Update GaussianPulse for consistent parameter validation results whether validation occurs during or after construction.

qiskit/pulse/library/parametric_pulses.py

@@ -242,13 +242,23 @@ def __init__(
             limit_amplitude: If ``True``, then limit the amplitude of the
                              waveform to 1. The default is ``True`` and the
                              amplitude is constrained to 1.
+
+        Raises:
+            PulseError: If the parameters passed are not valid.
         """
         if not _is_parameterized(amp):
             amp = complex(amp)
         self._amp = amp
         self._sigma = sigma
         self._risefall_sigma_ratio = risefall_sigma_ratio
         self._width = width
+
+        if self.width is not None and self.risefall_sigma_ratio is not None:
+            raise PulseError(
+                "Either the pulse width or the risefall_sigma_ratio parameter can be specified"
+                " but not both."
+            )
+
         super().__init__(duration=duration, name=name, limit_amplitude=limit_amplitude)
 
     @property
@@ -284,11 +294,6 @@ def validate_parameters(self) -> None:
             )
         if not _is_parameterized(self.sigma) and self.sigma <= 0:
             raise PulseError("Sigma must be greater than 0.")
-        if self.width is not None and self.risefall_sigma_ratio is not None:
-            raise PulseError(
-                "Either the pulse width or the risefall_sigma_ratio parameter can be specified"
-                " but not both."
-            )
         if self.width is None and self.risefall_sigma_ratio is None:
             raise PulseError(
                 "Either the pulse width or the risefall_sigma_ratio parameter must be specified."

releasenotes/notes/consistent-validation-for-gaussian-square-pulse-461087a09ff339a4.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Parameter validation for :class:`~.pulse.library.GaussianSquare` is now
+    consistent before and after construction.
+    Refer to `#7882 <https://github.com/Qiskit/qiskit-terra/issues/7882>`__ for more details.

test/python/pulse/test_pulse_lib.py

@@ -162,6 +162,11 @@ def test_gauss_square_extremes(self):
             gaus_square.get_waveform().samples[2:-2], const.get_waveform().samples[2:-2]
         )
 
+    def test_gauss_square_passes_validation_after_construction(self):
+        """Test that parameter validation is consistent before and after construction."""
+        pulse = GaussianSquare(duration=125, sigma=4, amp=0.5j, width=100)
+        pulse.validate_parameters()
+
     def test_drag_pulse(self):
         """Test that the Drag sample pulse matches the pulse library."""
         drag = Drag(duration=25, sigma=4, amp=0.5j, beta=1)