Add ndindex() and promote_dtype() helper functions

The latter should probably be used in some more places in the dtype tests, but
for now I am not refactoring those. The conversion between dtype objects and
their string representation is finicky, and probably should be removed
entirely. It's not simple though, because the spec doesn't require dtype
objects to be usable as dict keys (and, e.g., NumPy does weird stuff like
float64 == None, which we have to be careful to guard against). Also we need
to make sure that the tests will work even if a dtype object is
_UndefinedStub.

Author: asmeurer

array_api_tests/array_helpers.py

@@ -1,3 +1,5 @@
+import itertools
+
 from ._array_module import (isnan, all, any, equal, not_equal, logical_and,
                             logical_or, isfinite, greater, less, less_equal,
                             zeros, ones, full, bool, int8, int16, int32,
@@ -22,7 +24,7 @@
            'assert_isinf', 'positive_mathematical_sign',
            'assert_positive_mathematical_sign', 'negative_mathematical_sign',
            'assert_negative_mathematical_sign', 'same_sign',
-           'assert_same_sign']
+           'assert_same_sign', 'ndindex', 'promote_dtypes']
 
 def zero(shape, dtype):
     """
@@ -333,3 +335,32 @@ def int_to_dtype(x, n, signed):
         if x & highest_bit:
             x = -((~x & mask) + 1)
     return x
+
+def ndindex(shape):
+    """
+    Iterator of n-D indices to an array
+
+    Yields tuples of integers to index every element of an array of shape
+    `shape`. Same as np.ndindex().
+
+    """
+    return itertools.product(*[range(i) for i in shape])
+
+def promote_dtypes(dtype1, dtype2):
+    """
+    Special case of result_type() which uses the exact type promotion table
+    from the spec.
+    """
+    from .test_type_promotion import dtype_mapping, promotion_table
+
+    # Equivalent to this, but some libraries may not work properly with using
+    # dtype objects as dict keys
+    #
+    # d1, d2 = reverse_dtype_mapping[dtype1], reverse_dtype_mapping[dtype2]
+
+    d1 = [i for i in dtype_mapping if dtype_mapping[i] == dtype1][0]
+    d2 = [i for i in dtype_mapping if dtype_mapping[i] == dtype2][0]
+
+    if (d1, d2) not in promotion_table:
+        raise ValueError(f"{d1} and {d2} are not type promotable according to the spec (this may indicate a bug in the test suite).")
+    return dtype_mapping[promotion_table[d1, d2]]