Actually implement exact vs factors-don't-matter comparisons in chemcalc

common/lib/capa/capa/chem/chemcalc.py

@@ -312,12 +312,14 @@ def divide_chemical_expression(s1, s2, ignore_state=False):
         return Fraction(treedic['1 factors'][0] / treedic['2 factors'][0])
 
 
-def chemical_equations_equal(eq1, eq2, ignoreFactor=True):
+def chemical_equations_equal(eq1, eq2, exact=False):
     """
-    Check whether two chemical equations are the same.  If ignoreFactor is True,
-    then they are considered equal if they differ by a constant factor.
+    Check whether two chemical equations are the same.
 
-    arrows matter: ->, and <-> are different.
+    If exact is False, then they are considered equal if they differ by a
+    constant factor.
+
+    arrows matter: -> and <-> are different.
 
     e.g.
     chemical_equations_equal('H2 + O2 -> H2O2', 'O2 + H2 -> H2O2') -> True
@@ -325,6 +327,10 @@ def chemical_equations_equal(eq1, eq2, ignoreFactor=True):
 
     chemical_equations_equal('H2 + O2 -> H2O2', 'O2 + H2 <-> H2O2') -> False
 
+    chemical_equations_equal('H2 + O2 -> H2O2', '2 H2 + 2 O2 -> 2 H2O2') -> True
+    chemical_equations_equal('H2 + O2 -> H2O2', '2 H2 + 2 O2 -> 2 H2O2', exact=True) -> False
+
+
     If there's a syntax error, we raise pyparsing.ParseException.
     """
     # for now, we do a manual parse for the arrow.
@@ -359,4 +365,8 @@ def chemical_equations_equal(eq1, eq2, ignoreFactor=True):
         # factors don't match (molecule counts to add up)
         return False
 
+    if exact and factor_left != 1:
+        # want an exact match.
+        return False
+
     return True

common/lib/capa/capa/chem/tests.py

@@ -44,6 +44,17 @@ class Test_Compare_Equations(unittest.TestCase):
         self.assertFalse(chemical_equations_equal('H2 + O2 -> H2O2',
                                                   'O2 + H2 <-> 2H2O2'))
 
+    def test_exact_match(self):
+        self.assertTrue(chemical_equations_equal('H2 + O2 -> H2O2',
+                                                 '2O2 + 2H2 -> 2H2O2'))
+
+        self.assertFalse(chemical_equations_equal('H2 + O2 -> H2O2',
+                                                 '2O2 + 2H2 -> 2H2O2', exact=True))
+
+        # order still doesn't matter
+        self.assertTrue(chemical_equations_equal('H2 + O2 -> H2O2',
+                                                 'O2 + H2 -> H2O2', exact=True))
+
         
     def test_syntax_errors(self):
         self.assertRaises(ParseException, chemical_equations_equal,