Custom MibScalarInstance returns noSuchInstance¶

I subclassed MibScalarInstance and override getValue() to compute the value on every read, but my Agent returns noSuchInstance (or genErr for SNMPv1) and getValue() is never called. Why?

The instance was constructed with a schema-only syntax object.

In pyasn1, a type constructed without an argument (for example OctetString() or Integer32()) is a schema: its isValue attribute is False. MibScalarInstance.readTest and readGet short-circuit and raise NoSuchInstanceError whenever self.syntax.isValue is False — they never call getValue(). This guards against truly uninitialized instances leaking noSuchInstance semantics through the agent.

The fix is to pass an initial value to the instance constructor, even if your getValue() override is going to compute the real value at read time. Any value of the right type will do — an empty string, zero, etc. — its purpose is only to flip isValue to True.
```
class DynamicScalar(MibScalarInstance):
    def getValue(self, name, **context):
        return self.getSyntax().clone(compute_value_now())

# WRONG: schema-only syntax, getValue() is never called
DynamicScalar((1, 3, 6, 1, 4, 1, 60069, 9, 1), (0,), OctetString())

# RIGHT: initialized syntax; getValue() is called on every read
DynamicScalar((1, 3, 6, 1, 4, 1, 60069, 9, 1), (0,), OctetString(""))
```
The same applies to Integer32(0), ObjectIdentifier((0, 0)), and any other SMI type used as the syntax argument of a MibScalarInstance.

The schema/value distinction is intentional and is also what the companion MibScalar object relies on to advertise type information without claiming a value. Do not work around the check by stripping the isValue test — change the fixture instead.