Raise error if two StagingInput are nested (#899)

tensorpack/input_source/input_source.py

@@ -29,7 +29,7 @@ __all__ = ['PlaceholderInput', 'FeedInput', 'FeedfreeInput',
            'QueueInput', 'BatchQueueInput',
            'DummyConstantInput', 'TensorInput',
            'ZMQInput', 'TFDatasetInput',
-           'StagingInputWrapper', 'StagingInput']
+           'StagingInput']
 
 
 def _get_reset_callback(df):
@@ -527,6 +527,8 @@ class StagingInput(FeedfreeInput):
     This means that in multi-GPU training, you should ensure that each call on `hooked_sess.run`
     depends on either all input tensors on all GPUs, or no input tensors at all.
     As a result you cannot use this InputSource for :class:`InferenceRunner`.
+
+    More than one StagingInput cannot be used together.
     """
     class StagingCallback(Callback):
         """
@@ -562,6 +564,7 @@ class StagingInput(FeedfreeInput):
             # Only step the stagingarea when the input is evaluated in this sess.run
             fetches = ctx.original_args.fetches
             if dependency_of_fetches(fetches, self._check_dependency_op):
+                # note: this disable nesting of StagingInput
                 return self.fetches
 
     def __init__(self, input, nr_stage=1, device=None):
@@ -576,6 +579,9 @@ class StagingInput(FeedfreeInput):
         """
         if not isinstance(input, FeedfreeInput):
             raise ValueError("StagingInput takes a FeedfreeInput! Got {}".format(input))
+        if isinstance(input, StagingInput):
+            raise ValueError("StagingInput cannot be nested!")
+
         self._input = input
 
         self._nr_stage = nr_stage
@@ -660,8 +666,3 @@ class StagingInput(FeedfreeInput):
             run_before=False,
             run_as_trigger=False,
             run_step=True)
-
-
-@deprecated("Renamed to StagingInput", "2018-08-01")
-def StagingInputWrapper(*args, **kwargs):
-    return StagingInput(*args, **kwargs)