Restore ExitCode to being a namedtuple

Changing the `ExitCode` from a namedtuple to a class breaks backwards
compatibility since the class won't have the functionalities of a tuple
such as expanding an instance in its attributes. By subclassing from the
namedtuple definition we keep all that functionality while still being
able to implement additional methods, such as in this case `format`.

Co-Authored-By: Dominik Gresch <greschd@users.noreply.github.com>

aiida/engine/processes/exit_code.py

@@ -8,18 +8,22 @@
 # For further information please visit http://www.aiida.net               #
 ###########################################################################
 """A namedtuple and namespace for ExitCodes that can be used to exit from Processes."""
+from collections import namedtuple
 from aiida.common.extendeddicts import AttributeDict
 
 __all__ = ('ExitCode', 'ExitCodesNamespace')
 
 
-class ExitCode:
+class ExitCode(namedtuple('ExitCode', ['status', 'message', 'invalidates_cache'])):
     """A simple data class to define an exit code for a :class:`~aiida.engine.processes.process.Process`.
 
     When an instance of this clas is returned from a `Process._run()` call, it will be interpreted that the `Process`
     should be terminated and that the exit status and message of the namedtuple should be set to the corresponding
     attributes of the node.
 
+    .. note:: this class explicitly sub-classes a namedtuple to not break backwards compatibility and to have it behave
+        exactly as a tuple.
+
     :param status: positive integer exit status, where a non-zero value indicated the process failed, default is `0`
     :type status: int
 
@@ -30,11 +34,6 @@ class ExitCode:
     :type invalidates_cache: bool
     """
 
-    def __init__(self, status=0, message=None, invalidates_cache=False):
-        self.status = status
-        self.message = message
-        self.invalidates_cache = invalidates_cache
-
     def format(self, **kwargs):
         """Create a clone of this exit code where the template message is replaced by the keyword arguments.
 
@@ -53,6 +52,10 @@ def __eq__(self, other):
         return all(getattr(self, attr) == getattr(other, attr) for attr in ['status', 'message', 'invalidates_cache'])
 
 
+# Set the defaults for the `ExitCode` attributes
+ExitCode.__new__.__defaults__ = (0, None, False)
+
+
 class ExitCodesNamespace(AttributeDict):
     """A namespace of `ExitCode` instances that can be accessed through getattr as well as getitem.
 

tests/engine/processes/text_exit_code.py

@@ -53,3 +53,16 @@ def test_exit_code_template_message():
 
     assert exit_code_base != exit_code_called  # Calling the exit code should return a new instance
     assert exit_code_called.message == message_template.format(parameter=parameter_name)
+
+
+def test_exit_code_expand_tuple():
+    """Test that an exit code instance can be expanded in its attributes like a tuple."""
+    status = 418
+    message = 'I am a teapot'
+    invalidates_cache = True
+
+    status_exp, message_exp, invalidates_cache_exp = ExitCode(418, message, True)
+
+    assert status == status_exp
+    assert message == message_exp
+    assert invalidates_cache == invalidates_cache_exp