Distinct now maintains order (EntilZha#151)

* Distinct now maintains order.

* Tests now pass Black

* making distinct more explicit and lazy

functional/test/test_functional.py

@@ -413,14 +413,17 @@ def test_reverse(self):
         self.assert_type(result)
 
     def test_distinct(self):
-        l = [1, 1, 2, 3, 2, 3]
-        expect = [1, 2, 3]
+        l = [1, 3, 1, 2, 2, 3]
+        expect = [1, 3, 2]
         s = self.seq(l)
         result = s.distinct()
+        self.assertEqual(result.size(), len(expect))
+        for er in zip(expect, result):
+            self.assertEqual(
+                er[0], er[1], "Order was not preserved after running distinct!"
+            )
         for e in result:
             self.assertTrue(e in expect)
-        result = s.distinct()
-        self.assertEqual(result.size(), len(expect))
         self.assert_type(result)
 
     def test_distinct_by(self):

functional/transformations.py

@@ -143,7 +143,12 @@ def distinct_t():
     """
 
     def distinct(sequence):
-        return iter(set(sequence))
+        seen = set()
+        for element in sequence:
+            if element in seen:
+                continue
+            seen.add(element)
+            yield element
 
     return Transformation("distinct", distinct, None)