Normalize ListParameter to be Immutable (spotify#1759)

* Add normalize to ListParameter

* Add test for ListParameter normalization

Author: dlstadther

luigi/parameter.py

@@ -853,6 +853,15 @@ def run(self):
 
         $ luigi --module my_tasks MyTask --grades '[100,70]'
     """
+    def normalize(self, x):
+        """
+        Ensure that list parameter is converted to a tuple so it can be hashed.
+
+        :param str x: the value to parse.
+        :return: the normalized (hashable/immutable) value.
+        """
+        return tuple(x)
+
     def parse(self, x):
         """
         Parse an individual value from the input.

test/parameter_test.py

@@ -347,6 +347,13 @@ class Foo(luigi.Task):
         p = luigi.parameter.DictParameter()
         self.assertEqual(hash(Foo(args=dict(foo=1, bar="hello")).args), hash(p.parse('{"foo":1,"bar":"hello"}')))
 
+    def test_list(self):
+        class Foo(luigi.Task):
+            args = luigi.parameter.ListParameter()
+
+        p = luigi.parameter.ListParameter()
+        self.assertEqual(hash(Foo(args=[1, "hello"]).args), hash(p.normalize(p.parse('[1,"hello"]'))))
+
     def test_tuple(self):
         class Foo(luigi.Task):
             args = luigi.parameter.TupleParameter()