zheng-da opened a new issue #12154: the cond operator doesn't support a function that doesn't require inputs. URL: https://github.com/apache/incubator-mxnet/issues/12154 The python functions of the cond operator has to access some variables outside the function. ```python def test_no_inputs(): class TestBlock(gluon.HybridBlock): def __init__(self, prefix=None, params=None): super(TestBlock, self).__init__(prefix=prefix, params=params) def hybrid_forward(self, F, data): def then_func(): return F.ones((1)).expand_dims(0) def else_func(): return data return F.contrib.cond(data == 0, then_func, else_func) block = TestBlock() block.initialize(ctx=default_context()) block.hybridize() data = mx.nd.ones((1)).expand_dims(0) print(data) block(data) ``` The following code will run into an error as follows: ``` ====================================================================== ERROR: __main__.test_no_inputs ---------------------------------------------------------------------- Traceback (most recent call last): File "/usr/local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest self.test(*self.arg) File "tests/python/unittest/test_contrib_control_flow.py", line 47, in test_no_inputs block(data) File "/Users/dzzhen/Workspace/incubator-mxnet/python/mxnet/gluon/block.py", line 541, in __call__ out = self.forward(*args) File "/Users/dzzhen/Workspace/incubator-mxnet/python/mxnet/gluon/block.py", line 908, in forward return self._call_cached_op(x, *args) File "/Users/dzzhen/Workspace/incubator-mxnet/python/mxnet/gluon/block.py", line 814, in _call_cached_op out = self._cached_op(*cargs) File "/Users/dzzhen/Workspace/incubator-mxnet/python/mxnet/_ctypes/ndarray.py", line 150, in __call__ ctypes.byref(out_stypes))) File "/Users/dzzhen/Workspace/incubator-mxnet/python/mxnet/base.py", line 255, in check_call raise MXNetError(py_str(_LIB.MXGetLastError())) MXNetError: [16:00:03] src/imperative/cached_op.cc:127: Check failed: idx.input_nodes().size() >= 1 (0 vs. 1) CachedOp requires at least 1 input Stack trace returned 10 entries: [bt] (0) 0 libmxnet.so 0x00000001053cfe50 dmlc::StackTrace() + 272 [bt] (1) 1 libmxnet.so 0x00000001053cfbff dmlc::LogMessageFatal::~LogMessageFatal() + 47 [bt] (2) 2 libmxnet.so 0x000000010673209c mxnet::CachedOp::CachedOp(nnvm::Symbol const&, std::__1::vector<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > > const&) + 3164 [bt] (3) 3 libmxnet.so 0x00000001068d6865 mxnet::op::LoopState::MakeSharedOp(nnvm::Symbol const&) + 453 [bt] (4) 4 libmxnet.so 0x0000000106cbdb74 mxnet::op::LoopState::LoopState(nnvm::Symbol const&) + 164 [bt] (5) 5 libmxnet.so 0x00000001068da510 mxnet::op::CondState::CondState(mxnet::op::CondParam const&, nnvm::Symbol const&, nnvm::Symbol const&, nnvm::Symbol const&) + 64 [bt] (6) 6 libmxnet.so 0x00000001068da41b mxnet::OpStatePtr mxnet::OpStatePtr::Create<mxnet::op::CondState, mxnet::op::CondParam const&, nnvm::Symbol&, nnvm::Symbol&, nnvm::Symbol&>(mxnet::op::CondParam const&&&, nnvm::Symbol&&&, nnvm::Symbol&&&, nnvm::Symbol&&&) + 75 [bt] (7) 7 libmxnet.so 0x00000001068d1731 mxnet::op::CreateCondState(nnvm::NodeAttrs const&, mxnet::Context, std::__1::vector<nnvm::TShape, std::__1::allocator<nnvm::TShape> > const&, std::__1::vector<int, std::__1::allocator<int> > const&) + 49 [bt] (8) 8 libmxnet.so 0x0000000105550b4e std::__1::__function::__func<mxnet::OpStatePtr (*)(nnvm::NodeAttrs const&, mxnet::Context, std::__1::vector<nnvm::TShape, std::__1::allocator<nnvm::TShape> > const&, std::__1::vector<int, std::__1::allocator<int> > const&), std::__1::allocator<mxnet::OpStatePtr (*)(nnvm::NodeAttrs const&, mxnet::Context, std::__1::vector<nnvm::TShape, std::__1::allocator<nnvm::TShape> > const&, std::__1::vector<int, std::__1::allocator<int> > const&)>, mxnet::OpStatePtr (nnvm::NodeAttrs const&, mxnet::Context, std::__1::vector<nnvm::TShape, std::__1::allocator<nnvm::TShape> > const&, std::__1::vector<int, std::__1::allocator<int> > const&)>::operator()(nnvm::NodeAttrs const&, mxnet::Context&&, std::__1::vector<nnvm::TShape, std::__1::allocator<nnvm::TShape> > const&, std::__1::vector<int, std::__1::allocator<int> > const&) + 30 [bt] (9) 9 libmxnet.so 0x000000010676ca6e mxnet::imperative::RunGraph(bool, nnvm::IndexedGraph const&, std::__1::vector<mxnet::NDArray*, std::__1::allocator<mxnet::NDArray*> >, unsigned long, unsigned long, std::__1::vector<mxnet::OpReqType, std::__1::allocator<mxnet::OpReqType> >&&, std::__1::vector<unsigned int, std::__1::allocator<unsigned int> >&&, std::__1::vector<mxnet::OpStatePtr, std::__1::allocator<mxnet::OpStatePtr> >*, std::__1::vector<mxnet::DispatchMode, std::__1::allocator<mxnet::DispatchMode> > const&, bool) + 2462 ```
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