Added distinct_by transformation

Author: EntilZha

functional/chain.py

@@ -1094,6 +1094,16 @@ def distinct(self):
         """
         return self._transform(transformations.distinct_t())
 
+    def distinct_by(self, func):
+        """
+        Returns sequence of elements who are distinct by the passed function. The return
+        value of func must be hashable. When two elements are distinct by func, the first is taken.
+        :param func: function to use for determining distinctness
+        :return: elements distinct by func
+        :rtype FunctionalSequence
+        """
+        return self._transform(transformations.distinct_by_t(func))
+
     def slice(self, start, until):
         """
         Takes a slice of the sequence starting at start and until but not including until.

functional/transformations.py

@@ -53,6 +53,17 @@ def distinct(sequence):
     return Transformation('distinct', distinct, None)
 
 
+def distinct_by_t(func):
+    def distinct_by(sequence):
+        distinct_lookup = {}
+        for element in sequence:
+            key = func(element)
+            if key not in distinct_lookup:
+                distinct_lookup[key] = element
+        return distinct_lookup.values()
+    return Transformation('distinct_by({0})'.format(func.__name__), distinct_by, None)
+
+
 def sorted_t(key=None, reverse=False):
     return Transformation(
         'sorted',

test/test_functional.py

@@ -1,7 +1,10 @@
 import unittest
+from collections import namedtuple
 from functional.chain import seq, FunctionalSequence, _wrap, is_iterable
 
 
+Data = namedtuple('Data', 'x y')
+
 class TestChain(unittest.TestCase):
     def assert_type(self, s):
         self.assertTrue(isinstance(s, FunctionalSequence))
@@ -324,6 +327,13 @@ def test_distinct(self):
         self.assertEqual(result.size(), len(expect))
         self.assert_type(result)
 
+    def test_distinct_by(self):
+        s = seq(Data(1, 2), Data(1, 3), Data(2, 0), Data(3, -1), Data(1, 5))
+        expect = {Data(1, 2), Data(2, 0), Data(3, -1)}
+        result = s.distinct_by(lambda data: data.x)
+        self.assertSetEqual(set(result), expect)
+        self.assert_type(result)
+
     def test_slice(self):
         s = seq([1, 2, 3, 4])
         result = s.slice(1, 2)