Fix some abstract class stuff

blocks/algorithms/__init__.py

@@ -38,7 +38,6 @@ def process_batch(self, batch):
         pass
 
 
-@add_metaclass(ABCMeta)
 class DifferentiableCostMinimizer(TrainingAlgorithm):
     """Minimizes a differentiable cost given as a Theano expression.
 
@@ -184,6 +183,7 @@ def process_batch(self, batch):
         self._function(*ordered_batch)
 
 
+@add_metaclass(ABCMeta)
 class StepRule(object):
     """A rule to compute a step for a gradient descent algorithm."""
     @abstractmethod

blocks/bricks/sequence_generators.py

@@ -13,7 +13,6 @@
 from blocks.utils import dict_subset, dict_union, update_instance
 
 
-@add_metaclass(ABCMeta)
 class BaseSequenceGenerator(Initializable):
     """A generic sequence generator.
 
@@ -243,6 +242,7 @@ def initial_state(self, name, batch_size, *args, **kwargs):
             assert False
 
 
+@add_metaclass(ABCMeta)
 class AbstractEmitter(Brick):
     """The interface for the emitter component of a readout."""
     @abstractmethod

blocks/datasets/__init__.py

@@ -130,7 +130,6 @@ def get_default_stream(self):
         return DataStream(self, iteration_scheme=self.default_scheme)
 
 
-@add_metaclass(ABCMeta)
 class ContainerDataset(Dataset):
     """Equips a Python container with the dataset interface.
 
@@ -173,6 +172,7 @@ def get_data(self, state, request=None):
         return next(state)
 
 
+@add_metaclass(ABCMeta)
 class AbstractDataStream(object):
     """A stream of data separated into epochs.
 

blocks/main_loop.py

@@ -1,13 +1,8 @@
 """The event-based main loop of Blocks."""
-from abc import ABCMeta
-
-from six import add_metaclass
-
 from blocks.log import TrainingLog
 from blocks.utils import update_instance
 
 
-@add_metaclass(ABCMeta)
 class MainLoop(object):
     """The standard main loop of Blocks.