NLv2: fix 0 operands in pow expressions

pyomo/repn/plugins/nl_writer.py

@@ -1649,7 +1649,7 @@ def compile_repn(self, visitor, prefix='', args=None, named_exprs=None):
         elif nterms == 1:
             return prefix + nl_sum, args, named_exprs
         else: # nterms == 0
-            return prefix + (template.const % 0), [], named_exprs
+            return prefix + (template.const % 0), args, named_exprs
 
     def compile_nonlinear_fragment(self, visitor):
         if not self.nonlinear:
@@ -1924,8 +1924,13 @@ def handle_division_node(visitor, node, arg1, arg2):
     return (_GENERAL, AMPLRepn(0, None, nonlin))
 
 def handle_pow_node(visitor, node, arg1, arg2):
-    if arg1[0] is _CONSTANT and arg2[0] is _CONSTANT:
-        return _apply_node_operation(node, (arg1[1], arg2[1]))
+    if arg2[0] is _CONSTANT:
+        if arg1[0] is _CONSTANT:
+            return _apply_node_operation(node, (arg1[1], arg2[1]))
+        elif not arg2[1]:
+            return _CONSTANT, 1
+        elif arg2[1] == 1:
+            return arg1
     nonlin = node_result_to_amplrepn(arg1).compile_repn(
         visitor, visitor.template.pow)
     nonlin = node_result_to_amplrepn(arg2).compile_repn(visitor, *nonlin)

pyomo/repn/tests/ampl/test_nlv2.py

@@ -213,6 +213,43 @@ def test_errors_divide_by_0(self):
         self.assertEqual(repn.linear, {})
         self.assertEqual(repn.nonlinear, None)
 
+    def test_pow(self):
+        m = ConcreteModel()
+        m.p = Param(mutable=True, initialize=2)
+        m.x = Var()
+
+        info = INFO()
+        with LoggingIntercept() as LOG:
+            repn = info.visitor.walk_expression((m.x**m.p, None, None))
+        self.assertEqual(LOG.getvalue(), "")
+        self.assertEqual(repn.nl, None)
+        self.assertEqual(repn.mult, 1)
+        self.assertEqual(repn.const, 0)
+        self.assertEqual(repn.linear, {})
+        self.assertEqual(repn.nonlinear, ('o5\nv%s\nn2\n', [id(m.x)]))
+
+        m.p = 1
+        info = INFO()
+        with LoggingIntercept() as LOG:
+            repn = info.visitor.walk_expression((m.x**m.p, None, None))
+        self.assertEqual(LOG.getvalue(), "")
+        self.assertEqual(repn.nl, None)
+        self.assertEqual(repn.mult, 1)
+        self.assertEqual(repn.const, 0)
+        self.assertEqual(repn.linear, {id(m.x): 1})
+        self.assertEqual(repn.nonlinear, None)
+
+        m.p = 0
+        info = INFO()
+        with LoggingIntercept() as LOG:
+            repn = info.visitor.walk_expression((m.x**m.p, None, None))
+        self.assertEqual(LOG.getvalue(), "")
+        self.assertEqual(repn.nl, None)
+        self.assertEqual(repn.mult, 1)
+        self.assertEqual(repn.const, 1)
+        self.assertEqual(repn.linear, {})
+        self.assertEqual(repn.nonlinear, None)
+
     def test_errors_divide_by_0_mult_by_0(self):
         # Note: we may elect to deprecate this functionality in the future
         #
@@ -662,6 +699,31 @@ class CustomExpression(ScalarExpression):
         self.assertEqual(repn.nonlinear, None)
         self.assertEqual(info, [None, None, False])
 
+    def test_nested_operator_zero_arg(self):
+        # This tests an error encountered when developing the nlv2
+        # writer where var ids were being dropped then the second
+        # argument in a binary operator was 0.  The original case was
+        # for expr**p where p was a variable fixed to 0.  However, since
+        # then, _handle_pow_operator contains special handling for **0
+        # and **1.
+        m = ConcreteModel()
+        m.x = Var()
+        m.p = Param(initialize=0, mutable=True)
+        expr = (1/m.x) == m.p
+
+        info = INFO()
+        with LoggingIntercept() as LOG:
+            repn = info.visitor.walk_expression((expr, None, None))
+        self.assertEqual(LOG.getvalue(), "")
+        self.assertEqual(repn.nl, None)
+        self.assertEqual(repn.mult, 1)
+        self.assertEqual(repn.const, 0)
+        self.assertEqual(repn.linear, {})
+        self.assertEqual(
+            repn.nonlinear,
+            ('o24\no3\nn1\nv%s\nn0\n', [id(m.x)])
+        )
+
 class Test_NLWriter(unittest.TestCase):
     def test_external_function_str_args(self):
         m = ConcreteModel()