add parmod.Par

the reasonly class frappy_psi.parmod.Par represents a parameter
or a component of a tuple parameter

Change-Id: I47208c9d7a6fc377cd56b82cc6a9e8cdb433fe8e

frappy_psi/parmod.py

@@ -23,12 +23,36 @@
 """modules to access parameters"""
 
 from frappy.core import Drivable, IDLE, Attached, StringType, Property, \
-    Parameter, FloatRange
+    Parameter, FloatRange, Readable
 from frappy.errors import ConfigError
 from frappy_psi.convergence import HasConvergence
 from frappy_psi.mixins import HasRamp
 
 
+class Par(Readable):
+    value = Parameter(datatype=FloatRange(unit='$'))
+    read = Attached(description='<module>.<parameter> for read')
+    unit = Property('main unit', StringType())
+
+    def setProperty(self, key, value):
+        if key == 'read':
+            value, param = value.split('.')
+            setattr(self, f'{key}_param', param)
+        super().setProperty(key, value)
+
+    def checkProperties(self):
+        self.applyMainUnit(self.unit)
+        if self.read == self.name :
+            raise ConfigError('illegal recursive read/write module')
+        super().checkProperties()
+
+    def read_value(self):
+        return getattr(self.read, f'{self.read_param}')
+
+    def read_status(self):
+        return IDLE, ''
+
+
 class Driv(Drivable):
     value = Parameter(datatype=FloatRange(unit='$'))
     target = Parameter(datatype=FloatRange(unit='$'))