Implement subscribing predicates found in yaml

lib/tfw/yaml_fsm.py

@@ -1,5 +1,6 @@
-from subprocess import Popen
+from subprocess import Popen, run
 from functools import partial
+from contextlib import suppress
 
 import yaml
 from transitions import State
@@ -13,6 +14,7 @@ class YamlFSM(FSMBase):
         self.for_config_states_and_transitions_do(self.patch_config_callbacks)
         self.setup_states()
         super().__init__()  # FSMBase.__init__() requires states
+        self.for_config_states_and_transitions_do(self.subscribe_and_remove_predicates)
         self.setup_transitions()
 
     @staticmethod
@@ -38,6 +40,24 @@ class YamlFSM(FSMBase):
             if key in topatch:
                 json_obj[key] = partial(self.run, json_obj[key])
 
+    def subscribe_and_remove_predicates(self, json_obj):
+        for key in json_obj:
+            if key == 'predicates':
+                for predicate in json_obj[key]:
+                    self.subscribe_predicate(
+                        json_obj['trigger'],
+                        partial(
+                            self.statuscode_is_zero,
+                            predicate
+                        )
+                    )
+        with suppress(KeyError):
+            json_obj.pop('predicates')
+
     @staticmethod
     def run(command, event):
         Popen(command, shell=True)
+
+    @staticmethod
+    def statuscode_is_zero(command):
+        return run(command, shell=True).returncode == 0