diff --git a/examples/nas/.gitignore b/examples/nas/.gitignore
index 8705cba4d6..9ba06a7ca3 100644
--- a/examples/nas/.gitignore
+++ b/examples/nas/.gitignore
@@ -1 +1,2 @@
-data
+data
+checkpoints
diff --git a/examples/nas/darts/model.py b/examples/nas/darts/model.py
index 5c284b5a46..6a9afe6ff3 100644
--- a/examples/nas/darts/model.py
+++ b/examples/nas/darts/model.py
@@ -2,7 +2,7 @@
 import torch.nn as nn
 
 import ops
-from nni.nas.pytorch import mutables, darts
+from nni.nas.pytorch import mutables
 
 
 class AuxiliaryHead(nn.Module):
@@ -31,12 +31,14 @@ def forward(self, x):
         return logits
 
 
-class Node(darts.DartsNode):
-    def __init__(self, node_id, num_prev_nodes, channels, num_downsample_connect, drop_path_prob=0.):
-        super().__init__(node_id, limitation=2)
+class Node(nn.Module):
+    def __init__(self, node_id, num_prev_nodes, channels, num_downsample_connect):
+        super().__init__()
         self.ops = nn.ModuleList()
+        choice_keys = []
         for i in range(num_prev_nodes):
             stride = 2 if i < num_downsample_connect else 1
+            choice_keys.append("{}_p{}".format(node_id, i))
             self.ops.append(
                 mutables.LayerChoice(
                     [
@@ -48,18 +50,19 @@ def __init__(self, node_id, num_prev_nodes, channels, num_downsample_connect, dr
                         ops.DilConv(channels, channels, 3, stride, 2, 2, affine=False),
                         ops.DilConv(channels, channels, 5, stride, 4, 2, affine=False),
                     ],
-                    key="{}_p{}".format(node_id, i)))
-        self.drop_path = ops.DropPath_(drop_path_prob)
+                    key=choice_keys[-1]))
+        self.drop_path = ops.DropPath_()
+        self.input_switch = mutables.InputChoice(choose_from=choice_keys, n_chosen=2, key="{}_switch".format(node_id))
 
     def forward(self, prev_nodes):
         assert len(self.ops) == len(prev_nodes)
         out = [op(node) for op, node in zip(self.ops, prev_nodes)]
-        return sum(self.drop_path(o) for o in out if o is not None)
+        return self.input_switch(out)
 
 
 class Cell(nn.Module):
 
-    def __init__(self, n_nodes, channels_pp, channels_p, channels, reduction_p, reduction, drop_path_prob=0.):
+    def __init__(self, n_nodes, channels_pp, channels_p, channels, reduction_p, reduction):
         super().__init__()
         self.reduction = reduction
         self.n_nodes = n_nodes
@@ -74,10 +77,9 @@ def __init__(self, n_nodes, channels_pp, channels_p, channels, reduction_p, redu
 
         # generate dag
         self.mutable_ops = nn.ModuleList()
-        for depth in range(self.n_nodes):
-            self.mutable_ops.append(Node("r{:d}_n{}".format(reduction, depth),
-                                         depth + 2, channels, 2 if reduction else 0,
-                                         drop_path_prob=drop_path_prob))
+        for depth in range(2, self.n_nodes + 2):
+            self.mutable_ops.append(Node("{}_n{}".format("reduce" if reduction else "normal", depth),
+                                         depth, channels, 2 if reduction else 0))
 
     def forward(self, s0, s1):
         # s0, s1 are the outputs of previous previous cell and previous cell, respectively.
@@ -93,7 +95,7 @@ def forward(self, s0, s1):
 class CNN(nn.Module):
 
     def __init__(self, input_size, in_channels, channels, n_classes, n_layers, n_nodes=4,
-                 stem_multiplier=3, auxiliary=False, drop_path_prob=0.):
+                 stem_multiplier=3, auxiliary=False):
         super().__init__()
         self.in_channels = in_channels
         self.channels = channels
@@ -120,7 +122,7 @@ def __init__(self, input_size, in_channels, channels, n_classes, n_layers, n_nod
                 c_cur *= 2
                 reduction = True
 
