sync from Atlas repo

operator/interest.py

@@ -0,0 +1,25 @@
+"""
+Compound interest function with ``decimal.Decimal``
+
+"""
+
+def compound_interest(principal, rate, periods):
+    return principal * ((1 + rate) ** periods - 1)
+
+def test(verbose=False):
+    from decimal import Decimal, getcontext
+    getcontext().prec = 8
+    fixture = [(1000, Decimal('0.05'), 1, Decimal('50')),
+               (1000, Decimal('0.10'), 5, Decimal('610.51')),
+               (1000, Decimal('0.10'), 15, Decimal('3177.2482')),
+               (1000, Decimal('0.06'), 5, Decimal('338.2256')),
+              ]
+    for principal, rate, periods, future_value in fixture:
+        computed = compound_interest(principal, rate, periods)
+        if verbose:
+            print('{!r}, {!r}, {!r} -> {!r}'.format(
+                  principal, rate, periods, computed))
+        assert future_value == computed, '{!r} != {!r}'.format(future_value, computed)
+
+if __name__ == '__main__':
+    test(True)