add let().

test/test_case_test.py

@@ -1,8 +1,11 @@
+import itertools
+
 from testify import TestCase
 from testify import assert_equal
 from testify import class_setup
 from testify import class_setup_teardown
 from testify import class_teardown
+from testify import let
 from testify import run
 from testify import setup
 from testify import setup_teardown
@@ -245,5 +248,30 @@ def test_method_1(self):
     def test_method_2(self):
         pass
 
+class LetTest(TestCase):
+
+    @let
+    def counter(self):
+        return itertools.count(0)
+
+    def test_first_call_is_not_cached(self):
+        assert_equal(self.counter.next(), 0)
+
+    def test_subsequent_calls_are_cached(self):
+        assert_equal(self.counter.next(), 0)
+        assert_equal(self.counter.next(), 1)
+
+class LetWithLambdaTest(TestCase):
+
+    counter = let(lambda self: itertools.count(0))
+
+    def test_first_call_is_not_cached(self):
+        assert_equal(self.counter.next(), 0)
+
+    def test_subsequent_calls_are_cached(self):
+        assert_equal(self.counter.next(), 0)
+        assert_equal(self.counter.next(), 1)
+
+
 if __name__ == '__main__':
     run()

testify/__init__.py

@@ -42,7 +42,8 @@
                         class_teardown,
                         setup_teardown,
                         class_setup_teardown,
-                        suite)
+                        suite,
+                        let)
 
 from utils import turtle
 

testify/test_case.py

@@ -470,3 +470,33 @@ def fixture_method(func):
 
 setup_teardown = __fixture_decorator_factory('setup_teardown')
 class_setup_teardown = __fixture_decorator_factory('class_setup_teardown')
+
+class let(object):
+    """Decorator that creates a lazy-evaluated helper property. The value is
+    cached across multiple calls in the same test, but not across multiple
+    tests.
+    """
+
+    class _unsaved(object): pass
+
+    def __init__(self, func):
+        self._func = func
+        self._result = self._unsaved
+
+    def __get__(self, test_case, cls):
+        if test_case is None:
+            return self
+        if self._result is self._unsaved:
+            self._save_result(self._func(test_case))
+            self._register_reset_after_test_completion(test_case)
+        return self._result
+
+    def _save_result(self, result):
+        self._result = result
+
+    def _register_reset_after_test_completion(self, test_case):
+        test_case.register_callback(TestCase.EVENT_ON_COMPLETE_TEST_METHOD,
+                                    lambda _: self._reset_value())
+
+    def _reset_value(self):
+        self._result = self._unsaved