general get/set of array field length

devsupApp/src/dbfield.c

@@ -196,6 +196,32 @@ static PyObject *pyField_getarray(pyField *self)
 #endif
 }
 
+static PyObject *pyField_setlen(pyField *self, PyObject *args)
+{
+    rset *prset = dbGetRset(&self->addr);
+    Py_ssize_t len;
+
+    if(!PyArg_ParseTuple(args, "n", &len))
+        return NULL;
+
+    if(self->addr.special!=SPC_DBADDR ||
+       !prset ||
+       !prset->put_array_info)
+    {
+        PyErr_SetString(PyExc_TypeError, "Not an array field");
+    }
+
+    if(len<1 || len > self->addr.no_elements) {
+        PyErr_Format(PyExc_ValueError, "Requested length %ld out of range [1,%lu)",
+                        (long)len, (unsigned long)self->addr.no_elements);
+        return NULL;
+    }
+
+    prset->put_array_info(&self->addr, len);
+
+    Py_RETURN_NONE;
+}
+
 static PyObject* pyField_getTime(pyField *self)
 {
     DBLINK *plink = self->addr.pfield;
@@ -239,6 +265,11 @@ static PyObject* pyField_getAlarm(pyField *self)
     return Py_BuildValue("ll", (long)sevr, (long)stat);
 }
 
+static PyObject *pyField_len(pyField *self)
+{
+    return PyInt_FromLong(self->addr.no_elements);
+}
+
 static PyMethodDef pyField_methods[] = {
     {"name", (PyCFunction)pyField_name, METH_NOARGS,
      "Return Names (\"record\",\"field\")"},
@@ -250,10 +281,14 @@ static PyMethodDef pyField_methods[] = {
      "Sets field value from a scalar"},
     {"getarray", (PyCFunction)pyField_getarray, METH_NOARGS,
      "Return a numpy ndarray refering to this field for in-place operations."},
+    {"putarraylen", (PyCFunction)pyField_setlen, METH_VARARGS,
+     "Set array field length."},
     {"getTime", (PyCFunction)pyField_getTime, METH_NOARGS,
      "Return link target timestamp as a tuple (sec, nsec)."},
     {"getAlarm", (PyCFunction)pyField_getAlarm, METH_NOARGS,
      "Return link target alarm condtions as a tuple (severity, status)."},
+    {"__len__", (PyCFunction)pyField_len, METH_NOARGS,
+     "Maximum number of elements storable in this field"},
     {NULL, NULL, 0, NULL}
 };
 

devsupApp/src/devsup/_nullapi.py

@@ -168,10 +168,23 @@ class _Field(object):
           lock is held (ie withing :meth:`process <DeviceSupport.process>`).
         """
 
+    def putarraylen(self, len):
+        """Set the number of active elements in field's array.
+
+        Requires that the underlying field be an array.
+        Must be less than the maximum length of the field.
+        """
+
     def getAlarm(self):
         """Returns a tuple (severity, status) with the condtion of the linked field.
         
         Only works for fields of type DBF_INLINK.
         """
 
+    def __len__(self):
+        """Returns the maximum number of elements which may be stored in the field.
+
+        This is always 1 for scalar fields.
+        """
+
 _hooks = {}

testApp/test3.py

@@ -1,3 +1,4 @@
+from __future__ import print_function
 
 import numpy as np
 import scipy as sp
@@ -5,10 +6,13 @@ from numpy.random import randint, uniform
 
 class WfSup(object):
     def __init__(self, rec, args):
-        self.arr = rec.field('VAL').getarray()
-        self.x = np.arange(rec.NELM)
+        self.fld = rec.field('VAL')
+        self.arr = self.fld.getarray()
+        assert(len(self.fld)==rec.NELM)
+        self.x = np.arange(len(self.fld))
 
         self.phase = 0.0
+        print("start",self)
 
     def detach(self, rec):
         pass
@@ -17,7 +21,7 @@ class WfSup(object):
         pha = self.phase*np.pi/180.0
         self.phase = np.fmod(self.phase+10.0, 360.0)
 
-        N=randint(1, rec.NELM)
+        N=randint(1, len(self.fld))
         
         val=self.arr[:N]
         x=self.x[:N]
@@ -28,8 +32,8 @@ class WfSup(object):
         np.sin(val, out=val)
         val[:]*=uniform(0.5,2.0)
         val[:]+=2
-        
-        self.NORD = N
+
+        self.fld.putarraylen(N)
 
 def build(rec, args):
     return WfSup(rec, args)