build/parser: adding a method to search default value for an argument into ArgParse._actions

litex/build/parser.py

@@ -174,6 +174,8 @@ class LiteXArgumentParser(argparse.ArgumentParser):
         # When platform is None try to search for a user input
         if self._platform is None:
             platform = self.get_value_from_key("--platform", None)
+            if platform is None: # no user selection: try default
+                platform = self.get_default_value_from_actions("platform", None)
             if platform is not None:
                 self.set_platform(importlib.import_module(platform).Platform)
                 self.add_target_group()
@@ -244,3 +246,23 @@ class LiteXArgumentParser(argparse.ArgumentParser):
         except IndexError:
             value = default
         return value
+
+    def get_default_value_from_actions(self, key, default=None):
+        """
+        search key into ArgumentParser _actions list
+
+        Parameters
+        ==========
+        key: str
+            key to search
+        default: str
+            default value when key is not in _actions list
+
+        Return
+        ======
+            default value or default when key is not present
+        """
+        for act in self._actions:
+            if act.dest == key:
+                return act.default
+        return default