fix: Program declarations should be empty after copy_everything_excep…

…t_instructions (#1614)

* Backported test which fails, as expected.

* Fix: Don't copy declarations member when copying program.

Migrated the v4 test to a simpler, more direct test of the Program class.

* Fix: the shallow copy() method needs to copy the declarations

pyquil/quil.py

@@ -186,7 +186,6 @@ def copy_everything_except_instructions(self) -> "Program":
         """
         new_prog = Program()
         new_prog._calibrations = self.calibrations.copy()
-        new_prog._declarations = self._declarations.copy()
         new_prog._waveforms = self.waveforms.copy()
         new_prog._defined_gates = self._defined_gates.copy()
         new_prog._frames = self.frames.copy()
@@ -206,8 +205,9 @@ def copy(self) -> "Program":
 
         :return: a new Program
         """
-        new_prog = self.copy_everything_except_instructions()
+        new_prog = self.copy_everything_except_instructions()  # and declarations, which is a view
         new_prog._instructions = self._instructions.copy()
+        new_prog._declarations = self._declarations.copy()
         return new_prog
 
     @property

test/unit/test_quil.py

@@ -1409,3 +1409,20 @@ def test_params_pi_and_precedence():
     prog = Program(f"RX({more_less_trivial_pi}) 0")
     exp = str(prog[0].params[0])
     assert _eval_as_np_pi(more_less_trivial_pi) == _eval_as_np_pi(exp)
+
+
+def test_copy_everything_except_instructions():
+    """Test for https://github.com/rigetti/pyquil/issues/1613"""
+    program = Program(
+        """
+DECLARE beta REAL[1]
+RZ(0.5) 0
+CPHASE(pi) 0 1
+DECLARE ro BIT[2]
+MEASURE 0 ro[0]
+MEASURE 1 ro[1]
+"""
+    )
+    program = program.copy_everything_except_instructions()
+    assert len(program.instructions) == 0  # the purpose of copy_everything_except_instructions()
+    assert len(program.declarations) == 0  # this is a view on the instructions member; must be consistent