diff --git a/conftest.py b/conftest.py
new file mode 100644
index 000000000..b7f607923
--- /dev/null
+++ b/conftest.py
@@ -0,0 +1,18 @@
+# (C) Copyright IBM 2023.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+import pytest
+
+import ffsim
+
+
+@pytest.fixture(autouse=True)
+def add_ffsim(doctest_namespace):
+    doctest_namespace["ffsim"] = ffsim
diff --git a/tox.ini b/tox.ini
index 3c1c9439e..745289bc8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -15,7 +15,7 @@ wheel_build_env = .pkg
 extras =
     dev
 commands =
-    pytest tests/ {posargs}
+    pytest --doctest-modules python/ tests/ {posargs}
 
 [testenv:coverage]
 description = check test coverage