-            cell = Cell(n_nodes, channels_pp, channels_p, c_cur, reduction_p, reduction, drop_path_prob=drop_path_prob)
+            cell = Cell(n_nodes, channels_pp, channels_p, c_cur, reduction_p, reduction)
             self.cells.append(cell)
             c_cur_out = c_cur * n_nodes
             channels_pp, channels_p = channels_p, c_cur_out
@@ -147,3 +149,8 @@ def forward(self, x):
         if aux_logits is not None:
             return logits, aux_logits
         return logits
+
+    def drop_path_prob(self, p):
+        for module in self.modules():
+            if isinstance(module, ops.DropPath_):
+                module.p = p
diff --git a/examples/nas/darts/retrain.py b/examples/nas/darts/retrain.py
new file mode 100644
index 0000000000..5c8fabf8d0
--- /dev/null
+++ b/examples/nas/darts/retrain.py
@@ -0,0 +1,143 @@
+import logging
+from argparse import ArgumentParser
+
+import torch
+import torch.nn as nn
+
+import datasets
+import utils
+from model import CNN
+from nni.nas.pytorch.fixed import apply_fixed_architecture
+from nni.nas.pytorch.utils import AverageMeter
+
+logging.basicConfig(level=logging.INFO)
+logger = logging.getLogger(__name__)
+device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
+
+
+def train(config, train_loader, model, optimizer, criterion, epoch):
+    top1 = AverageMeter("top1")
+    top5 = AverageMeter("top5")
+    losses = AverageMeter("losses")
+
+    cur_step = epoch * len(train_loader)
+    cur_lr = optimizer.param_groups[0]['lr']
+    logger.info("Epoch %d LR %.6f", epoch, cur_lr)
+
+    model.train()
+
+    for step, (x, y) in enumerate(train_loader):
+        x, y = x.to(device, non_blocking=True), y.to(device, non_blocking=True)
+        bs = x.size(0)
+
+        optimizer.zero_grad()
+        logits, aux_logits = model(x)
+        loss = criterion(logits, y)
+        if config.aux_weight > 0.:
+            loss += config.aux_weight * criterion(aux_logits, y)
+        loss.backward()
+        # gradient clipping
+        nn.utils.clip_grad_norm_(model.parameters(), config.grad_clip)
+        optimizer.step()
+
+        accuracy = utils.accuracy(logits, y, topk=(1, 5))
+        losses.update(loss.item(), bs)
+        top1.update(accuracy["acc1"], bs)
+        top5.update(accuracy["acc5"], bs)
+
+        if step % config.log_frequency == 0 or step == len(train_loader) - 1:
+            logger.info(
+                "Train: [{:3d}/{}] Step {:03d}/{:03d} Loss {losses.avg:.3f} "
+                "Prec@(1,5) ({top1.avg:.1%}, {top5.avg:.1%})".format(
+                    epoch + 1, config.epochs, step, len(train_loader) - 1, losses=losses,
+                    top1=top1, top5=top5))
+
+        cur_step += 1
+
+    logger.info("Train: [{:3d}/{}] Final Prec@1 {:.4%}".format(epoch + 1, config.epochs, top1.avg))
+
+
+def validate(config, valid_loader, model, criterion, epoch, cur_step):
+    top1 = AverageMeter("top1")
+    top5 = AverageMeter("top5")
+    losses = AverageMeter("losses")
+
+    model.eval()
+
+    with torch.no_grad():
+        for step, (X, y) in enumerate(valid_loader):
+            X, y = X.to(device, non_blocking=True), y.to(device, non_blocking=True)
+            N = X.size(0)
+
+            logits = model(X)
+            loss = criterion(logits, y)
+
+            accuracy = utils.accuracy(logits, y, topk=(1, 5))
+            losses.update(loss.item(), N)
+            top1.update(accuracy["acc1"], N)
+            top5.update(accuracy["acc5"], N)
+
+            if step % config.log_frequency == 0 or step == len(valid_loader) - 1:
+                logger.info(
+                    "Valid: [{:3d}/{}] Step {:03d}/{:03d} Loss {losses.avg:.3f} "
+                    "Prec@(1,5) ({top1.avg:.1%}, {top5.avg:.1%})".format(
+                        epoch + 1, config.epochs, step, len(valid_loader) - 1, losses=losses,
+                        top1=top1, top5=top5))
+
+    logger.info("Valid: [{:3d}/{}] Final Prec@1 {:.4%}".format(epoch + 1, config.epochs, top1.avg))
+
+    return top1.avg
+
+
+if __name__ == "__main__":
+    parser = ArgumentParser("darts")
+    parser.add_argument("--layers", default=20, type=int)
+    parser.add_argument("--batch-size", default=96, type=int)
+    parser.add_argument("--log-frequency", default=10, type=int)
+    parser.add_argument("--epochs", default=600, type=int)
+    parser.add_argument("--aux-weight", default=0.4, type=float)
+    parser.add_argument("--drop-path-prob", default=0.2, type=float)
+    parser.add_argument("--workers", default=4)
+    parser.add_argument("--grad-clip", default=5., type=float)
+    parser.add_argument("--arc-checkpoint", default="./checkpoints/epoch_0.json")
+
+    args = parser.parse_args()
+    dataset_train, dataset_valid = datasets.get_dataset("cifar10", cutout_length=16)
+
+    model = CNN(32, 3, 36, 10, args.layers, auxiliary=True)
+    apply_fixed_architecture(model, args.arc_checkpoint, device=device)
+    criterion = nn.CrossEntropyLoss()
+
+    model.to(device)
+    criterion.to(device)
+
+    optimizer = torch.optim.SGD(model.parameters(), 0.025, momentum=0.9, weight_decay=3.0E-4)
+    lr_scheduler = torch.optim.lr_scheduler.CosineAnnealingLR(optimizer, args.epochs, eta_min=1E-6)
+
+    train_loader = torch.utils.data.DataLoader(dataset_train,
+                                               batch_size=args.batch_size,
+                                               shuffle=True,
+                                               num_workers=args.workers,
+                                               pin_memory=True)
+    valid_loader = torch.utils.data.DataLoader(dataset_valid,
+                                               batch_size=args.batch_size,
+                                               shuffle=False,
+                                               num_workers=args.workers,
+                                               pin_memory=True)
+
+    best_top1 = 0.
+    for epoch in range(args.epochs):
+        drop_prob = args.drop_path_prob * epoch / args.epochs
+        model.drop_path_prob(drop_prob)
+
+        # training
+        train(args, train_loader, model, optimizer, criterion, epoch)
+
+        # validation
+        cur_step = (epoch + 1) * len(train_loader)
+        top1 = validate(args, valid_loader, model, criterion, epoch, cur_step)
+        best_top1 = max(best_top1, top1)
+
+        lr_scheduler.step()
+
+    logger.info("Final best Prec@1 = {:.4%}".format(best_top1))
diff --git a/examples/nas/darts/search.py b/examples/nas/darts/search.py
index 75773cf5e0..02c720a60c 100644
--- a/examples/nas/darts/search.py
+++ b/examples/nas/darts/search.py
@@ -13,7 +13,7 @@
 if __name__ == "__main__":
     parser = ArgumentParser("darts")
     parser.add_argument("--layers", default=8, type=int)
-    parser.add_argument("--batch-size", default=96, type=int)
+    parser.add_argument("--batch-size", default=64, type=int)
     parser.add_argument("--log-frequency", default=10, type=int)
     parser.add_argument("--epochs", default=50, type=int)
     args = parser.parse_args()
@@ -36,4 +36,4 @@
                            batch_size=args.batch_size,
                            log_frequency=args.log_frequency,
                            callbacks=[LearningRateScheduler(lr_scheduler), ArchitectureCheckpoint("./checkpoints")])
-    trainer.train_and_validate()
+    trainer.train()
diff --git a/examples/nas/enas/macro.py b/examples/nas/enas/macro.py
index 48fcaaf03d..a9309f9079 100644
--- a/examples/nas/enas/macro.py
+++ b/examples/nas/enas/macro.py
@@ -6,7 +6,7 @@
 
 class ENASLayer(mutables.MutableScope):
 
-    def __init__(self, key, num_prev_layers, in_filters, out_filters):
+    def __init__(self, key, prev_labels, in_filters, out_filters):
         super().__init__(key)
         self.in_filters = in_filters
         self.out_filters = out_filters
@@ -18,16 +18,16 @@ def __init__(self, key, num_prev_layers, in_filters, out_filters):
             PoolBranch('avg', in_filters, out_filters, 3, 1, 1),
             PoolBranch('max', in_filters, out_filters, 3, 1, 1)
         ])
-        if num_prev_layers > 0:
-            self.skipconnect = mutables.InputChoice(num_prev_layers, n_selected=None, reduction="sum")
+        if len(prev_labels) > 0:
+            self.skipconnect = mutables.InputChoice(choose_from=prev_labels, n_chosen=None, reduction="sum")
         else:
             self.skipconnect = None
         self.batch_norm = nn.BatchNorm2d(out_filters, affine=False)
 
-    def forward(self, prev_layers, prev_labels):
+    def forward(self, prev_layers):
         out = self.mutable(prev_layers[-1])
         if self.skipconnect is not None:
-            connection = self.skipconnect(prev_layers[:-1], tags=prev_labels)
+            connection = self.skipconnect(prev_layers[:-1])
             if connection is not None:
                 out += connection
         return self.batch_norm(out)
@@ -53,11 +53,12 @@ def __init__(self, num_layers=12, out_filters=24, in_channels=3, num_classes=10,
 
         self.layers = nn.ModuleList()
         self.pool_layers = nn.ModuleList()
+        labels = []
         for layer_id in range(self.num_layers):
+            labels.append("layer_{}".format(layer_id))
             if layer_id in self.pool_layers_idx:
                 self.pool_layers.append(FactorizedReduce(self.out_filters, self.out_filters))
-            self.layers.append(ENASLayer("layer_{}".format(layer_id), layer_id,
-                                         self.out_filters, self.out_filters))
+            self.layers.append(ENASLayer(labels[-1], labels[:-1], self.out_filters, self.out_filters))
 
         self.gap = nn.AdaptiveAvgPool2d(1)
         self.dense = nn.Linear(self.out_filters, self.num_classes)
@@ -66,12 +67,11 @@ def forward(self, x):
         bs = x.size(0)
         cur = self.stem(x)
 
