Resolve #6: use standard Python semantics for negative indices

unpythonic/collections.py

@@ -654,6 +654,11 @@ def convert(k):
         if k is not None:
             if not isinstance(k, int):
                 raise TypeError("k must be int, got {} with value {}".format(type(k), k))
+            # Almost standard semantics for negative indices. Usually -l < k < l,
+            # but here we must allow for conversion of the end position, for
+            # which the last valid value is one past the end.
+            if not -l <= k <= l:
+                raise IndexError("Should have -n <= k <= n, but n = len(args) = {}, and k = {}".format(l, k))
             return apply_conversion(k) if k < 0 else k
     return convert
 

unpythonic/fun.py

@@ -273,6 +273,8 @@ def rotated(*args, **kwargs):
             n = len(args)
             if not n:
                 raise TypeError("Expected at least one argument")
+            if not -n < k < n:  # standard semantics for negative indices
+                raise IndexError("Should have -n < k < n, but n = len(args) = {}, and k = {}".format(n, k))
             j = -k % n
             rargs = args[-j:] + args[:-j]
             return lazycall(f, *rargs, **kwargs)
@@ -577,7 +579,9 @@ def apply_f_to_kth_arg(*args):
         n = len(args)
         if not n:
             raise TypeError("Expected at least one argument")
-        j = k % n  # handle also negative values
+        if not -n < k < n:  # standard semantics for negative indices
+            raise IndexError("Should have -n < k < n, but n = len(args) = {}, and k = {}".format(n, k))
+        j = k % n
         m = j + 1
         if n < m:
             raise TypeError("Expected at least {:d} arguments, got {:d}".format(m, n))