From 5382c86c1c3e7112975473441e521c98a235829f Mon Sep 17 00:00:00 2001
From: John Eskew <jeskew@edx.org>
Date: Mon, 16 Nov 2015 09:46:20 -0500
Subject: [PATCH] Protect against divide by zero in DataDog event. Add test for
 zero total/max score in a problem.

---
 common/lib/xmodule/xmodule/capa_base.py            |  9 +++++----
 .../lib/xmodule/xmodule/tests/test_capa_module.py  | 14 ++++++++++++++
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/common/lib/xmodule/xmodule/capa_base.py b/common/lib/xmodule/xmodule/capa_base.py
index 0ebdcea492..e491e72119 100644
--- a/common/lib/xmodule/xmodule/capa_base.py
+++ b/common/lib/xmodule/xmodule/capa_base.py
@@ -1117,10 +1117,11 @@ class CapaMixin(CapaFields):
 
         if dog_stats_api:
             dog_stats_api.increment(metric_name('checks'), tags=[u'result:success'])
-            dog_stats_api.histogram(
-                metric_name('correct_pct'),
-                float(published_grade['grade']) / published_grade['max_grade'],
-            )
+            if published_grade['max_grade'] != 0:
+                dog_stats_api.histogram(
+                    metric_name('correct_pct'),
+                    float(published_grade['grade']) / published_grade['max_grade'],
+                )
             dog_stats_api.histogram(
                 metric_name('attempts'),
                 self.attempts,
diff --git a/common/lib/xmodule/xmodule/tests/test_capa_module.py b/common/lib/xmodule/xmodule/tests/test_capa_module.py
index 354629cbd7..ad130868f8 100644
--- a/common/lib/xmodule/xmodule/tests/test_capa_module.py
+++ b/common/lib/xmodule/xmodule/tests/test_capa_module.py
@@ -731,6 +731,20 @@ class CapaModuleTest(unittest.TestCase):
         # Expect an AJAX alert message in 'success'
         self.assertIn(error_msg, result['success'])
 
+    def test_check_problem_zero_max_grade(self):
+        """
+        Test that a capa problem with a max grade of zero doesn't generate an error.
+        """
+        # Create the module
+        module = CapaFactory.create(attempts=1)
+
+        # Override the problem score to have a total of zero.
+        module.lcp.get_score = lambda: {'score': 0, 'total': 0}
+
+        # Check the problem
+        get_request_dict = {CapaFactory.input_key(): '3.14'}
+        module.check_problem(get_request_dict)
+
     def test_check_problem_error_nonascii(self):
 
         # Try each exception that capa_module should handle