-        layers, labels = [cur], []
+        layers = [cur]
 
         for layer_id in range(self.num_layers):
-            cur = self.layers[layer_id](layers, labels)
+            cur = self.layers[layer_id](layers)
             layers.append(cur)
-            labels.append(self.layers[layer_id].key)
             if layer_id in self.pool_layers_idx:
                 for i, layer in enumerate(layers):
                     layers[i] = self.pool_layers[self.pool_layers_idx.index(layer_id)](layer)
diff --git a/examples/nas/enas/micro.py b/examples/nas/enas/micro.py
index 209abf2405..fabd3919ca 100644
--- a/examples/nas/enas/micro.py
+++ b/examples/nas/enas/micro.py
@@ -32,9 +32,9 @@ def forward(self, x):
 
 
 class Cell(nn.Module):
-    def __init__(self, cell_name, num_prev_layers, channels):
+    def __init__(self, cell_name, prev_labels, channels):
         super().__init__()
-        self.input_choice = mutables.InputChoice(num_prev_layers, n_selected=1, return_mask=True,
+        self.input_choice = mutables.InputChoice(choose_from=prev_labels, n_chosen=1, return_mask=True,
                                                  key=cell_name + "_input")
         self.op_choice = mutables.LayerChoice([
             SepConvBN(channels, channels, 3, 1),
@@ -44,21 +44,21 @@ def __init__(self, cell_name, num_prev_layers, channels):
             nn.Identity()
         ], key=cell_name + "_op")
 
-    def forward(self, prev_layers, prev_labels):
-        chosen_input, chosen_mask = self.input_choice(prev_layers, tags=prev_labels)
+    def forward(self, prev_layers):
+        chosen_input, chosen_mask = self.input_choice(prev_layers)
         cell_out = self.op_choice(chosen_input)
         return cell_out, chosen_mask
 
 
 class Node(mutables.MutableScope):
-    def __init__(self, node_name, num_prev_layers, channels):
+    def __init__(self, node_name, prev_node_names, channels):
         super().__init__(node_name)
-        self.cell_x = Cell(node_name + "_x", num_prev_layers, channels)
-        self.cell_y = Cell(node_name + "_y", num_prev_layers, channels)
+        self.cell_x = Cell(node_name + "_x", prev_node_names, channels)
+        self.cell_y = Cell(node_name + "_y", prev_node_names, channels)
 
-    def forward(self, prev_layers, prev_labels):
-        out_x, mask_x = self.cell_x(prev_layers, prev_labels)
-        out_y, mask_y = self.cell_y(prev_layers, prev_labels)
+    def forward(self, prev_layers):
+        out_x, mask_x = self.cell_x(prev_layers)
+        out_y, mask_y = self.cell_y(prev_layers)
         return out_x + out_y, mask_x | mask_y
 
 
@@ -93,8 +93,11 @@ def __init__(self, num_nodes, in_channels_pp, in_channels_p, out_channels, reduc
 
         self.num_nodes = num_nodes
         name_prefix = "reduce" if reduction else "normal"
-        self.nodes = nn.ModuleList([Node("{}_node_{}".format(name_prefix, i),
-                                         i + 2, out_channels) for i in range(num_nodes)])
+        self.nodes = nn.ModuleList()
+        node_labels = [mutables.InputChoice.NO_KEY, mutables.InputChoice.NO_KEY]
+        for i in range(num_nodes):
+            node_labels.append("{}_node_{}".format(name_prefix, i))
+            self.nodes.append(Node(node_labels[-1], node_labels[:-1], out_channels))
         self.final_conv_w = nn.Parameter(torch.zeros(out_channels, self.num_nodes + 2, out_channels, 1, 1), requires_grad=True)
         self.bn = nn.BatchNorm2d(out_channels, affine=False)
         self.reset_parameters()
@@ -106,14 +109,12 @@ def forward(self, pprev, prev):
         pprev_, prev_ = self.preproc0(pprev), self.preproc1(prev)    
 
         prev_nodes_out = [pprev_, prev_]
-        prev_nodes_labels = ["prev1", "prev2"]
         nodes_used_mask = torch.zeros(self.num_nodes + 2, dtype=torch.bool, device=prev.device)
         for i in range(self.num_nodes):
-            node_out, mask = self.nodes[i](prev_nodes_out, prev_nodes_labels)
+            node_out, mask = self.nodes[i](prev_nodes_out)
             nodes_used_mask[:mask.size(0)] |= mask
             prev_nodes_out.append(node_out)
-            prev_nodes_labels.append(self.nodes[i].key)
-        
+
         unused_nodes = torch.cat([out for used, out in zip(nodes_used_mask, prev_nodes_out) if not used], 1)
         unused_nodes = F.relu(unused_nodes)
         conv_weight = self.final_conv_w[:, ~nodes_used_mask, :, :, :]
diff --git a/examples/nas/enas/search.py b/examples/nas/enas/search.py
index 6e1bdec34c..35bc930333 100644
--- a/examples/nas/enas/search.py
+++ b/examples/nas/enas/search.py
@@ -13,7 +13,7 @@
 if __name__ == "__main__":
     parser = ArgumentParser("enas")
     parser.add_argument("--batch-size", default=128, type=int)
-    parser.add_argument("--log-frequency", default=1, type=int)
+    parser.add_argument("--log-frequency", default=10, type=int)
     parser.add_argument("--search-for", choices=["macro", "micro"], default="macro")
     args = parser.parse_args()
 
@@ -43,5 +43,6 @@
                                num_epochs=num_epochs,
                                dataset_train=dataset_train,
                                dataset_valid=dataset_valid,
-                               log_frequency=args.log_frequency)
-    trainer.train_and_validate()
+                               log_frequency=args.log_frequency,
+                               mutator=mutator)
+    trainer.train()
diff --git a/src/sdk/pynni/nni/nas/pytorch/base_mutator.py b/src/sdk/pynni/nni/nas/pytorch/base_mutator.py
index dd2b844d24..550e449dfc 100644
--- a/src/sdk/pynni/nni/nas/pytorch/base_mutator.py
+++ b/src/sdk/pynni/nni/nas/pytorch/base_mutator.py
@@ -1,70 +1,127 @@
 import logging
 
 import torch.nn as nn
-
-from nni.nas.pytorch.mutables import Mutable
+from nni.nas.pytorch.mutables import Mutable, MutableScope, InputChoice
+from nni.nas.pytorch.utils import StructuredMutableTreeNode
 
 logger = logging.getLogger(__name__)
 
 
 class BaseMutator(nn.Module):
+    """
+    A mutator is responsible for mutating a graph by obtaining the search space from the network and implementing
+    callbacks that are called in ``forward`` in Mutables.
+    """
+
     def __init__(self, model):
         super().__init__()
         self.__dict__["model"] = model
-        self.before_parse_search_space()
-        self._parse_search_space()
-        self.after_parse_search_space()
-
-    def before_parse_search_space(self):
-        pass
-
-    def after_parse_search_space(self):
-        pass
-
-    def _parse_search_space(self):
-        for name, mutable, _ in self.named_mutables(distinct=False):
-            mutable.name = name
-            mutable.set_mutator(self)
+        self._structured_mutables = self._parse_search_space(self.model)
 
