Add scipy array API check in init

Author: amacati

drone_controllers/__init__.py

@@ -6,12 +6,27 @@
 broadcasting to support batching of arbitrary leading dimensions.
 """
 
+import os
+import sys
 from typing import Callable
 
+# SciPy array API check. We use the most recent array API features, which require the
+# SCIPY_ARRAY_API environment variable to be set to "1". This flag MUST be set before importing
+# scipy, because scipy's C extensions cannot be unloaded once they have been imported. Therefore, we
+# have to error out if the flag is not set. Otherwise, we immediately import scipy to ensure that no
+# other package sets the flag to a different value before importing scipy.
+
+if "scipy" in sys.modules and os.environ.get("SCIPY_ARRAY_API") != "1":
+    msg = """scipy has already been imported and the 'SCIPY_ARRAY_API' environment variable has not
+    been set. Please restart your Python session and set SCIPY_ARRAY_API="1" before importing any
+    packages that depend on scipy, or import this package first to automatically set the flag."""
+    raise RuntimeError(msg)
+
+os.environ["SCIPY_ARRAY_API"] = "1"
+import scipy  # noqa: F401, ensure scipy uses array API features
+
 from drone_controllers.core import parametrize
-from drone_controllers.mellinger import (
-    attitude2force_torque as mellinger_attitude2force_torque,
-)
+from drone_controllers.mellinger import attitude2force_torque as mellinger_attitude2force_torque
 from drone_controllers.mellinger import state2attitude as mellinger_state2attitude
 
 available_controller: dict[str, Callable] = {