diff --git a/python_module/megengine/core/tensor.py b/python_module/megengine/core/tensor.py
index 5856ff1db642fde7b96c40564d4fa197ae7eaf8c..8aefcb31d36c04aeba86d46daba066634790d3a8 100644
--- a/python_module/megengine/core/tensor.py
+++ b/python_module/megengine/core/tensor.py
@@ -425,7 +425,7 @@ class Tensor:
     def __getitem__(self, idx):
         return wrap_io_tensor(self._symvar.__getitem__)(_wrap_idx(idx))
 
-    def set_subtensor(self, val: "Tensor"):
+    def set_subtensor(self, val: "Tensor") -> _MGBIndexWrapper:
         r"""
         Return a object which supports using ``__getitem__`` to set subtensor.
 
@@ -433,7 +433,7 @@ class Tensor:
         """
         return _MGBIndexWrapper(self, mgb.opr.set_subtensor, val)
 
-    def incr_subtensor(self, val: "Tensor"):
+    def incr_subtensor(self, val: "Tensor") -> _MGBIndexWrapper:
         r"""
         Return a object which supports using ``__getitem__`` to increase subtensor.
 
@@ -442,7 +442,7 @@ class Tensor:
         return _MGBIndexWrapper(self, mgb.opr.incr_subtensor, val)
 
     @property
-    def ai(self):
+    def ai(self) -> _MGBIndexWrapper:
         r"""
         Return a object which supports complex index method to get subtensor.
 
@@ -465,20 +465,20 @@ class Tensor:
         """
         return _MGBIndexWrapper(self, mgb.opr.advanced_indexing)
 
-    def set_ai(self, val: "Tensor"):
+    def set_ai(self, val: "Tensor") -> _MGBIndexWrapper:
         r"""
         Equal to :meth:`~.Tensor.set_subtensor` which supports advanced indexing.
         """
         return _MGBIndexWrapper(self, mgb.opr.set_advanced_indexing, val)
 
-    def incr_ai(self, val: "Tensor"):
+    def incr_ai(self, val: "Tensor") -> _MGBIndexWrapper:
         r"""
         Equal to :meth:`~.Tensor.incr_subtensor` which supports advanced indexing.
         """
         return _MGBIndexWrapper(self, mgb.opr.incr_advanced_indexing, val)
 
     @property
-    def mi(self):
+    def mi(self) -> _MGBIndexWrapper:
         r"""
         Return a object which supports getting subtensor by
         the coordinates which is Cartesian product of given index.
@@ -502,20 +502,20 @@ class Tensor:
         """
         return _MGBIndexWrapper(self, mgb.opr.mesh_indexing)
 
-    def set_mi(self, val: "Tensor"):
+    def set_mi(self, val: "Tensor") -> _MGBIndexWrapper:
         r"""
         Equal to :meth:`~.Tensor.set_subtensor` which using mesh indexing.
         """
         return _MGBIndexWrapper(self, mgb.opr.set_mesh_indexing, val)
 
-    def incr_mi(self, val: "Tensor"):
+    def incr_mi(self, val: "Tensor") -> _MGBIndexWrapper:
         r"""
         Equal to :meth:`~.Tensor.incr_subtensor` which using mesh indexing.
         """
         return _MGBIndexWrapper(self, mgb.opr.incr_mesh_indexing, val)
 
     @property
-    def batched_mi(self):
+    def batched_mi(self) -> _MGBIndexWrapper:
         r"""
         Return a object which supports getting subtensor by
         batched mesh indexing.
@@ -555,13 +555,13 @@ class Tensor:
         """
         return _MGBIndexWrapper(self, mgb.opr.batched_mesh_indexing)
 
-    def batched_set_mi(self, val: "Tensor"):
+    def batched_set_mi(self, val: "Tensor") -> _MGBIndexWrapper:
         r"""
         Equal to :meth:`~.Tensor.incr_subtensor` which using batched mesh indexing.
         """
         return _MGBIndexWrapper(self, mgb.opr.batched_set_mesh_indexing, val)
 
-    def batched_incr_mi(self, val: "Tensor"):
+    def batched_incr_mi(self, val: "Tensor") -> _MGBIndexWrapper:
         r"""
         Equal to :meth:`~.Tensor.incr_subtensor` which using batched mesh indexing.
         """
@@ -680,18 +680,31 @@ def tensor(
     return Tensor(shared_nd, requires_grad=requires_grad)
 
 
-class Dict(collections.MutableMapping):
-    def __init__(self, *args, key=None, **kwargs):
+class TensorDict(collections.MutableMapping):
+    r"""
+    A helper class to maintain dict with Tensor key.
+    """
+
+    def __init__(self, *args, **kwargs):
         self.data = {}
-        if key:
-            self.keyfn = key
         for i in args:
             self.update(i)
         self.update(**kwargs)
 
-    @staticmethod
-    def keyfn(key):  # pylint: disable=method-hidden
-        return key
+    class keyfn:
+        def __new__(cls, x: Tensor):
+            if not isinstance(x, Tensor):
+                return x
+            return super().__new__(cls)
+
+        def __init__(self, x: Tensor):
+            self._data = x  # do not save id directly to make pickle work
+
+        def __hash__(self):
+            return id(self._data)
+
+        def __eq__(self, other):
+            return isinstance(other, type(self)) and id(self._data) == id(other._data)
 
     def __getitem__(self, key):
         _, v = self.data[self.keyfn(key)]
@@ -709,24 +722,3 @@ class Dict(collections.MutableMapping):
 
     def __len__(self):
         return len(self.data)
-
-
-class TensorDict(Dict):  # pylint: disable=too-many-ancestors
-    class keyfn:
-        def __new__(cls, x: Tensor):
-            if not isinstance(x, Tensor):
-                return x
-            return super().__new__(cls)
-
-        def __init__(self, x: Tensor):
-            self._data = x  # do not save id directly to make pickle work
-
-        def __hash__(self):
-            return id(self._data)
-
-        def __eq__(self, other):
-            # pylint: disable=undefined-variable
-            return isinstance(other, __class__) and id(self._data) == id(other._data)
-
-    def __init__(self, *args):
-        super().__init__(*args)