cunit < w2.wcs.cunit