Merge pull request Pyomo#3410 from jsiirola/xpress-import

Fix error when xpress is imported before xpress_direct

pyomo/solvers/plugins/solvers/xpress_direct.py

@@ -197,21 +197,6 @@ def __call__(self):
         return xpress
 
 
-_xpress_importer = _xpress_importer_class()
-xpress, xpress_available = attempt_import(
-    'xpress',
-    error_message=_xpress_importer,
-    # Other forms of exceptions can be thrown by the xpress python
-    # import.  For example, an xpress.InterfaceError exception is thrown
-    # if the Xpress license is not valid.  Unfortunately, you can't
-    # import without a license, which means we can't test for that
-    # explicit exception!
-    catch_exceptions=(Exception,),
-    importer=_xpress_importer,
-    callback=_finalize_xpress_import,
-)
-
-
 @SolverFactory.register('xpress_direct', doc='Direct python interface to XPRESS')
 class XpressDirect(DirectSolver):
     _name = None
@@ -1222,3 +1207,21 @@ def load_slacks(self, cons_to_load=None):
 
         """
         self._load_slacks(cons_to_load)
+
+
+# Note: because _finalize_xpress_import references XpressDirect, we need
+# to make sure to not attempt the xpress import until after the
+# XpressDirect class is fully declared.
+_xpress_importer = _xpress_importer_class()
+xpress, xpress_available = attempt_import(
+    'xpress',
+    error_message=_xpress_importer,
+    # Other forms of exceptions can be thrown by the xpress python
+    # import.  For example, an xpress.InterfaceError exception is thrown
+    # if the Xpress license is not valid.  Unfortunately, you can't
+    # import without a license, which means we can't test for that
+    # explicit exception!
+    catch_exceptions=(Exception,),
+    importer=_xpress_importer,
+    callback=_finalize_xpress_import,
+)