Fix and test MultichannelProgram init

qctoolkit/_program/_loop.py

@@ -300,6 +300,7 @@ def __init__(self, instruction_block: Union[AbstractInstructionBlock, Loop], cha
             self._init_from_instruction_block(instruction_block, channels)
         elif isinstance(instruction_block, Loop):
             assert channels is None
+            self._init_from_loop(loop=instruction_block)
         else:
             raise TypeError('Invalid program type', type(instruction_block), instruction_block)
 
@@ -308,7 +309,7 @@ def _init_from_loop(self, loop: Loop):
 
         assert first_waveform is not None
 
-        self._programs[first_waveform.defined_channels] = loop
+        self._programs[frozenset(first_waveform.defined_channels)] = loop
 
     def _init_from_instruction_block(self, instruction_block, channels):
         if channels is None:

tests/_program/loop_tests.py

@@ -488,6 +488,15 @@ def test_via_repr(self):
         self.assertEqual(root_loopA.__repr__(), reprA)
         self.assertEqual(root_loopB.__repr__(), reprB)
 
+    def test_init_from_loop(self):
+        program = Loop(waveform=DummyWaveform(defined_channels={'A', 'B'}))
+
+        mcp = MultiChannelProgram(program)
+        self.assertEqual(mcp.programs, {frozenset('AB'): program})
+
+        with self.assertRaises(TypeError):
+            MultiChannelProgram(mcp)
+
 
 class ProgramWaveformCompatibilityTest(unittest.TestCase):
     def test_is_compatible_incompatible(self):