Implement optional Jinja2 templating support for YamlFSM

lib/tfw/yaml_fsm.py

@@ -1,25 +1,21 @@
 from subprocess import Popen, run
-from functools import partial
+from functools import partial, singledispatch
 from contextlib import suppress
 
 import yaml
+import jinja2
 from transitions import State
 
 from tfw import FSMBase
 
 
 class YamlFSM(FSMBase):
-    def __init__(self, config_file):
-        self.config = self.parse_config(config_file)
+    def __init__(self, config_file, jinja2_variables=None):
+        self.config = ConfigParser(config_file, jinja2_variables).config
         self.setup_states()
         super().__init__()  # FSMBase.__init__() requires states
         self.setup_transitions()
 
-    @staticmethod
-    def parse_config(config_file):
-        with open(config_file, 'r') as ifile:
-            return yaml.safe_load(ifile)
-
     def setup_states(self):
         self.for_config_states_and_transitions_do(self.wrap_callbacks_with_subprocess_call)
         self.states = [State(**state) for state in self.config['states']]
@@ -62,3 +58,40 @@ def run_command_async(command, event):
 
 def command_statuscode_is_zero(command):
     return run(command, shell=True).returncode == 0
+
+
+class ConfigParser:
+    def __init__(self, config_file, jinja2_variables):
+        self.read_variables = singledispatch(self.read_variables)
+        self.read_variables.register(dict, self._read_variables_dict)
+        self.read_variables.register(str, self._read_variables_str)
+
+        self.config = self.parse_config(config_file, jinja2_variables)
+
+    def parse_config(self, config_file, jinja2_variables):
+        config_string = self.read_file(config_file)
+        if jinja2_variables is not None:
+            variables = self.read_variables(jinja2_variables)
+            template = jinja2.Environment(loader=jinja2.BaseLoader).from_string(config_string)
+            config_string = template.render(**variables)
+        return yaml.safe_load(config_string)
+
+    @staticmethod
+    def read_file(filename):
+        with open(filename, 'r') as ifile:
+            return ifile.read()
+
+    @staticmethod
+    def read_variables(variables):
+        raise TypeError(f'Invalid variables type {type(variables)}')
+
+    @staticmethod
+    def _read_variables_str(variables):
+        if isinstance(variables, str):
+            with open(variables, 'r') as ifile:
+                return yaml.safe_load(ifile)
+
+    @staticmethod
+    def _read_variables_dict(variables):
+            return variables
+