-    def named_mutables(self, root=None, distinct=True):
+    def _parse_search_space(self, module, root=None, prefix="", memo=None, nested_detection=None):
+        if memo is None:
+            memo = set()
         if root is None:
-            root = self.model
-        # if distinct is true, the method will filter out those with duplicated keys
-        key2module = dict()
-        for name, module in root.named_modules():
+            root = StructuredMutableTreeNode(None)
+        if module not in memo:
+            memo.add(module)
             if isinstance(module, Mutable):
-                module_distinct = False
-                if module.key in key2module:
-                    assert key2module[module.key].similar(module), \
-                        "Mutable \"{}\" that share the same key must be similar to each other".format(module.key)
-                else:
-                    module_distinct = True
-                    key2module[module.key] = module
-                if distinct:
-                    if module_distinct:
-                        yield name, module
-                else:
-                    yield name, module, module_distinct
-
-    def __setattr__(self, key, value):
-        if key in ["model", "net", "network"]:
-            logger.warning("Think twice if you are including the network into mutator.")
-        return super().__setattr__(key, value)
-
+                if nested_detection is not None:
+                    raise RuntimeError("Cannot have nested search space. Error at {} in {}"
+                                       .format(module, nested_detection))
+                module.name = prefix
+                module.set_mutator(self)
+                root = root.add_child(module)
+                if not isinstance(module, MutableScope):
+                    nested_detection = module
+                if isinstance(module, InputChoice):
+                    for k in module.choose_from:
+                        if k != InputChoice.NO_KEY and k not in [m.key for m in memo if isinstance(m, Mutable)]:
+                            raise RuntimeError("'{}' required by '{}' not found in keys that appeared before, and is not NO_KEY."
+                                               .format(k, module.key))
+            for name, submodule in module._modules.items():
+                if submodule is None:
+                    continue
+                submodule_prefix = prefix + ("." if prefix else "") + name
+                self._parse_search_space(submodule, root, submodule_prefix, memo=memo,
+                                         nested_detection=nested_detection)
+        return root
+
+    @property
+    def mutables(self):
+        return self._structured_mutables
+
+    @property
     def forward(self, *inputs):
-        raise NotImplementedError("Mutator is not forward-able")
+        raise RuntimeError("Forward is undefined for mutators.")
 
     def enter_mutable_scope(self, mutable_scope):
+        """
+        Callback when forward of a MutableScope is entered.
+
+        Parameters
+        ----------
+        mutable_scope: MutableScope
+
+        Returns
+        -------
+        None
+        """
         pass
 
     def exit_mutable_scope(self, mutable_scope):
+        """
+        Callback when forward of a MutableScope is exited.
+
+        Parameters
+        ----------
+        mutable_scope: MutableScope
+
+        Returns
+        -------
+        None
+        """
         pass
 
     def on_forward_layer_choice(self, mutable, *inputs):
+        """
+        Callbacks of forward in LayerChoice.
+
+        Parameters
+        ----------
+        mutable: LayerChoice
+        inputs: list of torch.Tensor
+
+        Returns
+        -------
+        tuple of torch.Tensor and torch.Tensor
+            output tensor and mask
+        """
         raise NotImplementedError
 
-    def on_forward_input_choice(self, mutable, tensor_list, tags):
+    def on_forward_input_choice(self, mutable, tensor_list):
+        """
+        Callbacks of forward in InputChoice.
+
+        Parameters
+        ----------
+        mutable: InputChoice
+        tensor_list: list of torch.Tensor
+
+        Returns
+        -------
+        tuple of torch.Tensor and torch.Tensor
+            output tensor and mask
+        """
         raise NotImplementedError
 
     def export(self):
+        """
+        Export the data of all decisions. This should output the decisions of all the mutables, so that the whole
+        network can be fully determined with these decisions for further training from scratch.
+
+        Returns
+        -------
+        dict
+        """
         raise NotImplementedError
diff --git a/src/sdk/pynni/nni/nas/pytorch/base_trainer.py b/src/sdk/pynni/nni/nas/pytorch/base_trainer.py
index 1248cc09e2..db1b033073 100644
--- a/src/sdk/pynni/nni/nas/pytorch/base_trainer.py
+++ b/src/sdk/pynni/nni/nas/pytorch/base_trainer.py
@@ -12,5 +12,9 @@ def validate(self):
         raise NotImplementedError
 
     @abstractmethod
-    def train_and_validate(self):
+    def export(self, file):
+        raise NotImplementedError
+
+    @abstractmethod
+    def checkpoint(self):
         raise NotImplementedError
diff --git a/src/sdk/pynni/nni/nas/pytorch/callbacks.py b/src/sdk/pynni/nni/nas/pytorch/callbacks.py
index 2a76b3dab8..83ae62cde0 100644
--- a/src/sdk/pynni/nni/nas/pytorch/callbacks.py
+++ b/src/sdk/pynni/nni/nas/pytorch/callbacks.py
@@ -1,9 +1,6 @@
-import json
 import logging
 import os
 
-import torch
-
 _logger = logging.getLogger(__name__)
 
 
@@ -44,26 +41,11 @@ def on_epoch_end(self, epoch):
 
 
 class ArchitectureCheckpoint(Callback):
-    class TorchTensorEncoder(json.JSONEncoder):
-        def default(self, o):  # pylint: disable=method-hidden
-            if isinstance(o, torch.Tensor):
-                olist = o.tolist()
-                if "bool" not in o.type().lower() and all(map(lambda d: d == 0 or d == 1, olist)):
-                    _logger.warning("Every element in %s is either 0 or 1. "
-                                    "You might consider convert it into bool.", olist)
-                return olist
-            return super().default(o)
-
     def __init__(self, checkpoint_dir, every="epoch"):
         super().__init__()
         assert every == "epoch"
         self.checkpoint_dir = checkpoint_dir
         os.makedirs(self.checkpoint_dir, exist_ok=True)
 
-    def _export_to_file(self, file):
-        mutator_export = self.mutator.export()
-        with open(file, "w") as f:
-            json.dump(mutator_export, f, indent=2, sort_keys=True, cls=self.TorchTensorEncoder)
-
     def on_epoch_end(self, epoch):
-        self._export_to_file(os.path.join(self.checkpoint_dir, "epoch_{}.json".format(epoch)))
+        self.trainer.export(os.path.join(self.checkpoint_dir, "epoch_{}.json".format(epoch)))
diff --git a/src/sdk/pynni/nni/nas/pytorch/darts/__init__.py b/src/sdk/pynni/nni/nas/pytorch/darts/__init__.py
index 7f2c9f9675..3bf08d285c 100644
--- a/src/sdk/pynni/nni/nas/pytorch/darts/__init__.py
+++ b/src/sdk/pynni/nni/nas/pytorch/darts/__init__.py
@@ -1,3 +1,2 @@
 from .mutator import DartsMutator
-from .trainer import DartsTrainer
-from .scope import DartsNode
\ No newline at end of file
+from .trainer import DartsTrainer
\ No newline at end of file
diff --git a/src/sdk/pynni/nni/nas/pytorch/darts/mutator.py b/src/sdk/pynni/nni/nas/pytorch/darts/mutator.py
index 589847d2b6..91d739c0a3 100644
--- a/src/sdk/pynni/nni/nas/pytorch/darts/mutator.py
+++ b/src/sdk/pynni/nni/nas/pytorch/darts/mutator.py
@@ -2,35 +2,47 @@
 from torch import nn as nn
 from torch.nn import functional as F
 
-from nni.nas.pytorch.mutables import LayerChoice
 from nni.nas.pytorch.mutator import Mutator
-from .scope import DartsNode
+from nni.nas.pytorch.mutables import LayerChoice, InputChoice
 
 
 class DartsMutator(Mutator):
-
-    def after_parse_search_space(self):
+    def __init__(self, model):
+        super().__init__(model)
         self.choices = nn.ParameterDict()
-        for _, mutable in self.named_mutables():
+        for mutable in self.mutables:
             if isinstance(mutable, LayerChoice):
-                self.choices[mutable.key] = nn.Parameter(1.0E-3 * torch.randn(len(mutable) + 1))
+                self.choices[mutable.key] = nn.Parameter(1.0E-3 * torch.randn(mutable.length + 1))
 
