diff --git a/imperative/python/megengine/core/tensor/utils.py b/imperative/python/megengine/core/tensor/utils.py
index 875147eb..2c2a309d 100644
--- a/imperative/python/megengine/core/tensor/utils.py
+++ b/imperative/python/megengine/core/tensor/utils.py
@@ -15,7 +15,6 @@ from .._imperative_rt.core2 import Tensor, apply, dtype_promotion, get_device
 from ..ops import builtin
 from ..ops.special import Const
 from .dtype import is_dtype_equal, is_quantize
-from .megbrain_graph import VarNode
 
 _enable_convert_inputs = True
 
@@ -60,7 +59,7 @@ def astype(x, dtype):
 
 
 def convert_single_value(v, *, dtype=None, device=None):
-    if isinstance(v, (Tensor, VarNode)):
+    if isinstance(v, Tensor):
         if not is_quantize(v.dtype):
             v = astype(v, dtype)
     else:
diff --git a/imperative/python/megengine/functional/elemwise.py b/imperative/python/megengine/functional/elemwise.py
index 158e826a..47de381d 100644
--- a/imperative/python/megengine/functional/elemwise.py
+++ b/imperative/python/megengine/functional/elemwise.py
@@ -14,7 +14,7 @@ import numpy as np
 from ..core._imperative_rt.core2 import apply
 from ..core.ops import builtin
 from ..core.ops.builtin import Elemwise
-from ..core.tensor import megbrain_graph, utils
+from ..core.tensor import utils
 from ..core.tensor.array_method import _elwise_apply
 from ..core.tensor.utils import isscalar, setscalar
 from ..device import get_default_device
@@ -77,9 +77,7 @@ __all__ = [
 
 
 def _elwise(*args, mode):
-    tensor_args = list(
-        filter(lambda x: isinstance(x, (Tensor, megbrain_graph.VarNode)), args)
-    )
+    tensor_args = list(filter(lambda x: isinstance(x, Tensor), args))
     if len(tensor_args) == 0:
         dtype = utils.dtype_promotion(args)
         first_arg = Tensor(args[0], dtype=dtype, device=get_default_device())
diff --git a/imperative/python/megengine/functional/nn.py b/imperative/python/megengine/functional/nn.py
index e3e52bbd..fb6d4ae5 100644
--- a/imperative/python/megengine/functional/nn.py
+++ b/imperative/python/megengine/functional/nn.py
@@ -15,7 +15,7 @@ from ..core._trace_option import use_symbolic_shape
 from ..core.ops import builtin
 from ..core.ops.builtin import BatchNorm
 from ..core.ops.special import Const
-from ..core.tensor import megbrain_graph, utils
+from ..core.tensor import utils
 from ..core.tensor.utils import astensor1d, setscalar
 from ..distributed import WORLD, is_distributed
 from ..jit.tracing import is_tracing
@@ -416,7 +416,6 @@ def adaptive_max_pool2d(
     :param oshp: `(OH, OW)` size of the output shape.
     :return: output tensor.
     """
-    assert isinstance(inp, (Tensor, megbrain_graph.VarNode)), "inp must be Tensor type"
     if isinstance(oshp, int):
         oshp = (oshp, oshp)
 
@@ -438,7 +437,6 @@ def adaptive_avg_pool2d(
     :param oshp: `(OH, OW)` size of the output shape.
     :return: output tensor.
     """
-    assert isinstance(inp, (Tensor, megbrain_graph.VarNode)), "inp must be Tensor type"
     if isinstance(oshp, int):
         oshp = (oshp, oshp)
 
@@ -1181,7 +1179,6 @@ def remap(
     op = builtin.Remap(
         imode=interp_mode, border_type=border_mode, format="NCHW", scalar=scalar
     )
-    assert isinstance(inp, (Tensor, megbrain_graph.VarNode)), "inp must be Tensor type"
     (result,) = apply(op, inp, map_xy)
     return result
 
@@ -1759,7 +1756,6 @@ def nvof(src: Tensor, precision: int = 1) -> Tensor:
         print(result.numpy())
 
     """
-    assert isinstance(src, (Tensor, megbrain_graph.VarNode)), "src must be Tensor type"
     assert src.ndim == 5 and src.shape[4] == 4
 
     src = src.detach()
diff --git a/imperative/python/megengine/quantization/utils.py b/imperative/python/megengine/quantization/utils.py
index f622ce0e..de1bb0fc 100644
--- a/imperative/python/megengine/quantization/utils.py
+++ b/imperative/python/megengine/quantization/utils.py
@@ -15,7 +15,6 @@ from .. import functional as F
 from ..autodiff import Function
 from ..core._imperative_rt.core2 import apply
 from ..core.ops import builtin
-from ..core.tensor import megbrain_graph
 from ..core.tensor.dtype import _metadata_dict
 from ..tensor import Tensor
 
@@ -94,14 +93,6 @@ def fake_quant_tensor(inp: Tensor, qmin: int, qmax: int, q_dict: Dict) -> Tensor
     if q_dict["mode"] == QuantMode.ASYMMERTIC:
         zero_point = q_dict["zero_point"]
 
-    assert isinstance(inp, (Tensor, megbrain_graph.VarNode)), "inp must be Tensor type"
-    assert isinstance(
-        scale, (Tensor, megbrain_graph.VarNode)
-    ), "scale must be Tensor type"
-    assert isinstance(
-        zero_point, (Tensor, megbrain_graph.VarNode)
-    ), "zero point must be Tensor type"
-
     op = builtin.FakeQuant(qmin=qmin, qmax=qmax)
     return apply(op, inp, scale, zero_point)[0]
 
diff --git a/imperative/python/src/tensor.cpp b/imperative/python/src/tensor.cpp
index 2fb3cefd..1cc7657a 100644
--- a/imperative/python/src/tensor.cpp
+++ b/imperative/python/src/tensor.cpp
@@ -12,6 +12,7 @@
 #include "megbrain/dtype.h"
 #include "megbrain/common.h"
 #include "megbrain/imperative/ops/utility.h"
+#include "megbrain/imperative/ops/backward_graph.h"
 
 #include "./tensor.h"
 #include "./grad.h"
@@ -156,7 +157,7 @@ PyObject* py_apply(PyObject* self, PyObject*const* args, size_t nargs/* , PyObje
         ctx.args = &tensors[0];
         ctx.nargs = nargs;
         ctx.pytype = pytype;
-        if (strstr(op->ob_type->tp_name, "BackwardGraph")) {
+        if (ctx.op->same_type<BackwardGraph>()) {
             ctx.backward = true;
         }
 
@@ -165,7 +166,9 @@ PyObject* py_apply(PyObject* self, PyObject*const* args, size_t nargs/* , PyObje
                 auto* t = tensors[i] = tw->m_tensor.get();
                 ctx.flags |= t->m_flags;
             } else {
-                PyErr_SetString(PyExc_TypeError, "expect Tensor");
+                PyErr_SetString(PyExc_TypeError,
+                    ssprintf("op %s expect type Tensor as inputs, got %s actually",
+                        ctx.op->make_name().c_str(), Py_TYPE(args[i])->tp_name).c_str());
                 return nullptr;
             }
         }