Support constant expressions in EquationFactory.

Avoid errors for expressions that evaluate to numbers instead
of BaseBuilder instances.

diffpy/srfit/equation/builder.py

@@ -89,6 +89,7 @@
 _builders = {}
 
 
+import numbers
 import numpy
 
 import diffpy.srfit.equation.literals as literals
@@ -145,8 +146,13 @@ def makeEquation(self, eqstr, buildargs = True, argclass =
         """
         self._prepareBuilders(eqstr, buildargs, argclass, argkw)
         beq = eval(eqstr, {}, self.builders)
-        eq = beq.getEquation()
-        self.equations.add(eq)
+        # handle scalar numbers or numpy arrays
+        if isinstance(beq, (numbers.Number, numpy.ndarray)):
+            lit = literals.Argument(value=beq, const=True)
+            eq = Equation(name='', root=lit)
+        else:
+            eq = beq.getEquation()
+            self.equations.add(eq)
         return eq
 
     def registerConstant(self, name, value):

diffpy/srfit/tests/testbuilder.py

@@ -201,6 +201,19 @@ def testParseEquation(self):
         self.assertTrue(noObserversInGlobalBuilders())
         return
 
+
+    def test_parse_constant(self):
+        """Verify parsing of constant numeric expressions.
+        """
+        factory = builder.EquationFactory()
+        eq = factory.makeEquation('3.12 + 2')
+        self.assertTrue(isinstance(eq, builder.Equation))
+        self.assertEqual(set(), factory.equations)
+        self.assertEqual(5.12, eq())
+        self.assertRaises(ValueError, eq, 3)
+        return
+
+
     def testBuildEquation(self):
 
         from numpy import array_equal