Make CompositeDevice._named a frozendict (and add frozendict to compat.py)

This prevents it being modified post-construction (just like the way
CompositeDevice._all and CompositeDevice._order are already 'frozen' by being
tuples)

gpiozero/compat.py

@@ -9,6 +9,9 @@ from __future__ import (
 str = type('')
 
 import cmath
+import collections
+import operator
+import functools
 
 
 # Back-ported from python 3.5; see
@@ -51,3 +54,30 @@ def median(data):
         i = n // 2
         return (data[i - 1] + data[i]) / 2
 
+
+# Copied from the MIT-licensed https://github.com/slezica/python-frozendict
+class frozendict(collections.Mapping):
+    def __init__(self, *args, **kwargs):
+        self.__dict = dict(*args, **kwargs)
+        self.__hash = None
+
+    def __getitem__(self, key):
+        return self.__dict[key]
+
+    def copy(self, **add_or_replace):
+        return frozendict(self, **add_or_replace)
+
+    def __iter__(self):
+        return iter(self.__dict)
+
+    def __len__(self):
+        return len(self.__dict)
+
+    def __repr__(self):
+        return '<frozendict %s>' % repr(self.__dict)
+
+    def __hash__(self):
+        if self.__hash is None:
+            hashes = map(hash, self.items())
+            self.__hash = functools.reduce(operator.xor, hashes, 0)
+        return self.__hash

gpiozero/devices.py

@@ -28,6 +28,7 @@ from .exc import (
     GPIOPinInUse,
     GPIODeviceClosed,
     )
+from .compat import frozendict
 
 # Get a pin implementation to use as the default; we prefer RPi.GPIO's here
 # as it supports PWM, and all Pi revisions. If no third-party libraries are
@@ -263,7 +264,7 @@ class CompositeDevice(Device):
     """
     def __init__(self, *args, **kwargs):
         self._all = ()
-        self._named = {}
+        self._named = frozendict({})
         self._namedtuple = None
         self._order = kwargs.pop('_order', None)
         if self._order is None:
@@ -278,7 +279,7 @@ class CompositeDevice(Device):
         for dev in self._all:
             if not isinstance(dev, Device):
                 raise CompositeDeviceBadDevice("%s doesn't inherit from Device" % dev)
-        self._named = kwargs
+        self._named = frozendict(kwargs)
         self._namedtuple = namedtuple('%sValue' % self.__class__.__name__, chain(
             (str(i) for i in range(len(args))), self._order),
             rename=True)
@@ -286,7 +287,7 @@ class CompositeDevice(Device):
     def __getattr__(self, name):
         # if _named doesn't exist yet, pretend it's an empty dict
         if name == '_named':
-            return {}
+            return frozendict({})
         try:
             return self._named[name]
         except KeyError: