Workchains: Raise if if_/while_ predicate does not return boolean

src/plumpy/workchains.py

@@ -388,7 +388,12 @@ def predicate(self) -> PREDICATE_TYPE:
         return self._predicate
 
     def is_true(self, workflow: 'WorkChain') -> bool:
-        return self._predicate(workflow)
+        result = self._predicate(workflow)
+
+        if not isinstance(result, bool):
+            raise TypeError(f'The conditional predicate `{self._predicate.__name__}` did not return a boolean')
+
+        return result
 
     def __call__(self, *instructions: Union[_Instruction, WC_COMMAND_TYPE]) -> _Instruction:
         assert self._body is None, 'Instructions have already been set'

test/test_workchains.py

@@ -618,3 +618,14 @@ def step_two(self):
 
         workchain = Wf(inputs=dict(subspace={'one': 1, 'two': 2}))
         workchain.execute()
+
+
+@pytest.mark.parametrize('construct', (if_, while_))
+def test_conditional_return_type(construct):
+    """Test that a conditional passed to the ``if_`` and ``while_`` functions that does not return a ``bool`` raises."""
+
+    def invalid_conditional(self):
+        return 'true'
+
+    with pytest.raises(TypeError, match='The conditional predicate `invalid_conditional` did not return a boolean'):
+        construct(invalid_conditional)[0].is_true(None)