divisor: implement slow version

riemann/divisor.py

@@ -1,4 +1,7 @@
 '''Compute the sum of divisors of a number.'''
 
 def divisor_sum(n: int) -> int:
-    raise ValueError("Not implemented.")
+    if n <= 0:
+        raise ValueError("Non-positive numbers are not supported.")
+
+    return sum(i for i in range(1, n+1) if n%i == 0)

test/divisor_test.py

@@ -1,4 +1,19 @@
+import pytest
 from riemann.divisor import divisor_sum
 
+def test_sum_of_divisors_of_1():
+    assert 1 == divisor_sum(1)
+
 def test_sum_of_divisors_of_72():
     assert 195 == divisor_sum(72)
+
+def test_illegal_input_0():
+    with pytest.raises(ValueError):
+        divisor_sum(0)
+
+def test_illegal_input_0():
+    with pytest.raises(ValueError):
+        divisor_sum(0)
+def test_illegal_input_negative():
+    with pytest.raises(ValueError):
+        divisor_sum(-5)