diff --git a/reframe/core/buildsystems.py b/reframe/core/buildsystems.py
index d4020d7540..0801d74cf7 100644
--- a/reframe/core/buildsystems.py
+++ b/reframe/core/buildsystems.py
@@ -6,7 +6,7 @@
 from reframe.core.exceptions import BuildSystemError
 
 
-class BuildSystem:
+class BuildSystem(abc.ABC):
     """The abstract base class of any build system.
 
     Concrete build systems inherit from this class and must override the
diff --git a/reframe/frontend/executors/__init__.py b/reframe/frontend/executors/__init__.py
index 7d8abfa0ec..0a1f31d551 100644
--- a/reframe/frontend/executors/__init__.py
+++ b/reframe/frontend/executors/__init__.py
@@ -212,7 +212,7 @@ def abort(self, cause=None):
             self.fail((type(exc), exc, None))
 
 
-class TaskEventListener:
+class TaskEventListener(abc.ABC):
     @abc.abstractmethod
     def on_task_run(self, task):
         """Called whenever the run() method of a RegressionTask is called."""
@@ -320,7 +320,7 @@ def print_separator(check, prefix):
         self._policy.exit()
 
 
-class ExecutionPolicy:
+class ExecutionPolicy(abc.ABC):
     """Base abstract class for execution policies.
 
     An execution policy implements the regression check pipeline."""
@@ -369,7 +369,3 @@ def runcase(self, case):
 
         if self.force_local:
             case.check.local = True
-
-    @abc.abstractmethod
-    def getstats(self):
-        """Return test case statistics of the run."""
diff --git a/reframe/utility/versioning.py b/reframe/utility/versioning.py
index 143cf18984..9f3617de06 100644
--- a/reframe/utility/versioning.py
+++ b/reframe/utility/versioning.py
@@ -67,7 +67,7 @@ def __str__(self):
         return base + '-dev%s' % self._dev_number
 
 
-class _ValidatorImpl:
+class _ValidatorImpl(abc.ABC):
     """Abstract base class for the validation of version ranges."""
     @abc.abstractmethod
     def validate(version):
@@ -81,6 +81,7 @@ class _IntervalValidator(_ValidatorImpl):
     ``validate`` returns ``True`` if a given version string is inside
     the interval including the endpoints.
     """
+
     def __init__(self, condition):
         try:
             min_version_str, max_version_str = condition.split('..')
@@ -108,6 +109,7 @@ class _RelationalValidator(_ValidatorImpl):
     ``<bool_operator><version>``, and its method ``validate`` returns
     ``True`` if a given version string satisfies the relation.
     """
+
     def __init__(self, condition):
         self._op_actions = {
             ">":  lambda x, y: x > y,
diff --git a/unittests/test_buildsystems.py b/unittests/test_buildsystems.py
index 81a2e1cc0d..3a1d902b30 100644
--- a/unittests/test_buildsystems.py
+++ b/unittests/test_buildsystems.py
@@ -6,7 +6,7 @@
 from reframe.core.exceptions import BuildSystemError
 
 
-class _BuildSystemTest:
+class _BuildSystemTest(abc.ABC):
     @abc.abstractmethod
     def create_build_system(self):
         pass
diff --git a/unittests/test_modules.py b/unittests/test_modules.py
index d91afe9990..10e487664d 100644
--- a/unittests/test_modules.py
+++ b/unittests/test_modules.py
@@ -10,7 +10,7 @@
 from unittests.fixtures import TEST_MODULES
 
 
-class _TestModulesSystem:
+class _TestModulesSystem(abc.ABC):
     def setUp(self):
         self.environ_save = EnvironmentSnapshot()
         self.modules_system.searchpath_add(TEST_MODULES)
diff --git a/unittests/test_schedulers.py b/unittests/test_schedulers.py
index 3242ff3324..71dd558642 100644
--- a/unittests/test_schedulers.py
+++ b/unittests/test_schedulers.py
@@ -18,7 +18,7 @@
 from reframe.core.schedulers.slurm import SlurmNode
 
 
-class _TestJob:
+class _TestJob(abc.ABC):
     def setUp(self):
         self.workdir = tempfile.mkdtemp(dir='unittests')
         self.testjob = self.job_type(
@@ -71,7 +71,6 @@ def setup_user(self, msg=None):
 
         self.testjob.options += partition.access
 
-    @abc.abstractmethod
     def assertScriptSanity(self, script_file):
         """Assert the sanity of the produced script file."""
         with open(self.testjob.script_filename) as fp: