Add close method and context manager

Permit devices to be explicitly closed and used as context managers.
Also deal with cleanup properly at script end and ensure objects don't
step on the global cleanup function.

gpiozero/input_devices.py

@@ -9,7 +9,7 @@ from threading import Event
 from RPi import GPIO
 from w1thermsensor import W1ThermSensor
 
-from .devices import GPIODeviceError, GPIODevice, GPIOQueue
+from .devices import GPIODeviceError, GPIODeviceClosed, GPIODevice, GPIOQueue
 
 
 def _alias(key):
@@ -33,8 +33,12 @@ class InputDevice(GPIODevice):
                 'GPIO pins 2 and 3 are fitted with physical pull up '
                 'resistors; you cannot initialize them with pull_up=False'
             )
-        super(InputDevice, self).__init__(pin)
+        # _pull_up should be assigned first as __repr__ relies upon it to
+        # support the case where __repr__ is called during debugging of an
+        # instance that has failed to initialize (due to an exception in the
+        # super-class __init__)
         self._pull_up = pull_up
+        super(InputDevice, self).__init__(pin)
         self._active_edge = GPIO.FALLING if pull_up else GPIO.RISING
         self._inactive_edge = GPIO.RISING if pull_up else GPIO.FALLING
         self._active_state = GPIO.LOW if pull_up else GPIO.HIGH
@@ -59,8 +63,11 @@ class InputDevice(GPIODevice):
         return self._pull_up
 
     def __repr__(self):
-        return "<gpiozero.%s object on pin=%d, pull_up=%s, is_active=%s>" % (
-            self.__class__.__name__, self.pin, self.pull_up, self.is_active)
+        try:
+            return "<gpiozero.%s object on pin=%d, pull_up=%s, is_active=%s>" % (
+                self.__class__.__name__, self.pin, self.pull_up, self.is_active)
+        except:
+            return super(InputDevice, self).__repr__()
 
 
 class WaitableInputDevice(InputDevice):
@@ -174,9 +181,6 @@ class DigitalInputDevice(WaitableInputDevice):
         # Call _fire_events once to set initial state of events
         super(DigitalInputDevice, self)._fire_events()
 
-    def __del__(self):
-        GPIO.remove_event_detect(self.pin)
-
     def _fire_events(self, channel):
         super(DigitalInputDevice, self)._fire_events()
 
@@ -188,27 +192,50 @@ class SmoothedInputDevice(WaitableInputDevice):
     def __init__(
             self, pin=None, pull_up=False, threshold=0.5,
             queue_len=5, sample_wait=0.0, partial=False):
+        self._queue = None
         super(SmoothedInputDevice, self).__init__(pin, pull_up)
         self._queue = GPIOQueue(self, queue_len, sample_wait, partial)
         self.threshold = float(threshold)
 
+    def close(self):
+        try:
+            self._queue.stop()
+        except AttributeError:
+            if self._queue is not None:
+                raise
+        except RuntimeError:
+            # Cannot join thread before it starts; we don't care about this
+            # because we're trying to close the thread anyway
+            pass
+        else:
+            self._queue = None
+        super(SmoothedInputDevice, self).close()
+
     def __repr__(self):
-        if self.partial or self._queue.full.wait(0):
+        try:
+            self._check_open()
+        except GPIODeviceClosed:
             return super(SmoothedInputDevice, self).__repr__()
         else:
-            return "<gpiozero.%s object on pin=%d, pull_up=%s>" % (
-                self.__class__.__name__, self.pin, self.pull_up)
+            if self.partial or self._queue.full.wait(0):
+                return super(SmoothedInputDevice, self).__repr__()
+            else:
+                return "<gpiozero.%s object on pin=%d, pull_up=%s>" % (
+                    self.__class__.__name__, self.pin, self.pull_up)
 
     @property
     def queue_len(self):
+        self._check_open()
         return self._queue.queue.maxlen
 
     @property
     def partial(self):
+        self._check_open()
         return self._queue.partial
 
     @property
     def value(self):
+        self._check_open()
         return self._queue.value
 
     def _get_threshold(self):
@@ -284,9 +311,6 @@ class LightSensor(SmoothedInputDevice):
         )
         self._queue.start()
 
-    def __del__(self):
-        GPIO.remove_event_detect(self.pin)
-
     @property
     def charge_time_limit(self):
         return self._charge_time_limit