Cython enums for null_equality and nan_equality redeclare 'UNEQUAL', raising a warning during builds.

[bdice]

The Cython enums null_equality and nan_equality both have an enum value named UNEQUAL.

cudf/python/cudf/cudf/_lib/cpp/types.pxd

Lines 45 to 51 in 74763ba

	ctypedef enum null_equality "cudf::null_equality":
	EQUAL "cudf::null_equality::EQUAL"
	UNEQUAL "cudf::null_equality::UNEQUAL"
	ctypedef enum nan_equality "cudf::nan_equality":
	ALL_EQUAL "cudf::nan_equality::ALL_EQUAL"
	UNEQUAL "cudf::nan_equality::UNEQUAL"

When building the cuDF Python code, I see an error message repeated many times:

warning: cudf/_lib/cpp/types.pxd:51:8: 'UNEQUAL' redeclared

This issue of repeated enum names has been previously discussed on StackOverflow: https://stackoverflow.com/questions/50653853/duplicate-enum-member-names-in-cython-redeclaration-error

Looking at the compiled C++, I see that this Cython code...

cudf/python/cudf/cudf/_lib/lists.pyx

Lines 90 to 95 in 74763ba

	cdef null_equality c_nulls_equal = (
	null_equality.EQUAL if nulls_equal else null_equality.UNEQUAL
	)
	cdef nan_equality c_nans_equal = (
	nan_equality.ALL_EQUAL if nans_all_equal else nan_equality.UNEQUAL
	)

... translates into this C++, which seems fine.

  /* "cudf/_lib/lists.pyx":91
 *     )
 *     cdef null_equality c_nulls_equal = (
 *         null_equality.EQUAL if nulls_equal else null_equality.UNEQUAL             # <<<<<<<<<<<<<<
 *     )
 *     cdef nan_equality c_nans_equal = (
 */
  if ((__pyx_v_nulls_equal != 0)) {
    __pyx_t_3 = cudf::null_equality::EQUAL;
  } else {
    __pyx_t_3 = cudf::null_equality::UNEQUAL;
  }
  __pyx_v_c_nulls_equal = __pyx_t_3;
  /* "cudf/_lib/lists.pyx":94
 *     )
 *     cdef nan_equality c_nans_equal = (
 *         nan_equality.ALL_EQUAL if nans_all_equal else nan_equality.UNEQUAL             # <<<<<<<<<<<<<<
 *     )
 *
 */
  if ((__pyx_v_nans_all_equal != 0)) {
    __pyx_t_4 = cudf::nan_equality::ALL_EQUAL;
  } else {
    __pyx_t_4 = cudf::nan_equality::UNEQUAL;
  }
  __pyx_v_c_nans_equal = __pyx_t_4;

Therefore, the issue that the warning is trying to alert us about is not a problem because of our use of explicit C names in the pxd file, like "cudf::nan_equality::UNEQUAL".

Discussing with @shwina, we identified two paths forward to remove this warning:

1. Wait for Cython 3.0, and start using enum class which does not have this scoping issue.
2. Prefix the enum's Cython names to something like NAN_ALL_EQUAL, NAN_UNEQUAL.

For now, @shwina and I agreed that we should take no action and wait for Cython 3.0 (option 1).

[vyasr]

Yep I think we've discussed this before and come to the conclusion that waiting for scoped enum support in Cython is fine unless this conflict actually starts causing problems.

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