Fixes Qiskit#7078: Introduce ClassicalIOChannel as a subclass of puls…

…e.Channel

RegisterSlot, MemorySlot and SnapshotChannel now derive from this class. There are some tests to fix.

qiskit/pulse/channels.py

@@ -140,11 +140,6 @@ def name(self) -> str:
         """Return the shorthand alias for this channel, which is based on its type and index."""
         return f"{self.__class__.prefix}{self._index}"
 
-    @property
-    def is_schedulable(self) -> bool:
-        """Return whether this channel can be independently scheduled"""
-        return True
-
     def __repr__(self):
         return f"{self.__class__.__name__}({self._index})"
 
@@ -170,6 +165,12 @@ class PulseChannel(Channel, metaclass=ABCMeta):
     pass
 
 
+class ClassicalIOChannel(Channel, metaclass=ABCMeta):
+    """Base class of classical IO channels. These cannot have instructions scheduled on them."""
+
+    pass
+
+
 class DriveChannel(PulseChannel):
     """Drive channels transmit signals to qubits which enact gate operations."""
 
@@ -197,7 +198,7 @@ class AcquireChannel(Channel):
     prefix = "a"
 
 
-class SnapshotChannel(Channel):
+class SnapshotChannel(ClassicalIOChannel):
     """Snapshot channels are used to specify instructions for simulators."""
 
     prefix = "s"
@@ -207,25 +208,15 @@ def __init__(self):
         super().__init__(0)
 
 
-class MemorySlot(Channel):
+class MemorySlot(ClassicalIOChannel):
     """Memory slot channels represent classical memory storage."""
 
     prefix = "m"
 
-    @property
-    def is_schedulable(self) -> bool:
-        """Return whether this channel can be independently scheduled"""
-        return False
-
 
-class RegisterSlot(Channel):
+class RegisterSlot(ClassicalIOChannel):
     """Classical resister slot channels represent classical registers (low-latency classical
     memory).
     """
 
     prefix = "c"
-
-    @property
-    def is_schedulable(self) -> bool:
-        """Return whether this channel can be independently scheduled"""
-        return False

qiskit/pulse/instructions/delay.py

@@ -14,7 +14,7 @@
 from typing import Optional, Union, Tuple
 
 from qiskit.circuit import ParameterExpression
-from qiskit.pulse.channels import Channel
+from qiskit.pulse.channels import Channel, ClassicalIOChannel
 from qiskit.pulse.exceptions import PulseError
 from qiskit.pulse.instructions.instruction import Instruction
 
@@ -51,11 +51,15 @@ def __init__(
             name: Name of the delay for display purposes.
 
         Raises:
-            PulseError: If `channel` cannot be delayed because it cannot be scheduled.
+            PulseError: If `channel` cannot be delayed because it is a classical IO channel.
         """
         super().__init__(operands=(duration, channel), name=name)
-        if not channel.is_schedulable:
-            raise PulseError("Cannot delay chanel because it is not schedulable.")
+        if isinstance(channel, ClassicalIOChannel):
+            raise PulseError(
+                "Cannot delay channel {0} because it is a classical IO channel.".format(
+                    channel.name
+                )
+            )
 
     @property
     def channel(self) -> Channel:

qiskit/pulse/instructions/directives.py

@@ -16,6 +16,8 @@
 from typing import Optional, Tuple
 
 from qiskit.pulse import channels as chans
+from qiskit.pulse.channels import ClassicalIOChannel
+from qiskit.pulse.exceptions import PulseError
 from qiskit.pulse.instructions import instruction
 
 
@@ -44,7 +46,17 @@ def __init__(self, *channels: chans.Channel, name: Optional[str] = None):
         Args:
             channels: The channel that the barrier applies to.
             name: Name of the directive for display purposes.
+        Raises:
+            PulseError: If one of the `channels` cannot be blocked because it is a classical IO channel.
         """
+        for channel in channels:
+            if isinstance(channel, ClassicalIOChannel):
+                raise PulseError(
+                    "Cannot block channel {0} because it is a classical IO channel.".format(
+                        channel.name
+                    )
+                )
+
         super().__init__(operands=tuple(channels), name=name)
 
     @property

qiskit/pulse/transforms/canonicalization.py

@@ -18,6 +18,7 @@
 import numpy as np
 
 from qiskit.pulse import channels as chans, exceptions, instructions
+from qiskit.pulse.channels import ClassicalIOChannel
 from qiskit.pulse.exceptions import PulseError
 from qiskit.pulse.exceptions import UnassignedDurationError
 from qiskit.pulse.instruction_schedule_map import InstructionScheduleMap
@@ -475,7 +476,7 @@ def pad(
     channels = channels or schedule.channels
 
     for channel in channels:
-        if not channel.is_schedulable:
+        if isinstance(channel, ClassicalIOChannel):
             continue
         if channel not in schedule.channels:
             schedule |= instructions.Delay(until, channel)