Move range check to start of function. Expand tests to include negati…

…ve indicies.
da2746caf25d5f2571b8915cf86e1d5190e6ffd4
commit da2746caf25d5f2571b8915cf86e1d5190e6ffd4
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
@@ -863,6 +863,8 @@ which incur interpreter overhead.
        'Equivalent to list(combinations(iterable, r))[index]'
        pool = tuple(iterable)
        n = len(pool)
+       if r < 0 or r > n:
+           raise ValueError
        c = 1
        k = min(r, n-r)
        for i in range(1, k+1):
@@ -871,8 +873,6 @@ which incur interpreter overhead.
            index += c
        if index < 0 or index >= c:
            raise IndexError
-       if r < 0 or r > n:
-           raise ValueError
        result = []
        while r:
            c, n, r = c*r//n, n-1, r-1

diff --git a/Lib/test/test_itertools.py b/Lib/test/test_itertools.py
@@ -2266,6 +2266,8 @@ def test_permutations_sizeof(self):
 ...     'Equivalent to list(combinations(iterable, r))[index]'
 ...     pool = tuple(iterable)
 ...     n = len(pool)
+...     if r < 0 or r > n:
+...         raise ValueError
 ...     c = 1
 ...     k = min(r, n-r)
 ...     for i in range(1, k+1):
@@ -2274,8 +2276,6 @@ def test_permutations_sizeof(self):
 ...         index += c
 ...     if index < 0 or index >= c:
 ...         raise IndexError
-...     if r < 0 or r > n:
-...         raise ValueError
 ...     result = []
 ...     while r:
 ...         c, n, r = c*r//n, n-1, r-1
@@ -2371,9 +2371,12 @@ def test_permutations_sizeof(self):
 
 >>> population = 'ABCDEFGH'
 >>> for r in range(len(population) + 1):
-...     for i, expected in enumerate(combinations(population, r)):
-...         actual = nth_combination(population, r, i)
-...         assert expected == actual
+...     seq = list(combinations(population, r))
+...     for i in range(len(seq)):
+...         assert nth_combination(population, r, i) == seq[i]
+...     for i in range(-len(seq), 0):
+...         assert nth_combination(population, r, i) == seq[i]
+
 
 """