fix symbolblock (#8050)

python/mxnet/gluon/block.py

@@ -487,8 +487,8 @@ def __init__(self, outputs, inputs, params=None):
         self._params = ParameterDict('', params)
         if isinstance(inputs, symbol.Symbol) and len(inputs.list_outputs()) == 1:
             inputs = [inputs]
-        if isinstance(outputs, symbol.Symbol) and len(outputs.list_outputs()) == 1:
-            outputs = [outputs]
+        if isinstance(outputs, (list, tuple)) and len(outputs) == 1:
+            outputs = outputs[0]
 
         syms, self._in_format = _flatten(inputs)
         out, self._out_format = _flatten(outputs)
@@ -523,7 +523,7 @@ def forward(self, x, *args):
         assert in_fmt == self._in_format, "Invalid input format"
         ret = copy.copy(self._cached_graph[1])
         ret._compose(**{k.name: v for k, v in zip(self._cached_graph[0], args)})
-        return _regroup(ret, self._out_format)[0]
+        return _regroup(list(ret), self._out_format)[0]
 
     def hybrid_forward(self, F, x, *args, **kwargs):
         raise NotImplementedError

tests/python/unittest/test_gluon.py

@@ -130,7 +130,27 @@ def test_symbol_block():
     assert len(smodel(mx.nd.zeros((16, 10)))) == 14
 
     out = smodel(mx.sym.var('in'))
-    assert len(out.get_internals().list_outputs()) == len(outputs.list_outputs())
+    assert len(out) == len(outputs.list_outputs())
+
+    class Net(nn.HybridBlock):
+        def __init__(self, model):
+            super(Net, self).__init__()
+            self.model = model
+
+        def hybrid_forward(self, F, x):
+            out = self.model(x)
+            return F.add_n(*[i.sum() for i in out])
+
+    net = Net(smodel)
+    net.hybridize()
+    assert isinstance(net(mx.nd.zeros((16, 10))), mx.nd.NDArray)
+
+    inputs = mx.sym.var('data')
+    outputs = model(inputs)
+    smodel = gluon.SymbolBlock(outputs, inputs, params=model.collect_params())
+    net = Net(smodel)
+    net.hybridize()
+    assert isinstance(net(mx.nd.zeros((16, 10))), mx.nd.NDArray)
 
 
 def check_layer_forward(layer, dshape):