-    def on_calc_layer_choice_mask(self, mutable: LayerChoice):
-        return F.softmax(self.choices[mutable.key], dim=-1)[:-1]
+    def device(self):
+        for v in self.choices.values():
+            return v.device
 
-    def export(self):
-        result = super().export()
-        for _, darts_node in self.named_mutables():
-            if isinstance(darts_node, DartsNode):
-                keys, edges_max = [], []  # key of all the layer choices in current node, and their best edge weight
-                for _, choice in self.named_mutables(darts_node):
-                    if isinstance(choice, LayerChoice):
-                        keys.append(choice.key)
-                        max_val, index = torch.max(result[choice.key], 0)
-                        edges_max.append(max_val)
-                        result[choice.key] = F.one_hot(index, num_classes=len(result[choice.key])).view(-1).bool()
-                _, topk_edge_indices = torch.topk(torch.tensor(edges_max).view(-1), darts_node.limitation)  # pylint: disable=not-callable
-                for i, key in enumerate(keys):
-                    if i not in topk_edge_indices:
-                        result[key] = torch.zeros_like(result[key])
+    def sample_search(self):
+        result = dict()
+        for mutable in self.mutables:
+            if isinstance(mutable, LayerChoice):
+                result[mutable.key] = F.softmax(self.choices[mutable.key], dim=-1)[:-1]
+            elif isinstance(mutable, InputChoice):
+                result[mutable.key] = torch.ones(mutable.n_candidates, dtype=torch.bool, device=self.device())
+        return result
+
+    def sample_final(self):
+        result = dict()
+        edges_max = dict()
+        for mutable in self.mutables:
+            if isinstance(mutable, LayerChoice):
+                max_val, index = torch.max(F.softmax(self.choices[mutable.key], dim=-1)[:-1], 0)
+                edges_max[mutable.key] = max_val
+                result[mutable.key] = F.one_hot(index, num_classes=mutable.length).view(-1).bool()
+        for mutable in self.mutables:
+            if isinstance(mutable, InputChoice):
+                weights = torch.tensor([edges_max.get(src_key, 0.) for src_key in mutable.choose_from])  # pylint: disable=not-callable
+                _, topk_edge_indices = torch.topk(weights, mutable.n_chosen or mutable.n_candidates)
+                selected_multihot = []
+                for i, src_key in enumerate(mutable.choose_from):
+                    if i not in topk_edge_indices and src_key in result:
+                        result[src_key] = torch.zeros_like(result[src_key])  # clear this choice to optimize calc graph
+                    selected_multihot.append(i in topk_edge_indices)
+                result[mutable.key] = torch.tensor(selected_multihot, dtype=torch.bool, device=self.device())  # pylint: disable=not-callable
         return result
diff --git a/src/sdk/pynni/nni/nas/pytorch/darts/scope.py b/src/sdk/pynni/nni/nas/pytorch/darts/scope.py
deleted file mode 100644
index a2bf2b3cff..0000000000
--- a/src/sdk/pynni/nni/nas/pytorch/darts/scope.py
+++ /dev/null
@@ -1,11 +0,0 @@
-from nni.nas.pytorch.mutables import MutableScope
-
-
-class DartsNode(MutableScope):
-    """
-    At most `limitation` choice is activated in a `DartsNode` when exporting.
-    """
-
-    def __init__(self, key, limitation):
-        super().__init__(key)
-        self.limitation = limitation
diff --git a/src/sdk/pynni/nni/nas/pytorch/darts/trainer.py b/src/sdk/pynni/nni/nas/pytorch/darts/trainer.py
index 464832eadf..c6b29de04a 100644
--- a/src/sdk/pynni/nni/nas/pytorch/darts/trainer.py
+++ b/src/sdk/pynni/nni/nas/pytorch/darts/trainer.py
@@ -4,7 +4,7 @@
 from torch import nn as nn
 
 from nni.nas.pytorch.trainer import Trainer
-from nni.nas.utils import AverageMeterGroup
+from nni.nas.pytorch.utils import AverageMeterGroup
 from .mutator import DartsMutator
 
 
@@ -13,9 +13,9 @@ def __init__(self, model, loss, metrics,
                  optimizer, num_epochs, dataset_train, dataset_valid,
                  mutator=None, batch_size=64, workers=4, device=None, log_frequency=None,
                  callbacks=None):
-        super().__init__(model, loss, metrics, optimizer, num_epochs,
-                         dataset_train, dataset_valid, batch_size, workers, device, log_frequency,
-                         mutator if mutator is not None else DartsMutator(model), callbacks)
+        super().__init__(model, mutator if mutator is not None else DartsMutator(model),
+                         loss, metrics, optimizer, num_epochs, dataset_train, dataset_valid,
+                         batch_size, workers, device, log_frequency, callbacks)
         self.ctrl_optim = torch.optim.Adam(self.mutator.parameters(), 3.0E-4, betas=(0.5, 0.999),
                                            weight_decay=1.0E-3)
         n_train = len(self.dataset_train)
@@ -31,6 +31,9 @@ def __init__(self, model, loss, metrics,
                                                         batch_size=batch_size,
                                                         sampler=valid_sampler,
                                                         num_workers=workers)
+        self.test_loader = torch.utils.data.DataLoader(self.dataset_valid,
+                                                       batch_size=batch_size,
+                                                       num_workers=workers)
 
     def train_one_epoch(self, epoch):
         self.model.train()
@@ -47,8 +50,8 @@ def train_one_epoch(self, epoch):
 
             # phase 1. child network step
             self.optimizer.zero_grad()
-            with self.mutator.forward_pass():
-                logits = self.model(trn_X)
+            self.mutator.reset()
+            logits = self.model(trn_X)
             loss = self.loss(logits, trn_y)
             loss.backward()
             # gradient clipping
@@ -76,10 +79,10 @@ def validate_one_epoch(self, epoch):
         self.mutator.eval()
         meters = AverageMeterGroup()
         with torch.no_grad():
-            for step, (X, y) in enumerate(self.valid_loader):
+            self.mutator.reset()
+            for step, (X, y) in enumerate(self.test_loader):
                 X, y = X.to(self.device), y.to(self.device)
-                with self.mutator.forward_pass():
-                    logits = self.model(X)
+                logits = self.model(X)
                 metrics = self.metrics(logits, y)
                 meters.update(metrics)
                 if self.log_frequency is not None and step % self.log_frequency == 0:
@@ -93,8 +96,8 @@ def _unrolled_backward(self, trn_X, trn_y, val_X, val_y, backup_model, lr):
         v_model: backup model before this step
         lr: learning rate for virtual gradient step (same as net lr)
         """
-        with self.mutator.forward_pass():
-            loss = self.loss(self.model(val_X), val_y)
+        self.mutator.reset()
+        loss = self.loss(self.model(val_X), val_y)
         w_model = tuple(self.model.parameters())
         w_ctrl = tuple(self.mutator.parameters())
         w_grads = torch.autograd.grad(loss, w_model + w_ctrl)
@@ -125,8 +128,8 @@ def _compute_hessian(self, model, dw, trn_X, trn_y):
                 for p, d in zip(self.model.parameters(), dw):
                     p += eps * d
 
