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

…259)

The `if_` and `while_` conditionals are constructed with a predicate.
The interface expects the predicate to be a callable that returns a
boolean, which if true, the body of the conditional is entered.

The problem is that the type of the value returned by the predicate was
not explicitly checked, and any value that would evaluate as truthy
would be accepted. This could potentially lead to unexpected behavior,
such as an infinite loop for the `while_` construct.

Here the `_Conditional.is_true` method is updated to explicitly check
the type of the value returned by the predicate. If anything but a
boolean is returned, a `TypeError` is raised.

Cherry-pick: 800bcf1

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)