Add mock for decorators of optional dependencies

src/ctapipe/exceptions.py

@@ -1,3 +1,12 @@
+__all__ = [
+    "CTAPipeException",
+    "TooFewEvents",
+    "OptionalDependencyMissing",
+    "InputMissing",
+    "MockOptionalDecorator",
+]
+
+
 class CTAPipeException(Exception):
     pass
 
@@ -17,3 +26,46 @@ def __init__(self, module):
 
 class InputMissing(ValueError):
     """Raised in case an input was not specified."""
+
+
+class MockOptionalDecorator:
+    """A decorator that can be used in-place of an imported decorator.
+
+    Will throw the corresponding OptionalDependencyMissing exception when
+    the decorated function is called.
+
+    Examples
+    --------
+    You might want to use this class for optional dependencies that provide
+    decorators. With decorators, it is hard to defer import of the dependency
+    to runtime. See this example of how one might make numba with njit optional:
+
+        from unittest import MagicMock
+        from ctapipe.exceptions import MockOptionalDecorator
+        try:
+            import numba
+        except ModuleNotFoundError:
+            numba = MagicMock()
+            numba.njit = MockOptionalDecorator("numba")
+
+        @numba.njit(cache=True)
+        def example(x):
+            return 5 * x
+    """
+
+    def __init__(self, module):
+        self.module = module
+
+    def __call__(self, *args, **kwargs):
+        def _raise(*args, **kwargs):
+            raise OptionalDependencyMissing(self.module)
+
+        # decorator called as @decorator without arguments
+        if len(args) == 1 and len(kwargs) == 0 and callable(args[0]):
+            return _raise
+
+        # decorator called as @decorator(*args, **kwargs)
+        def wrapper(func):
+            return _raise
+
+        return wrapper

src/ctapipe/tests/test_exceptions.py

@@ -0,0 +1,35 @@
+from unittest.mock import MagicMock
+
+import pytest
+
+
+def test_optional_decorator_no_args():
+    from ctapipe.exceptions import MockOptionalDecorator, OptionalDependencyMissing
+
+    dummy_opt_module = MagicMock()
+    dummy_opt_module.some_decorator = MockOptionalDecorator("dummy")
+
+    # defining the function should not throw
+    @dummy_opt_module.some_decorator
+    def func():
+        pass
+
+    with pytest.raises(OptionalDependencyMissing, match="'dummy' is required"):
+        # calling the function should raise
+        func()
+
+
+def test_optional_decorator_with_args():
+    from ctapipe.exceptions import MockOptionalDecorator, OptionalDependencyMissing
+
+    dummy_opt_module = MagicMock()
+    dummy_opt_module.some_decorator = MockOptionalDecorator("dummy")
+
+    # defining the function should not throw
+    @dummy_opt_module.some_decorator(foo=5)
+    def func():
+        pass
+
+    with pytest.raises(OptionalDependencyMissing, match="'dummy' is required"):
+        # calling the function should raise
+        func()