From ed90c80ccab59fbbf1ae2d00b0844ade79f0d70d Mon Sep 17 00:00:00 2001
From: Peter Pinch <pdpinch@mit.edu>
Date: Wed, 31 Jan 2018 11:18:59 -0500
Subject: [PATCH] catch mismatched parens

---
 common/lib/calc/calc/calc.py            | 32 +++++++++++++++++++++++++
 common/lib/calc/calc/tests/test_calc.py |  9 +++++++
 common/lib/capa/capa/responsetypes.py   | 16 +++++++++++--
 3 files changed, 55 insertions(+), 2 deletions(-)

diff --git a/common/lib/calc/calc/calc.py b/common/lib/calc/calc/calc.py
index 9024b9fc6d..fc45f107bc 100644
--- a/common/lib/calc/calc/calc.py
+++ b/common/lib/calc/calc/calc.py
@@ -100,6 +100,13 @@ class UndefinedVariable(Exception):
     pass
 
 
+class UnmatchedParenthesis(Exception):
+    """
+    Indicate when a student inputs a formula with mismatched parentheses.
+    """
+    pass
+
+
 def lower_dict(input_dict):
     """
     Convert all keys in a dictionary to lowercase; keep their original values.
@@ -249,6 +256,7 @@ def evaluator(variables, functions, math_expr, case_sensitive=False):
         return float('nan')
 
     # Parse the tree.
+    check_parens(math_expr)
     math_interpreter = ParseAugmenter(math_expr, case_sensitive)
     math_interpreter.parse_algebra()
 
@@ -278,6 +286,30 @@ def evaluator(variables, functions, math_expr, case_sensitive=False):
     return math_interpreter.reduce_tree(evaluate_actions)
 
 
+def check_parens(formula):
+    """
+    Check that any open parentheses are closed
+
+    Otherwise, raise an UnmatchedParenthesis exception
+    """
+    count = 0
+    delta = {
+        '(': +1,
+        ')': -1
+    }
+    for index, char in enumerate(formula):
+        if char in delta:
+            count += delta[char]
+            if count < 0:
+                msg = "Invalid Input: A closing parenthesis was found after segment " + \
+                      "{}, but there is no matching opening parenthesis before it."
+                raise UnmatchedParenthesis(msg.format(formula[0:index]))
+    if count > 0:
+        msg = "Invalid Input: Parentheses are unmatched. " + \
+              "{} parentheses were opened but never closed."
+        raise UnmatchedParenthesis(msg.format(count))
+
+
 class ParseAugmenter(object):
     """
     Holds the data for a particular parse.
diff --git a/common/lib/calc/calc/tests/test_calc.py b/common/lib/calc/calc/tests/test_calc.py
index 2519bfc171..d8da18c93f 100644
--- a/common/lib/calc/calc/tests/test_calc.py
+++ b/common/lib/calc/calc/tests/test_calc.py
@@ -554,3 +554,12 @@ class EvaluatorTest(unittest.TestCase):
             calc.evaluator({'r1': 5}, {}, "r1+r2")
         with self.assertRaisesRegexp(calc.UndefinedVariable, 'r1 r3'):
             calc.evaluator(variables, {}, "r1*r3", case_sensitive=True)
+
+    def test_mismatched_parens(self):
+        """
+        Check to see if the evaluator catches mismatched parens
+        """
+        with self.assertRaisesRegexp(calc.UnmatchedParenthesis, 'opened but never closed'):
+            calc.evaluator({}, {}, "(1+2")
+        with self.assertRaisesRegexp(calc.UnmatchedParenthesis, 'no matching opening parenthesis'):
+            calc.evaluator({}, {}, "(1+2))")
diff --git a/common/lib/capa/capa/responsetypes.py b/common/lib/capa/capa/responsetypes.py
index 993b8dc0f2..4a5ce19692 100644
--- a/common/lib/capa/capa/responsetypes.py
+++ b/common/lib/capa/capa/responsetypes.py
@@ -40,7 +40,7 @@ import capa.safe_exec as safe_exec
 import capa.xqueue_interface as xqueue_interface
 import dogstats_wrapper as dog_stats_api
 # specific library imports
-from calc import UndefinedVariable, evaluator
+from calc import UndefinedVariable, UnmatchedParenthesis, evaluator
 from cmath import isnan
 from openedx.core.djangolib.markup import HTML, Text
 
@@ -1604,6 +1604,10 @@ class NumericalResponse(LoncapaResponse):
                     bad_variables=text_type(undef_var),
                 )
             )
+        except UnmatchedParenthesis as err:
+            raise StudentInputError(
+                err.args[0]
+            )
         except ValueError as val_err:
             if 'factorial' in text_type(val_err):
                 # This is thrown when fact() or factorial() is used in an answer
@@ -1770,7 +1774,7 @@ class NumericalResponse(LoncapaResponse):
         try:
             evaluator(dict(), dict(), answer)
             return True
-        except (StudentInputError, UndefinedVariable):
+        except (StudentInputError, UndefinedVariable, UnmatchedParenthesis):
             return False
 
     def get_answers(self):
@@ -3108,6 +3112,14 @@ class FormulaResponse(LoncapaResponse):
                 raise StudentInputError(
                     _("Invalid input: {bad_input} not permitted in answer.").format(bad_input=text_type(err))
                 )
+            except UnmatchedParenthesis as err:
+                log.debug(
+                    'formularesponse: unmatched parenthesis in formula=%s',
+                    cgi.escape(answer)
+                )
+                raise StudentInputError(
+                    err.args[0]
+                )
             except ValueError as err:
                 if 'factorial' in text_type(err):
                     # This is thrown when fact() or factorial() is used in a formularesponse answer