Avoid to_dict() when possible in pulse system model (Qiskit#695)

* Avoid to_dict() when possible in pulse system model

As fall out from Qiskit/qiskit#4016 there was a bug in the
to_dict() method of the backend configuration that was resulting in
UChannelLO objects not getting converted to their dict form in the
output of to_dict(). While this is being fixed in
Qiskit/qiskit#4124 this commit side steps the issue by adapting
the access patterns in the pulse system model to rely on class attribute
access and getattr() instead of dict access and get().

Co-authored-by: Christopher J. Wood <[email protected]>

qiskit/providers/aer/pulse/pulse_system_model.py

@@ -110,23 +110,23 @@ def from_backend(cls, backend, subsystem_list=None):
             raise AerError("{} is not a Qiskit backend".format(backend))
 
         # get relevant information from backend
-        defaults = backend.defaults().to_dict()
-        config = backend.configuration().to_dict()
+        defaults = backend.defaults()
+        config = backend.configuration()
 
-        if not config['open_pulse']:
+        if not config.open_pulse:
             raise AerError('{} is not an open pulse backend'.format(backend))
 
         # draw defaults
-        qubit_freq_est = defaults.get('qubit_freq_est', None)
-        meas_freq_est = defaults.get('meas_freq_est', None)
+        qubit_freq_est = getattr(defaults, 'qubit_freq_est', None)
+        meas_freq_est = getattr(defaults, 'meas_freq_est', None)
 
         # draw from configuration
         # if no subsystem_list, use all for device
-        subsystem_list = subsystem_list or list(range(config['n_qubits']))
-        ham_string = config['hamiltonian']
+        subsystem_list = subsystem_list or list(range(config.n_qubits))
+        ham_string = config.hamiltonian
         hamiltonian = HamiltonianModel.from_dict(ham_string, subsystem_list)
-        u_channel_lo = config.get('u_channel_lo', None)
-        dt = config.get('dt', None)
+        u_channel_lo = getattr(config, 'u_channel_lo', None)
+        dt = getattr(config, 'dt', None)
 
         control_channel_labels = [None] * len(u_channel_lo)
         # populate control_channel_dict
@@ -152,11 +152,14 @@ def from_backend(cls, backend, subsystem_list=None):
                     # construct string for u channel
                     u_string = ''
                     for u_term_dict in u_lo:
-                        scale = u_term_dict.get('scale', [1.0, 0])
-                        q_idx = u_term_dict.get('q')
+                        scale = getattr(u_term_dict, 'scale', [1.0, 0])
+                        q_idx = getattr(u_term_dict, 'q')
                         if len(u_string) > 0:
                             u_string += ' + '
-                        u_string += str(scale[0] + scale[1] * 1j) + 'q' + str(q_idx)
+                        if isinstance(scale, complex):
+                            u_string += str(scale) + 'q' + str(q_idx)
+                        else:
+                            u_string += str(scale[0] + scale[1] * 1j) + 'q' + str(q_idx)
                     control_channel_labels[u_idx] = {'driven_q': drive_idx, 'freq': u_string}
 
         return cls(hamiltonian=hamiltonian,