Fixing bug in qcstyle that modified custom style dict (#11874)

* fixed qcstyle bug that modified style dict, added test

* black

* ensure that the keys in nested_attrs are not overwritten again

* release notes

* Update releasenotes/notes/qcstyle-bug-custom-style-dicts-22deab6c602ccd6a.yaml

---------

Co-authored-by: Julien Gacon <gaconju@gmail.com>
(cherry picked from commit d21127a)

qiskit/visualization/circuit/qcstyle.py

@@ -96,11 +96,11 @@ def update(self, other):
         nested_attrs = {"displaycolor", "displaytext"}
         for attr in nested_attrs.intersection(other.keys()):
             if attr in self.keys():
-                self[attr].update(other.pop(attr))
+                self[attr].update(other[attr])
             else:
-                self[attr] = other.pop(attr)
+                self[attr] = other[attr]
 
-        super().update(other)
+        super().update((key, value) for key, value in other.items() if key not in nested_attrs)
 
 
 class DefaultStyle:

releasenotes/notes/qcstyle-bug-custom-style-dicts-22deab6c602ccd6a.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed bug in :meth:`.QuantumCircuit.draw` that was causing custom style dictionaries 
+    for the Matplotlib drawer to be modified upon execution.

test/visual/mpl/circuit/test_circuit_matplotlib_drawer.py

@@ -1010,15 +1010,16 @@ def test_user_style(self):
         circuit.barrier(5, 6)
         circuit.reset(5)
 
+        style = {
+            "name": "user_style",
+            "displaytext": {"H2": "H_2"},
+            "displaycolor": {"H2": ("#EEDD00", "#FF0000")},
+        }
         fname = "user_style.png"
         self.circuit_drawer(
             circuit,
             output="mpl",
-            style={
-                "name": "user_style",
-                "displaytext": {"H2": "H_2"},
-                "displaycolor": {"H2": ("#EEDD00", "#FF0000")},
-            },
+            style=style,
             filename=fname,
         )
 
@@ -1029,7 +1030,19 @@ def test_user_style(self):
             FAILURE_DIFF_DIR,
             FAILURE_PREFIX,
         )
-        self.assertGreaterEqual(ratio, self.threshold)
+
+        with self.subTest(msg="check image"):
+            self.assertGreaterEqual(ratio, self.threshold)
+
+        with self.subTest(msg="check style dict unchanged"):
+            self.assertEqual(
+                style,
+                {
+                    "name": "user_style",
+                    "displaytext": {"H2": "H_2"},
+                    "displaycolor": {"H2": ("#EEDD00", "#FF0000")},
+                },
+            )
 
     def test_subfont_change(self):
         """Tests changing the subfont size"""