Fix handling of POW in Baron writer

pyomo/repn/plugins/baron_writer.py

@@ -46,14 +46,24 @@
 
 
 def _handle_PowExpression(visitor, node, values):
-    x,y = node.args
-    if type(x) not in native_types and not x.is_fixed() and \
-       type(y) not in native_types and not y.is_fixed():
-        # Per the BARON manual, x ^ y is allowed as long as x
-        # and y are not both variables
-        return f"exp(({values[0]}) * log({values[1]}))"
-    else:
+    # Per the BARON manual, x ^ y is allowed as long as x and y are not
+    # both variables.  There is an issue that if one of the arguments
+    # contains "0*var", Pyomo will see that as fixed, but Baron will see
+    # it as variable.  We will work around that by resolving any fixed
+    # expressions to their corresponding fixed value.
+    unfixed_count = 0
+    for i, arg in enumerate(node.args):
+        if type(arg) in native_types:
+            pass
+        elif arg.is_fixed():
+            values[i] = ftoa(value(arg))
+        else:
+            unfixed_count += 1
+
+    if unfixed_count < 2:
         return f"{values[0]} ^ {values[1]}"
+    else:
+        return f"exp(({values[0]}) * log({values[1]}))"
 
 _allowableUnaryFunctions = {
     'exp', 'log10', 'log', 'sqrt',

pyomo/repn/tests/baron/test_baron.py

@@ -17,13 +17,16 @@
 from io import StringIO
 
 import pyomo.common.unittest as unittest
-
+from pyomo.common.collections import OrderedSet
 from pyomo.common.fileutils import this_file_dir
+import pyomo.core.expr.current as EXPR
+from pyomo.core.base import SymbolMap
 
 from pyomo.environ import (
     ConcreteModel, Var, Param,  Constraint, Objective,  Block, sin,
     maximize, Binary, Suffix
 )
+from pyomo.repn.plugins.baron_writer import expression_to_string
 
 thisdir = this_file_dir()
 
@@ -216,6 +219,28 @@ def test_invalid_suffix(self):
             m.write(StringIO(), format='bar')
 
 
+class TestToBaronVisitor(unittest.TestCase):
+    def test_pow(self):
+        variables = OrderedSet()
+        smap = SymbolMap()
+
+        m = ConcreteModel()
+        m.x = Var(initialize=1)
+        m.y = Var(initialize=2)
+        m.p = Param(mutable=True, initialize=0)
+
+        e = m.x ** m.y
+        test = expression_to_string(e, variables, smap)
+        self.assertEqual(test, "exp((x) * log(y))")
+
+        e = m.x ** (3 + EXPR.ProductExpression((m.p, m.y)))
+        test = expression_to_string(e, variables, smap)
+        self.assertEqual(test, "x ^ 3")
+
+        e = (3 + EXPR.ProductExpression((m.p, m.y))) ** m.x
+        test = expression_to_string(e, variables, smap)
+        self.assertEqual(test, "3 ^ x")
+
 #class TestBaron_writer(unittest.TestCase):
 class XTestBaron_writer(object):
     """These tests verified that the BARON writer complained loudly for