Raise ValueError for tuple arguments to Tuple instead of NotImplement…

…edError

These only work in NumPy as fancy indices. However, there is too much of a
risk of API confusion in thinking that Tuple() works like Tuple((1, 2, 3))
instead of Tuple(1, 2, 3). So we make this an unconditional error with helpful
error messages. Fancy indices should use either a list or array.

c.f. issue Quansight-Labs#17.

ndindex/tests/test_tuple.py

@@ -13,6 +13,12 @@
 from .helpers import check_same, Tuples, prod, shapes, iterslice, ndindices
 
 
+def test_tuple_constructor():
+    # Test things in the Tuple constructor that are not tested by the other
+    # tests below.
+    raises(ValueError, lambda: Tuple((1, 2, 3)))
+    raises(ValueError, lambda: Tuple(0, (1, 2, 3)))
+
 def test_tuple_exhaustive():
     # Exhaustive tests here have to be very limited because of combinatorial
     # explosion.

ndindex/tuple.py

@@ -48,7 +48,9 @@ def _typecheck(self, *args):
         for arg in args:
             newarg = ndindex(arg)
             if isinstance(newarg, Tuple):
-                raise NotImplementedError("tuples of tuples are not yet supported")
+                if len(args) == 1:
+                    raise ValueError("tuples inside of tuple indices are not supported. Did you mean to call Tuple(*args) instead of Tuple(args)?")
+                raise ValueError("tuples inside of tuple indices are not supported. If you meant to use a fancy index, use a list or array instead.")
             newargs.append(newarg)
 
         if newargs.count(ellipsis()) > 1: