core: check that first argument to start() is not None

Previously this was handled by 'callback' setter,
but that one now accepts both callable and None, so
we need a separate check for None in start()

gevent/core.ppyx

@@ -580,6 +580,8 @@ cdef public class loop [object PyGeventLoopObject, type PyGeventLoop_Type]:
             return True if libev.ev_is_active(&self._watcher) else False
 
 #define START(TYPE) def start(self, object callback, *args):       \
+        if callback is None:                                       \
+            raise TypeError('callback must be callable, not None') \
         self.callback = callback                                   \
         self.args = args                                           \
         LIBEV_UNREF                                                \
@@ -649,6 +651,8 @@ cdef public class io(watcher) [object PyGeventIOObject, type PyGeventIO_Type]:
     WATCHER_BASE(io)
 
     def start(self, object callback, *args, pass_events=False):
+        if callback is None:
+            raise TypeError('callback must be callable, not None')
         self.callback = callback
         if pass_events:
             self.args = (GEVENT_CORE_EVENTS, ) + args
@@ -742,6 +746,8 @@ cdef public class timer(watcher) [object PyGeventTimerObject, type PyGeventTimer
     WATCHER_BASE(timer)
 
     def start(self, object callback, *args, update=True):
+        if callback is None:
+            raise TypeError('callback must be callable, not None')
         self.callback = callback
         self.args = args
         LIBEV_UNREF
@@ -934,6 +940,8 @@ cdef public class callback(watcher) [object PyGeventCallbackObject, type PyGeven
     INIT(prepare,,)
 
     def start(self, object callback, *args):
+        if callback is None:
+            raise TypeError('callback must be callable, not None')
         self.callback = callback
         self.args = args
         libev.ev_feed_event(self.loop._ptr, &self._watcher, libev.EV_CUSTOM)

greentest/test__core_watcher.py

@@ -17,8 +17,8 @@ class Test(unittest.TestCase):
         io.start(lambda *args: lst.append(args))
         self.assertEqual(io.args, ())
         try:
-            io.callback = None
-            raise AssertionError('"io.callback = None" must raise TypeError')
+            io.callback = False
+            raise AssertionError('"io.callback = False" must raise TypeError')
         except TypeError:
             pass
         try: