Fix #359, close #396

gpiozero/devices.py

@@ -7,6 +7,7 @@ from __future__ import (
 nstr = str
 str = type('')
 
+import os
 import atexit
 import weakref
 from collections import namedtuple
@@ -14,12 +15,16 @@ from itertools import chain
 from types import FunctionType
 from threading import RLock
 
+import pkg_resources
+
 from .threads import _threads_shutdown
+from .pins import _pins_shutdown
 from .mixins import (
     ValuesMixin,
     SharedMixin,
     )
 from .exc import (
+    BadPinFactory,
     DeviceClosed,
     CompositeDeviceBadName,
     CompositeDeviceBadOrder,
@@ -30,25 +35,32 @@ from .exc import (
     )
 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
-# available, however, we fall back to a pure Python implementation which
-# supports platforms like PyPy
-from .pins import _pins_shutdown
-try:
-    from .pins.rpigpio import RPiGPIOPin
-    pin_factory = RPiGPIOPin
-except ImportError:
-    try:
-        from .pins.rpio import RPIOPin
-        pin_factory = RPIOPin
-    except ImportError:
-        try:
-            from .pins.pigpiod import PiGPIOPin
-            pin_factory = PiGPIOPin
-        except ImportError:
-            from .pins.native import NativePin
-            pin_factory = NativePin
+
+def _default_pin_factory(name=os.getenv('GPIOZERO_PIN_FACTORY', None)):
+    group = 'gpiozero_pin_factories'
+    if name is None:
+        # If no factory is explicitly specified, try various names in
+        # "preferred" order. Note that in this case we only select from
+        # gpiozero distribution so without explicitly specifying a name (via
+        # the environment) it's impossible to auto-select a factory from
+        # outside the base distribution
+        #
+        # We prefer RPi.GPIO here as it supports PWM, and all Pi revisions.  If
+        # no third-party libraries are available, however, we fall back to a
+        # pure Python implementation which supports platforms like PyPy
+        dist = pkg_resources.get_distribution('gpiozero')
+        for name in ('RPiGPIOPin', 'RPIOPin', 'PiGPIOPin', 'NativePin'):
+            try:
+                return pkg_resources.load_entry_point(dist, group, name)
+            except ImportError:
+                pass
+        raise BadPinFactory('Unable to locate any default pin factory!')
+    else:
+        for factory in pkg_resources.iter_entry_points(group, name):
+            return factory.load()
+        raise BadPinFactory('Unable to locate pin factory "%s"' % name)
+
+pin_factory = _default_pin_factory()
 
 
 _PINS = set()

gpiozero/exc.py

@@ -22,6 +22,9 @@ class BadWaitTime(GPIOZeroError, ValueError):
 class BadQueueLen(GPIOZeroError, ValueError):
     "Error raised when non-positive queue length is specified"
 
+class BadPinFactory(GPIOZeroError, ImportError):
+    "Error raised when an unknown pin factory name is specified"
+
 class CompositeDeviceError(GPIOZeroError):
     "Base class for errors specific to the CompositeDevice hierarchy"
 

setup.py

@@ -61,6 +61,14 @@ if sys.version_info[:2] == (3, 2):
     __extra_requires__['test'][1] = 'coverage<4.0dev'
 
 __entry_points__ = {
+    'gpiozero_pin_factories': [
+        'PiGPIOPin  = gpiozero.pins.pigpiod:PiGPIOPin',
+        'RPiGPIOPin = gpiozero.pins.rpigpio:RPiGPIOPin',
+        'RPIOPin    = gpiozero.pins.rpio:RPIOPin',
+        'NativePin  = gpiozero.pins.native:NativePin',
+        'MockPin    = gpiozero.pins.mock:MockPin',
+        'MockPWMPin = gpiozero.pins.mock:MockPWMPin',
+    ],
 }