Reset backend options to default in get_backend

qiskit_ibm_provider/ibm_provider.py

@@ -109,6 +109,10 @@ class IBMProvider(Provider):
 
         simulator_backend = provider.get_backend('ibmq_qasm_simulator')
 
+    IBMBackend's are uniquely identified by their name. If you invoke :meth:`get_backend()` twice,
+    you will get the same IBMBackend instance, and any previously updated options will be reset
+    to the default values.
+
     It is also possible to use the ``backend`` attribute to reference a backend.
     As an example, to retrieve the same backend from the example above::
 
@@ -660,6 +664,7 @@ def get_backend(
             )
         if not backends:
             raise QiskitBackendNotFoundError("No backend matches the criteria")
+        backends[0]._options = IBMBackend._default_options()
         return backends[0]
 
     def __repr__(self) -> str:

releasenotes/notes/reset_backend_options-f7e02c9bfff58213.yaml

@@ -0,0 +1,12 @@
+---
+fixes:
+  - |
+    ``IBMProvider.get_backend()`` returns the backend with its default options. At the end of this
+    .. code-block::
+
+      backend1 = provider.get_backend("xxx")
+      backend1.options.shots = 100
+      backend2 = provider.get_backend("xxx")
+
+    ``backend2.options.shots`` has the default value (4000). ``backend1.options.shots`` 
+    also has the default value, because backend1 and backend2 are the same instance.

test/integration/test_ibm_provider.py

@@ -202,6 +202,19 @@ def test_get_backend(self):
         backend = self.dependencies.provider.get_backend(name=self.backend_name)
         self.assertEqual(backend.name, self.backend_name)
 
+    def test_get_backend_options(self):
+        """Test resetting backend options when calling get_backend."""
+        default_shots = 4000
+        backend1 = self.dependencies.provider.get_backend(name=self.backend_name)
+        self.assertEqual(backend1.options.shots, default_shots)
+        backend1.options.shots = 100
+        self.assertEqual(backend1.options.shots, 100)
+        backend2 = self.dependencies.provider.get_backend(name=self.backend_name)
+        # When getting a backend, it has the default options.
+        # backend1 and backend2 are the same instance.
+        self.assertEqual(backend2.options.shots, default_shots)
+        self.assertEqual(backend1.options.shots, default_shots)
+
     def test_jobs_filter(self):
         """Test limit filters when accessing jobs from the provider."""
         num_jobs = PAGE_SIZE + 1