use canning hook in dependent

allows the dependency function to be called even before the real task arguments are unpacked.

IPython/parallel/controller/dependency.py

@@ -59,10 +59,12 b' class dependent(object):'
         self.df = df
         self.dargs = dargs
         self.dkwargs = dkwargs
-
-    def __call__(self, *args, **kwargs):
+    
+    def check_dependency(self):
         if self.df(*self.dargs, **self.dkwargs) is False:
             raise UnmetDependency()
+    
+    def __call__(self, *args, **kwargs):
         return self.f(*args, **kwargs)
     
     if not py3compat.PY3:

IPython/utils/pickleutil.py

@@ -64,7 +64,7 b' class CannedObject(object):'
         obj = self.obj
         for key in self.keys:
             setattr(obj, key, uncan(getattr(obj, key), g))
-
+        
         if self.hook:
             self.hook = uncan(self.hook, g)
             self.hook(obj, g)
@@ -308,6 +308,11 b' def uncan_sequence(obj, g=None):'
     else:
         return obj
 
+def _uncan_dependent_hook(dep, g=None):
+    dep.check_dependency()
+    
+def can_dependent(obj):
+    return CannedObject(obj, keys=('f', 'df'), hook=_uncan_dependent_hook)
 
 #-------------------------------------------------------------------------------
 # API dictionaries
@@ -316,7 +321,7 b' def uncan_sequence(obj, g=None):'
 # These dicts can be extended for custom serialization of new objects
 
 can_map = {
-    'IPython.parallel.dependent' : lambda obj: CannedObject(obj, keys=('f','df')),
+    'IPython.parallel.dependent' : can_dependent,
     'numpy.ndarray' : CannedArray,
     FunctionType : CannedFunction,
     bytes : CannedBytes,