[RFC][caffe2] TaskGroup.__repr__ shouldn't have side effects

Summary: `__repr__` calling self.tasks() ends up marking the instance as "used", which doesn't seem appropriate. I was debugging a value being passed around and then ran into `Cannot add Task to an already used TaskGroup.` because the value had been logged once.

Test Plan:
Added a unit test -- didn't see a clean public method to test it, but I'm happy to add one if that makes sense.

Will wait for sandcastle to trigger everything else; I'm not at all familiar with this code so any other recommendations would be great!

Reviewed By: cryptopic

Differential Revision: D23541198

fbshipit-source-id: 5d1ec674a1ddaedf113140133b90e0da6afa7270

caffe2/python/task.py

@@ -1,10 +1,6 @@
 ## @package task
 # Module caffe2.python.task
 
-
-
-
-
 from caffe2.python import core, context
 from caffe2.python.schema import Field, from_blob_list
 from collections import defaultdict
@@ -354,7 +350,9 @@ class TaskGroup(object):
 
     def __repr__(self):
         return "TaskGroup(tasks={}, workspace_type={}, remote_nets={})".format(
-            self.tasks(), self.workspace_type(), self.remote_nets())
+            self._tasks + self._tasks_to_add,
+            self.workspace_type(),
+            self.remote_nets())
 
 
 class TaskOutput(object):

caffe2/python/task_test.py

@@ -1,8 +1,3 @@
-
-
-
-
-
 import unittest
 from caffe2.python import task
 
@@ -22,3 +17,8 @@ class TestTask(unittest.TestCase):
         ]
         for obj, want in cases:
             self.assertEqual(obj.__repr__(), want)
+
+    def testEffectlessRepr(self):
+        task_group = task.TaskGroup()
+        _repr = task_group.__repr__()
+        self.assertFalse(task_group._already_used)