Make multimethods work with inheritance. (#3411)

Previously, this would fail with a NoSuchMethodError:

    class Package(object):
        # this is the default implementation
        def some_method(self):
            ...

    class Foo(Package):
        @when('platform=cray')
        def some_method(self):
            ...

        @when('platform=linux')
        def some_method(self):
            ...

This fixes the implementation of `@when` so that the superclass method
will be invoked when no subclass method matches.

Adds tests to ensure this works, as well.

lib/spack/spack/multimethod.py

@@ -128,10 +128,16 @@ def __call__(self, package_self, *args, **kwargs):
 
         if self.default:
             return self.default(package_self, *args, **kwargs)
+
         else:
-            raise NoSuchMethodError(
-                type(package_self), self.__name__, spec,
-                [m[0] for m in self.method_list])
+            superclass = super(package_self.__class__, package_self)
+            superclass_fn = getattr(superclass, self.__name__, None)
+            if callable(superclass_fn):
+                return superclass_fn(*args, **kwargs)
+            else:
+                raise NoSuchMethodError(
+                    type(package_self), self.__name__, spec,
+                    [m[0] for m in self.method_list])
 
     def __str__(self):
         return "SpecMultiMethod {\n\tdefault: %s,\n\tspecs: %s\n}" % (

lib/spack/spack/test/multimethod.py

@@ -118,3 +118,11 @@ def test_virtual_dep_match(builtin_mock):
 
     pkg = spack.repo.get('multimethod^mpich@1.0')
     assert pkg.different_by_virtual_dep() == 1
+
+
+def test_multimethod_with_base_class(builtin_mock):
+    pkg = spack.repo.get('multimethod@3')
+    assert pkg.base_method() == "subclass_method"
+
+    pkg = spack.repo.get('multimethod@1')
+    assert pkg.base_method() == "base_method"

var/spack/repos/builtin.mock/packages/multimethod-base/package.py

@@ -0,0 +1,40 @@
+##############################################################################
+# Copyright (c) 2013-2017, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class MultimethodBase(Package):
+    """This is a base class for the Multimethod test case.
+
+    It tests whether mutlimethod properly invokes methods in a base
+    class when subclass multi-methods do not match.
+
+    """
+
+    homepage = 'http://www.example.com/'
+    url      = 'http://www.example.com/example-1.0.tar.gz'
+
+    def base_method(self):
+        return "base_method"

var/spack/repos/builtin.mock/packages/multimethod/package.py

@@ -25,8 +25,10 @@
 from spack import *
 import spack.architecture
 
+from spack.pkg.builtin.mock.multimethod_base import MultimethodBase
 
-class Multimethod(Package):
+
+class Multimethod(MultimethodBase):
     """This package is designed for use with Spack's multimethod test.
        It has a bunch of test cases for the @when decorator that the
        test uses.
@@ -132,3 +134,11 @@ def different_by_virtual_dep(self):
     @when('^mpi@2:')
     def different_by_virtual_dep(self):
         return 2
+
+    #
+    # Make sure methods with a default implementation in a superclass
+    # will invoke that method when none in the subclass match.
+    #
+    @when("@2:")
+    def base_method(self):
+        return "subclass_method"