-            with self.mutator.forward_pass():
-                loss = self.loss(self.model(trn_X), trn_y)
+            self.mutator.reset()
+            loss = self.loss(self.model(trn_X), trn_y)
             if e > 0:
                 dalpha_pos = torch.autograd.grad(loss, self.mutator.parameters())  # dalpha { L_trn(w+) }
             elif e < 0:
diff --git a/src/sdk/pynni/nni/nas/pytorch/enas/mutator.py b/src/sdk/pynni/nni/nas/pytorch/enas/mutator.py
index 3bd32459b4..9d9a176352 100644
--- a/src/sdk/pynni/nni/nas/pytorch/enas/mutator.py
+++ b/src/sdk/pynni/nni/nas/pytorch/enas/mutator.py
@@ -2,8 +2,8 @@
 import torch.nn as nn
 import torch.nn.functional as F
 
-from nni.nas.pytorch.mutables import LayerChoice
 from nni.nas.pytorch.mutator import Mutator
+from nni.nas.pytorch.mutables import LayerChoice, InputChoice, MutableScope
 
 
 class StackedLSTMCell(nn.Module):
@@ -27,15 +27,14 @@ def forward(self, inputs, hidden):
 class EnasMutator(Mutator):
     def __init__(self, model, lstm_size=64, lstm_num_layers=1, tanh_constant=1.5, cell_exit_extra_step=False,
                  skip_target=0.4, branch_bias=0.25):
+        super().__init__(model)
         self.lstm_size = lstm_size
         self.lstm_num_layers = lstm_num_layers
         self.tanh_constant = tanh_constant
         self.cell_exit_extra_step = cell_exit_extra_step
         self.skip_target = skip_target
         self.branch_bias = branch_bias
-        super().__init__(model)
 
-    def before_parse_search_space(self):
         self.lstm = StackedLSTMCell(self.lstm_num_layers, self.lstm_size, False)
         self.attn_anchor = nn.Linear(self.lstm_size, self.lstm_size, bias=False)
         self.attn_query = nn.Linear(self.lstm_size, self.lstm_size, bias=False)
@@ -45,9 +44,8 @@ def before_parse_search_space(self):
         self.cross_entropy_loss = nn.CrossEntropyLoss(reduction="none")
         self.bias_dict = nn.ParameterDict()
 
-    def after_parse_search_space(self):
         self.max_layer_choice = 0
-        for _, mutable in self.named_mutables():
+        for mutable in self.mutables:
             if isinstance(mutable, LayerChoice):
                 if self.max_layer_choice == 0:
                     self.max_layer_choice = mutable.length
@@ -64,8 +62,29 @@ def is_conv(choice):
         self.embedding = nn.Embedding(self.max_layer_choice + 1, self.lstm_size)
         self.soft = nn.Linear(self.lstm_size, self.max_layer_choice, bias=False)
 
-    def before_pass(self):
-        super().before_pass()
+    def sample_search(self):
+        self._initialize()
+        self._sample(self.mutables)
+        return self._choices
+
+    def sample_final(self):
+        return self.sample_search()
+
+    def _sample(self, tree):
+        mutable = tree.mutable
+        if isinstance(mutable, LayerChoice) and mutable.key not in self._choices:
+            self._choices[mutable.key] = self._sample_layer_choice(mutable)
+        elif isinstance(mutable, InputChoice) and mutable.key not in self._choices:
+            self._choices[mutable.key] = self._sample_input_choice(mutable)
+        for child in tree.children:
+            self._sample(child)
+        if isinstance(mutable, MutableScope) and mutable.key not in self._anchors_hid:
+            if self.cell_exit_extra_step:
+                self._lstm_next_step()
+            self._mark_anchor(mutable.key)
+
+    def _initialize(self):
+        self._choices = dict()
         self._anchors_hid = dict()
         self._inputs = self.g_emb.data
         self._c = [torch.zeros((1, self.lstm_size),
@@ -84,7 +103,7 @@ def _lstm_next_step(self):
     def _mark_anchor(self, key):
         self._anchors_hid[key] = self._h[-1]
 
-    def on_calc_layer_choice_mask(self, mutable):
+    def _sample_layer_choice(self, mutable):
         self._lstm_next_step()
         logit = self.soft(self._h[-1])
         if self.tanh_constant is not None:
@@ -94,14 +113,14 @@ def on_calc_layer_choice_mask(self, mutable):
         branch_id = torch.multinomial(F.softmax(logit, dim=-1), 1).view(-1)
         log_prob = self.cross_entropy_loss(logit, branch_id)
         self.sample_log_prob += torch.sum(log_prob)
-        entropy = (log_prob * torch.exp(-log_prob)).detach()
+        entropy = (log_prob * torch.exp(-log_prob)).detach()  # pylint: disable=invalid-unary-operand-type
         self.sample_entropy += torch.sum(entropy)
         self._inputs = self.embedding(branch_id)
         return F.one_hot(branch_id, num_classes=self.max_layer_choice).bool().view(-1)
 
-    def on_calc_input_choice_mask(self, mutable, tags):
+    def _sample_input_choice(self, mutable):
         query, anchors = [], []
-        for label in tags:
+        for label in mutable.choose_from:
             if label not in self._anchors_hid:
                 self._lstm_next_step()
                 self._mark_anchor(label)  # empty loop, fill not found
@@ -113,8 +132,8 @@ def on_calc_input_choice_mask(self, mutable, tags):
         if self.tanh_constant is not None:
             query = self.tanh_constant * torch.tanh(query)
 
-        if mutable.n_selected is None:
-            logit = torch.cat([-query, query], 1)
+        if mutable.n_chosen is None:
+            logit = torch.cat([-query, query], 1)  # pylint: disable=invalid-unary-operand-type
 
             skip = torch.multinomial(F.softmax(logit, dim=-1), 1).view(-1)
             skip_prob = torch.sigmoid(logit)
@@ -123,19 +142,14 @@ def on_calc_input_choice_mask(self, mutable, tags):
             log_prob = self.cross_entropy_loss(logit, skip)
             self._inputs = (torch.matmul(skip.float(), torch.cat(anchors, 0)) / (1. + torch.sum(skip))).unsqueeze(0)
         else:
-            assert mutable.n_selected == 1, "Input choice must select exactly one or any in ENAS."
+            assert mutable.n_chosen == 1, "Input choice must select exactly one or any in ENAS."
             logit = query.view(1, -1)
             index = torch.multinomial(F.softmax(logit, dim=-1), 1).view(-1)
-            skip = F.one_hot(index).view(-1)
+            skip = F.one_hot(index, num_classes=mutable.n_candidates).view(-1)
             log_prob = self.cross_entropy_loss(logit, index)
             self._inputs = anchors[index.item()]
 
         self.sample_log_prob += torch.sum(log_prob)
-        entropy = (log_prob * torch.exp(-log_prob)).detach()
+        entropy = (log_prob * torch.exp(-log_prob)).detach()  # pylint: disable=invalid-unary-operand-type
         self.sample_entropy += torch.sum(entropy)
         return skip.bool()
-
-    def exit_mutable_scope(self, mutable_scope):
-        if self.cell_exit_extra_step:
-            self._lstm_next_step()
-        self._mark_anchor(mutable_scope.key)
diff --git a/src/sdk/pynni/nni/nas/pytorch/enas/trainer.py b/src/sdk/pynni/nni/nas/pytorch/enas/trainer.py
index 7d3e493782..1ed302ac7b 100644
--- a/src/sdk/pynni/nni/nas/pytorch/enas/trainer.py
+++ b/src/sdk/pynni/nni/nas/pytorch/enas/trainer.py
@@ -2,7 +2,7 @@
 import torch.optim as optim
 
 from nni.nas.pytorch.trainer import Trainer
-from nni.nas.utils import AverageMeterGroup
+from nni.nas.pytorch.utils import AverageMeterGroup
 from .mutator import EnasMutator
 
