Update options to be a mapping instead of a simple namespace

This commit changes the options class to be a mapping instead of a
simple namespace. While we wanted to preserve attribute based access
primarily the simple namespace is too limited in practice and involves a
lot of hacking around the internals such as frequent calls to
Options.__dict__ to get an iterator of fields in the options. To improve
the usability of the class without breaking existing users this changes
the base class to be a MutableMapping and adds the necessary methods to
make it usable like a dict or a simple namespace.

qiskit/providers/basicaer/qasm_simulator.py

@@ -125,7 +125,7 @@ def __init__(self, configuration=None, provider=None, **fields):
         self._shots = 0
         self._memory = False
         self._initial_statevector = self.options.get("initial_statevector")
-        self._chop_threshold = self.options.get("chop_threashold")
+        self._chop_threshold = self.options.get("chop_threshold")
         self._qobj_config = None
         # TEMP
         self._sample_measure = False

qiskit/providers/options.py

@@ -12,10 +12,10 @@
 
 """Container class for backend options."""
 
-import types
+from collections.abc import MutableMapping
 
 
-class Options(types.SimpleNamespace):
+class Options(MutableMapping):
     """Base options object
 
     This class is the abstract class that all backend options are based
@@ -26,10 +26,63 @@ class Options(types.SimpleNamespace):
     options.
     """
 
+    _dict = {}
+
+    def __init__(self, **kwargs):
+        self._dict = {}
+        super().__init__()
+        self._dict = kwargs
+
+    def __getattr__(self, field):
+        if field == "_dict":
+            super().__getattribute__(field)
+        elif field not in self._dict:
+            raise AttributeError("%s not a valid option" % field)
+        self._dict[field]
+
+    def __getitem__(self, field):
+        if field not in self._dict:
+            raise KeyError("%s not a valid option" % field)
+        self._dict.get(field, default)
+
+    def __setitem__(self, field, value):
+        if field in self._dict:
+            self._dict[field] = value
+        else:
+            raise KeyError("%s not a valid option" % field)
+
+    def __delitem__(self, field):
+        del self._dict[field]
+
+    def __len__(self):
+        return len(self._dict)
+
+    def __iter__(self):
+        return iter(self._dict)
+
+    def __setattr__(self, field, value):
+        if field == "_dict":
+            super().__setattr__(field, value)
+        elif field in self._dict:
+            self._dict[field] = value
+        else:
+            raise AttributeError("%s not a valid option" % field)
+
+    def __repr__(self):
+        return "Options(%s)" % ", ".join("k=v" for k, v in self._dict.items())
+
     def update_options(self, **fields):
         """Update options with kwargs"""
-        self.__dict__.update(fields)
+        self._dict.update(fields)
 
     def get(self, field, default=None):
         """Get an option value for a given key."""
-        return getattr(self, field, default)
+        return self._dict.get(field, default)
+
+    def __dict__(self):
+        return self._dict
+
+    def __eq__(self, other):
+        if isinstance(other, Options):
+            return self._dict == other._dict
+        return NotImplementedError