DEV: Add zipline.utils.memoize.

- Moved zipline.utils.lazyval.
- Added `remember_last` which is just `lru_cache(1)` with simpler logic.

tests/modelling/test_engine.py

@@ -45,7 +45,7 @@ from zipline.modelling.factor.technical import (
     MaxDrawdown,
     SimpleMovingAverage,
 )
-from zipline.utils.lazyval import lazyval
+from zipline.utils.memoize import lazyval
 from zipline.utils.test_utils import (
     make_rotating_asset_info,
     make_simple_asset_info,

tests/test_doctests.py

@@ -9,7 +9,7 @@ from zipline.modelling import (
     expression,
 )
 from zipline.utils import (
-    lazyval,
+    memoize,
     test_utils,
 )
 
@@ -47,8 +47,8 @@ class DoctestTestCase(TestCase):
     def test_engine_docs(self):
         self._check_docs(engine)
 
-    def test_lazyval_docs(self):
-        self._check_docs(lazyval)
+    def test_memoize_docs(self):
+        self._check_docs(memoize)
 
     def test_test_utils_docs(self):
         self._check_docs(test_utils)

tests/test_memoize.py

@@ -0,0 +1,34 @@
+"""
+Tests for zipline.utils.memoize.
+"""
+from unittest import TestCase
+
+from zipline.utils.memoize import remember_last
+
+
+class TestRememberLast(TestCase):
+
+    def test_remember_last(self):
+
+        # Store the count in a list so we can mutate it from inside `func`.
+        call_count = [0]
+
+        @remember_last
+        def func(x):
+            call_count[0] += 1
+            return x
+
+        self.assertEqual((func(1), call_count[0]), (1, 1))
+
+        # Calling again with the same argument should just re-use the old
+        # value, which means func shouldn't get called again.
+        self.assertEqual((func(1), call_count[0]), (1, 1))
+        self.assertEqual((func(1), call_count[0]), (1, 1))
+
+        # Calling with a new value should increment the counter.
+        self.assertEqual((func(2), call_count[0]), (2, 2))
+        self.assertEqual((func(2), call_count[0]), (2, 2))
+
+        # Calling the old value should still increment the counter.
+        self.assertEqual((func(1), call_count[0]), (1, 3))
+        self.assertEqual((func(1), call_count[0]), (1, 3))