Add alternate constructor for values

fuzz/values.py

@@ -34,3 +34,21 @@ def __init__(self, value, error=0):
             raise ValueError("error {} is negative".format(error))
         self._value = value
         self._error = error
+
+
+    @staticmethod
+    def create(value, error=0):
+        """This is a static method, and serves as an alternate constructor for
+        Values. It tries to convert some value to an actual Value, and if it
+        can't because it is the wrong type, it just sends the object back
+        unaltered.
+
+        :param value: The value to convert.
+        :param error: The error associated with the value.
+        :returns: Either the converted :py:class:`.value` or the original\
+        object."""
+
+        try:
+            return Value(value, error)
+        except TypeError:
+            return value

tests/test_values.py

@@ -36,3 +36,26 @@ def test_error_requires_numbers(self):
     def test_error_must_be_positive(self):
         with self.assertRaises(ValueError):
             Value(23, -0.01)
+
+
+
+class ValueSafeCreateTests(TestCase):
+
+    def test_can_get_simple_value_safely(self):
+        val = Value.create(10)
+        self.assertIsInstance(val, Value)
+        self.assertEqual(val._value, 10)
+        self.assertEqual(val._error, 0)
+
+
+    def test_can_get_error_value_safely(self):
+        val = Value.create(10.5, 0.2)
+        self.assertIsInstance(val, Value)
+        self.assertEqual(val._value, 10.5)
+        self.assertEqual(val._error, 0.2)
+
+
+    def test_can_ignore_non_numbers(self):
+        s = "value"
+        val = Value.create(s, 0.2)
+        self.assertIs(val, s)