Make numeric response handle plus sign exponential correctly

NumericResponse was failing to accept as correct an answer with a plus
sign in the exponential. For example: 5.0e+1 was marked as incorrect
if the answer was 50. However, 5.0e1 was marked as correct. This
caused confusion, since the answer shown to students included the plus
sign.

LMS Lighthouse [#242]

common/lib/capa/capa/calc.py

@@ -183,7 +183,7 @@ def evaluator(variables, functions, string, cs=False):
 
     # 0.33k or -17
     number = (Optional(minus | plus) + inner_number
-              + Optional(CaselessLiteral("E") + Optional("-") + number_part)
+              + Optional(CaselessLiteral("E") + Optional((plus | minus)) + number_part)
               + Optional(number_suffix))
     number = number.setParseAction(number_parse_action)  # Convert to number
 

common/lib/capa/capa/tests/test_responsetypes.py

@@ -642,6 +642,15 @@ class NumericalResponseTest(ResponseTest):
         incorrect_responses = ["", "2.11", "1.89", "0"]
         self.assert_multiple_grade(problem, correct_responses, incorrect_responses)
 
+    def test_exponential_answer(self):
+        problem = self.build_problem(question_text="What 5 * 10?",
+                                     explanation="The answer is 50",
+                                     answer="5e+1")
+        correct_responses = ["50", "50.0", "5e1", "5e+1", "50e0", "500e-1"]
+        incorrect_responses = ["", "3.9", "4.1", "0", "5.01e1"]
+        self.assert_multiple_grade(problem, correct_responses, incorrect_responses)
+
+
 
 class CustomResponseTest(ResponseTest):
     from response_xml_factory import CustomResponseXMLFactory