Fix #107

Add forward_device and backward_device to Motor, left_motor and
right_motor to Robot, and ensure all CompositeDevice descendents have a
proper close() method and closed property. Also, add a few more
_check_open calls around the place to make sure GPIODeviceClosed is
properly raised in response to read and writing values.

gpiozero/boards.py

@@ -31,6 +31,10 @@ class LEDBoard(SourceMixin, CompositeDevice):
         for led in self.leds:
             led.close()
 
+    @property
+    def closed(self):
+        return all(led.closed for led in self.leds)
+
     @property
     def leds(self):
         """
@@ -188,6 +192,10 @@ class TrafficLightsBuzzer(SourceMixin, CompositeDevice):
         self.buzzer.close()
         self.button.close()
 
+    @property
+    def closed(self):
+        return all(o.closed for o in self.all)
+
     @property
     def all(self):
         """
@@ -305,6 +313,24 @@ class Robot(SourceMixin, CompositeDevice):
         self._left.close()
         self._right.close()
 
+    @property
+    def closed(self):
+        return self._left.closed and self._right.closed
+
+    @property
+    def left_motor(self):
+        """
+        Returns the `Motor` device representing the robot's left motor.
+        """
+        return self._left
+
+    @property
+    def right_motor(self):
+        """
+        Returns the `Motor` device representing the robot's right motor.
+        """
+        return self._right
+
     @property
     def value(self):
         """

gpiozero/devices.py

@@ -82,8 +82,20 @@ class GPIOBase(object):
         # safely called from subclasses without worrying whether super-class'
         # have it (which in turn is useful in conjunction with the SourceMixin
         # class).
+        """
+        Shut down the device and release all associated resources.
+        """
         pass
 
+    @property
+    def closed(self):
+        """
+        Returns `True` if the device is closed (see the `close` method). Once a
+        device is closed you can no longer use any other methods or properties
+        to control or query the device.
+        """
+        return False
+
     def __enter__(self):
         return self
 
@@ -150,7 +162,8 @@ class CompositeDevice(ValuesMixin, GPIOBase):
     Represents a device composed of multiple GPIO devices like simple HATs,
     H-bridge motor controllers, robots composed of multiple motors, etc.
     """
-    pass
+    def __repr__(self):
+        return "<gpiozero.%s object>" % (self.__class__.__name__)
 
 
 class GPIODevice(ValuesMixin, GPIOBase):
@@ -199,15 +212,6 @@ class GPIODevice(ValuesMixin, GPIOBase):
             raise GPIODeviceClosed(
                 '%s is closed or uninitialized' % self.__class__.__name__)
 
-    @property
-    def closed(self):
-        """
-        Returns `True` if the device is closed (see the `close` method). Once a
-        device is closed you can no longer use any other methods or properties
-        to control or query the device.
-        """
-        return self._pin is None
-
     def close(self):
         """
         Shut down the device and release all associated resources.
@@ -254,6 +258,10 @@ class GPIODevice(ValuesMixin, GPIOBase):
                 GPIO.remove_event_detect(pin)
                 GPIO.cleanup(pin)
 
+    @property
+    def closed(self):
+        return self._pin is None
+
     @property
     def pin(self):
         """

gpiozero/output_devices.py

@@ -62,7 +62,11 @@ class OutputDevice(SourceMixin, GPIODevice):
             raise
 
     def _write(self, value):
-        GPIO.output(self.pin, bool(value))
+        try:
+            GPIO.output(self.pin, bool(value))
+        except ValueError:
+            self._check_open()
+            raise
 
     def on(self):
         """
@@ -236,12 +240,17 @@ class PWMOutputDevice(DigitalOutputDevice):
         super(PWMOutputDevice, self).close()
 
     def _read(self):
+        self._check_open()
         return self._value
 
     def _write(self, value):
         if not 0 <= value <= 1:
             raise OutputDeviceError("PWM value must be between 0 and 1")
-        self._pwm.ChangeDutyCycle(value * 100)
+        try:
+            self._pwm.ChangeDutyCycle(value * 100)
+        except AttributeError:
+            self._check_open()
+            raise
         self._value = value
 
     @property
@@ -385,6 +394,30 @@ class Motor(SourceMixin, CompositeDevice):
         self._forward = PWMOutputDevice(forward)
         self._backward = PWMOutputDevice(backward)
 
+    def close(self):
+        self._forward.close()
+        self._backward.close()
+
+    @property
+    def closed(self):
+        return self._forward.closed and self._backward.closed
+
+    @property
+    def forward_device(self):
+        """
+        Returns the `PWMOutputDevice` representing the forward pin of the motor
+        controller.
+        """
+        return self._forward
+
+    @property
+    def backward_device(self):
+        """
+        Returns the `PWMOutputDevice` representing the backward pin of the
+        motor controller.
+        """
+        return self._backward
+
     @property
     def value(self):
         """