 
@@ -12,9 +12,9 @@ def __init__(self, model, loss, metrics, reward_function,
                  mutator=None, batch_size=64, workers=4, device=None, log_frequency=None, callbacks=None,
                  entropy_weight=0.0001, skip_weight=0.8, baseline_decay=0.999,
                  mutator_lr=0.00035, mutator_steps_aggregate=20, mutator_steps=50, aux_weight=0.4):
-        super().__init__(model, loss, metrics, optimizer, num_epochs,
-                         dataset_train, dataset_valid, batch_size, workers, device, log_frequency,
-                         mutator if mutator is not None else EnasMutator(model), callbacks)
+        super().__init__(model, mutator if mutator is not None else EnasMutator(model),
+                         loss, metrics, optimizer, num_epochs, dataset_train, dataset_valid,
+                         batch_size, workers, device, log_frequency, callbacks)
         self.reward_function = reward_function
         self.mutator_optim = optim.Adam(self.mutator.parameters(), lr=mutator_lr)
 
@@ -52,8 +52,9 @@ def train_one_epoch(self, epoch):
             x, y = x.to(self.device), y.to(self.device)
             self.optimizer.zero_grad()
 
-            with self.mutator.forward_pass():
-                logits = self.model(x)
+            with torch.no_grad():
+                self.mutator.reset()
+            logits = self.model(x)
 
             if isinstance(logits, tuple):
                 logits, aux_logits = logits
@@ -81,7 +82,8 @@ def train_one_epoch(self, epoch):
             for step, (x, y) in enumerate(self.valid_loader):
                 x, y = x.to(self.device), y.to(self.device)
 
-                with self.mutator.forward_pass():
+                self.mutator.reset()
+                with torch.no_grad():
                     logits = self.model(x)
                 metrics = self.metrics(logits, y)
                 reward = self.reward_function(logits, y)
@@ -107,9 +109,9 @@ def train_one_epoch(self, epoch):
                     self.mutator_optim.zero_grad()
 
                 if self.log_frequency is not None and step % self.log_frequency == 0:
-                    print("Mutator Epoch [{}/{}] Step [{}/{}]  {}".format(epoch, self.num_epochs,
-                                                                          mutator_step // self.mutator_steps_aggregate,
-                                                                          self.mutator_steps, meters))
+                    print("RL Epoch [{}/{}] Step [{}/{}]  {}".format(epoch, self.num_epochs,
+                                                                     mutator_step // self.mutator_steps_aggregate,
+                                                                     self.mutator_steps, meters))
                 mutator_step += 1
                 if mutator_step >= total_mutator_steps:
                     break
diff --git a/src/sdk/pynni/nni/nas/pytorch/fixed.py b/src/sdk/pynni/nni/nas/pytorch/fixed.py
index 526d66b610..6b83aa0800 100644
--- a/src/sdk/pynni/nni/nas/pytorch/fixed.py
+++ b/src/sdk/pynni/nni/nas/pytorch/fixed.py
@@ -2,10 +2,12 @@
 
 import torch
 
+from nni.nas.pytorch.mutables import MutableScope
 from nni.nas.pytorch.mutator import Mutator
 
 
 class FixedArchitecture(Mutator):
+
     def __init__(self, model, fixed_arc, strict=True):
         """
         Initialize a fixed architecture mutator.
@@ -20,39 +22,57 @@ def __init__(self, model, fixed_arc, strict=True):
             Force everything that appears in `fixed_arc` to be used at least once.
         """
         super().__init__(model)
-        if isinstance(fixed_arc, str):
-            with open(fixed_arc, "r") as f:
-                fixed_arc = json.load(f.read())
         self._fixed_arc = fixed_arc
-        self._strict = strict
-
-    def _encode_tensor(self, data):
-        if isinstance(data, list):
-            if all(map(lambda o: isinstance(o, bool), data)):
-                return torch.tensor(data, dtype=torch.bool)  # pylint: disable=not-callable
-            else:
-                return torch.tensor(data, dtype=torch.float)  # pylint: disable=not-callable
-        if isinstance(data, dict):
-            return {k: self._encode_tensor(v) for k, v in data.items()}
-        return data
-
-    def before_pass(self):
-        self._unused_key = set(self._fixed_arc.keys())
-
-    def after_pass(self):
-        if self._strict:
-            if self._unused_key:
-                raise ValueError("{} are never used by the network. "
-                                 "Set strict=False if you want to disable this check.".format(self._unused_key))
-
-    def _check_key(self, key):
-        if key not in self._fixed_arc:
-            raise ValueError("\"{}\" is demanded by the network, but not found in saved architecture.".format(key))
-
-    def on_calc_layer_choice_mask(self, mutable):
-        self._check_key(mutable.key)
-        return self._fixed_arc[mutable.key]
-
-    def on_calc_input_choice_mask(self, mutable, tags):
-        self._check_key(mutable.key)
-        return self._fixed_arc[mutable.key]
+
+        mutable_keys = set([mutable.key for mutable in self.mutables if not isinstance(mutable, MutableScope)])
+        fixed_arc_keys = set(self._fixed_arc.keys())
+        if fixed_arc_keys - mutable_keys:
+            raise RuntimeError("Unexpected keys found in fixed architecture: {}.".format(fixed_arc_keys - mutable_keys))
+        if mutable_keys - fixed_arc_keys:
+            raise RuntimeError("Missing keys in fixed architecture: {}.".format(mutable_keys - fixed_arc_keys))
+
+    def sample_search(self):
+        return self._fixed_arc
+
+    def sample_final(self):
+        return self._fixed_arc
+
+
+def _encode_tensor(data, device):
+    if isinstance(data, list):
+        if all(map(lambda o: isinstance(o, bool), data)):
+            return torch.tensor(data, dtype=torch.bool, device=device)  # pylint: disable=not-callable
+        else:
+            return torch.tensor(data, dtype=torch.float, device=device)  # pylint: disable=not-callable
+    if isinstance(data, dict):
+        return {k: _encode_tensor(v, device) for k, v in data.items()}
+    return data
+
+
+def apply_fixed_architecture(model, fixed_arc_path, device=None):
+    """
+    Load architecture from `fixed_arc_path` and apply to model.
+
+    Parameters
+    ----------
+    model: torch.nn.Module
+        Model with mutables.
+    fixed_arc_path: str
+        Path to the JSON that stores the architecture.
+    device: torch.device
+        Architecture weights will be transfered to `device`.
+
+    Returns
+    -------
+    FixedArchitecture
+    """
+
+    if device is None:
+        device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
+    if isinstance(fixed_arc_path, str):
+        with open(fixed_arc_path, "r") as f:
+            fixed_arc = json.load(f)
+    fixed_arc = _encode_tensor(fixed_arc, device)
+    architecture = FixedArchitecture(model, fixed_arc)
+    architecture.to(device)
+    architecture.reset()
diff --git a/src/sdk/pynni/nni/nas/pytorch/mutables.py b/src/sdk/pynni/nni/nas/pytorch/mutables.py
index 16b73b903d..79cde1cf3f 100644
--- a/src/sdk/pynni/nni/nas/pytorch/mutables.py
+++ b/src/sdk/pynni/nni/nas/pytorch/mutables.py
@@ -1,6 +1,6 @@
 import torch.nn as nn
 
