signal: init value can be a ValueInfo structure with dtype, shape and…

… default value to characterize the value

ophyd/signal.py

@@ -6,6 +6,7 @@
 import weakref
 
 import numpy as np
+from collections import namedtuple
 
 from . import get_cl
 from .ophydobj import Kind, OphydObject
@@ -50,14 +51,19 @@ class ConnectionTimeoutError(TimeoutError):
     ...
 
 
+ValueInfo = namedtuple(
+    "ValueInfo", ("dtype", "shape", "default"), defaults=(float, (), None)
+)
+
+
 class Signal(OphydObject):
     r"""A signal, which can have a read-write or read-only value.
 
     Parameters
     ----------
     name : string, keyword only
-    value : any, optional
-        The initial value
+    value : any acceptable value or a ValueInfo object, optional
+        The initial value, or the ValueInfo structure to characterize the value
     kind : a member the Kind IntEnum (or equivalent integer), optional
         Default is Kind.normal. See Kind for options.
     parent : Device, optional
@@ -85,6 +91,7 @@ class Signal(OphydObject):
     rtolerance : any, optional
         The relative tolerance associated with the value
     """
+
     SUB_VALUE = "value"
     SUB_META = "meta"
     _default_sub = SUB_VALUE
@@ -95,7 +102,7 @@ def __init__(
         self,
         *,
         name,
-        value=0.0,
+        value=DEFAULT_EPICSSIGNAL_VALUE,
         timestamp=None,
         parent=None,
         labels=None,
@@ -116,7 +123,19 @@ def __init__(
         self.cl = cl
         self._dispatcher = cl.get_dispatcher()
         self._metadata_thread_ctx = self._dispatcher.get_thread_context("monitor")
-        self._readback = value
+        if isinstance(value, ValueInfo):
+            self._value_dtype_str = np.dtype(value.dtype).name
+            self._value_shape = value.shape
+            if value.default is None:
+                self._readback = DEFAULT_EPICSSIGNAL_VALUE
+            else:
+                # ensure default value corresponds to dtype and shape...
+                self._readback = np.asanyarray(value.default, value.dtype)
+                self._readback.shape = value.shape
+        else:
+            self._value_dtype_str = None
+            self._value_shape = None
+            self._readback = value
 
         if timestamp is None:
             timestamp = time.time()
@@ -480,8 +499,10 @@ def describe(self):
             return {
                 self.name: {
                     "source": "SIM:{}".format(self.name),
-                    "dtype": data_type(val),
-                    "shape": data_shape(val),
+                    "dtype": (
+                        data_type(val) if self._value_dtype_str is None else self._value_dtype_str
+                    ),
+                    "shape": data_shape(val) if self._value_shape is None else self._value_shape,
                 }
             }
         except ValueError as ve: