Add 'yellow' as an alias of 'amber' for TrafficLights

Fixes #345

gpiozero/boards.py

@@ -536,11 +536,11 @@ class PiLiterBarGraph(LEDBarGraph):
 
 class TrafficLights(LEDBoard):
     """
-    Extends :class:`LEDBoard` for devices containing red, amber, and green
+    Extends :class:`LEDBoard` for devices containing red, yellow, and green
     LEDs.
 
     The following example initializes a device connected to GPIO pins 2, 3,
-    and 4, then lights the amber LED attached to GPIO 3::
+    and 4, then lights the amber (yellow) LED attached to GPIO 3::
 
         from gpiozero import TrafficLights
 
@@ -566,9 +566,20 @@ class TrafficLights(LEDBoard):
         ``None``, each device will be left in whatever state the pin is found
         in when configured for output (warning: this can be on). If ``True``,
         the device will be switched on initially.
+
+    :param int yellow:
+        The GPIO pin that the yellow LED is attached to. This is merely an
+        alias for the ``amber`` parameter - you can't specify both ``amber``
+        and ``yellow``.
     """
     def __init__(self, red=None, amber=None, green=None,
-                 pwm=False, initial_value=False):
+                 pwm=False, initial_value=False, yellow=None):
+        if amber is not None and yellow is not None:
+            raise OutputDeviceBadValue(
+                'Only one of amber or yellow can be specified'
+            )
+        if amber is None:
+            amber = yellow
         if not all(p is not None for p in [red, amber, green]):
             raise GPIOPinMissing(
                 'red, amber and green pins must be provided'
@@ -578,6 +589,16 @@ class TrafficLights(LEDBoard):
             pwm=pwm, initial_value=initial_value,
             _order=('red', 'amber', 'green'))
 
+    def __getattr__(self, name):
+        if name == 'yellow':
+            name = 'amber'
+        return super(TrafficLights, self).__getattr__(name)
+
+    def __setattr__(self, name, value):
+        if name == 'yellow':
+            name = 'amber'
+        return super(TrafficLights, self).__setattr__(name, value)
+
 
 class PiTraffic(TrafficLights):
     """

tests/test_boards.py

@@ -551,6 +551,11 @@ def test_traffic_lights():
         assert red_pin.state
         assert not amber_pin.state
         assert not green_pin.state
+    with TrafficLights(red=red_pin, yellow=amber_pin, green=green_pin) as board:
+        board.yellow.on()
+        assert not red_pin.state
+        assert amber_pin.state
+        assert not green_pin.state
 
 def test_traffic_lights_bad_init():
     with pytest.raises(ValueError):