-from nni.nas.utils import global_mutable_counting
+from nni.nas.pytorch.utils import global_mutable_counting
 
 
 class Mutable(nn.Module):
@@ -37,7 +37,7 @@ def set_mutator(self, mutator):
         self.__dict__["mutator"] = mutator
 
     def forward(self, *inputs):
-        raise NotImplementedError("Mutable forward must be implemented.")
+        raise NotImplementedError
 
     @property
     def key(self):
@@ -51,9 +51,6 @@ def name(self):
     def name(self, name):
         self._name = name
 
-    def similar(self, other):
-        return type(self) == type(other)
-
     def _check_built(self):
         if not hasattr(self, "mutator"):
             raise ValueError(
@@ -66,19 +63,17 @@ def __repr__(self):
 
 class MutableScope(Mutable):
     """
-    Mutable scope labels a subgraph to help mutators make better decisions. Mutators get notified when a mutable scope
-    is entered and exited. Mutators can override ``enter_mutable_scope`` and ``exit_mutable_scope`` to catch
-    corresponding events, and do status dump or update.
+    Mutable scope labels a subgraph/submodule to help mutators make better decisions.
+    Mutators get notified when a mutable scope is entered and exited. Mutators can override ``enter_mutable_scope``
+    and ``exit_mutable_scope`` to catch corresponding events, and do status dump or update.
     """
 
     def __init__(self, key):
         super().__init__(key=key)
 
-    def build(self):
-        self.mutator.on_init_mutable_scope(self)
-
     def __call__(self, *args, **kwargs):
         try:
+            self._check_built()
             self.mutator.enter_mutable_scope(self)
             return super().__call__(*args, **kwargs)
         finally:
@@ -93,43 +88,92 @@ def __init__(self, op_candidates, reduction="mean", return_mask=False, key=None)
         self.reduction = reduction
         self.return_mask = return_mask
 
-    def __len__(self):
-        return len(self.choices)
-
     def forward(self, *inputs):
         out, mask = self.mutator.on_forward_layer_choice(self, *inputs)
         if self.return_mask:
             return out, mask
         return out
 
-    def similar(self, other):
-        return type(self) == type(other) and self.length == other.length
-
 
 class InputChoice(Mutable):
-    def __init__(self, n_candidates, n_selected=None, reduction="mean", return_mask=False, key=None):
+    """
+    Input choice selects `n_chosen` inputs from `choose_from` (contains `n_candidates` keys). For beginners,
+    use `n_candidates` instead of `choose_from` is a safe option. To get the most power out of it, you might want to
+    know about `choose_from`.
+
+    The keys in `choose_from` can be keys that appear in past mutables, or ``NO_KEY`` if there are no suitable ones.
+    The keys are designed to be the keys of the sources. To help mutators make better decisions,
+    mutators might be interested in how the tensors to choose from come into place. For example, the tensor is the
+    output of some operator, some node, some cell, or some module. If this operator happens to be a mutable (e.g.,
+    ``LayerChoice`` or ``InputChoice``), it has a key naturally that can be used as a source key. If it's a
+    module/submodule, it needs to be annotated with a key: that's where a ``MutableScope`` is needed.
+    """
+
+    NO_KEY = ""
+
+    def __init__(self, n_candidates=None, choose_from=None, n_chosen=None,
+                 reduction="mean", return_mask=False, key=None):
+        """
+        Initialization.
+
+        Parameters
+        ----------
+        n_candidates: int
+            Number of inputs to choose from.
+        choose_from: list of str
+            List of source keys to choose from. At least of one of `choose_from` and `n_candidates` must be fulfilled.
+            If `n_candidates` has a value but `choose_from` is None, it will be automatically treated as `n_candidates`
+            number of empty string.
+        n_chosen: int
+            Recommended inputs to choose. If None, mutator is instructed to select any.
+        reduction: str
+            `mean`, `concat`, `sum` or `none`.
+        return_mask: bool
+            If `return_mask`, return output tensor and a mask. Otherwise return tensor only.
+        key: str
+            Key of the input choice.
+        """
         super().__init__(key=key)
+        # precondition check
+        assert n_candidates is not None or choose_from is not None, "At least one of `n_candidates` and `choose_from`" \
+                                                                    "must be not None."
+        if choose_from is not None and n_candidates is None:
+            n_candidates = len(choose_from)
+        elif choose_from is None and n_candidates is not None:
+            choose_from = [self.NO_KEY] * n_candidates
+        assert n_candidates == len(choose_from), "Number of candidates must be equal to the length of `choose_from`."
         assert n_candidates > 0, "Number of candidates must be greater than 0."
+        assert n_chosen is None or 0 <= n_chosen <= n_candidates, "Expected selected number must be None or no more " \
+                                                                  "than number of candidates."
+
         self.n_candidates = n_candidates
-        self.n_selected = n_selected
+        self.choose_from = choose_from
+        self.n_chosen = n_chosen
         self.reduction = reduction
         self.return_mask = return_mask
 
-    def build(self):
-        self.mutator.on_init_input_choice(self)
-
-    def forward(self, optional_inputs, tags=None):
+    def forward(self, optional_inputs):
+        """
+        Forward method of LayerChoice.
+
+        Parameters
+        ----------
+        optional_inputs: list or dict
+            Recommended to be a dict. As a dict, inputs will be converted to a list that follows the order of
+            `choose_from` in initialization. As a list, inputs must follow the semantic order that is the same as
+            `choose_from`.
+
+        Returns
+        -------
+        tuple of torch.Tensor and torch.Tensor or torch.Tensor
+        """
+        optional_input_list = optional_inputs
+        if isinstance(optional_inputs, dict):
+            optional_input_list = [optional_inputs[tag] for tag in self.choose_from]
+        assert isinstance(optional_input_list, list), "Optional input list must be a list"
         assert len(optional_inputs) == self.n_candidates, \
             "Length of the input list must be equal to number of candidates."
-        if tags is None:
-            tags = [""] * self.n_candidates
-        else:
-            assert len(tags) == self.n_candidates, "Length of tags must be equal to number of candidates."
-        out, mask = self.mutator.on_forward_input_choice(self, optional_inputs, tags)
+        out, mask = self.mutator.on_forward_input_choice(self, optional_input_list)
         if self.return_mask:
             return out, mask
         return out
-
-    def similar(self, other):
-        return type(self) == type(other) and \
-               self.n_candidates == other.n_candidates and self.n_selected and other.n_selected
diff --git a/src/sdk/pynni/nni/nas/pytorch/mutator.py b/src/sdk/pynni/nni/nas/pytorch/mutator.py
index 21d39545e7..80608c6925 100644
--- a/src/sdk/pynni/nni/nas/pytorch/mutator.py
+++ b/src/sdk/pynni/nni/nas/pytorch/mutator.py
@@ -1,46 +1,59 @@
-from contextlib import contextmanager
-
 import torch
-import torch.nn as nn
 
 from nni.nas.pytorch.base_mutator import BaseMutator
 
 
-class Mutator(BaseMutator, nn.Module):
+class Mutator(BaseMutator):
 
-    def export(self):
-        if self._in_forward_pass:
-            raise RuntimeError("Still in forward pass. Exporting might induce incompleteness.")
-        if not self._cache:
-            raise RuntimeError("No running history found. You need to call your model at least once before exporting. "
-                               "You might also want to check if there are no valid mutables in your model.")
-        return self._cache
-
-    @contextmanager
-    def forward_pass(self):
-        self._in_forward_pass = True
+    def __init__(self, model):
+        super().__init__(model)
         self._cache = dict()
-        self.before_pass()
-        try:
-            yield self
-        finally:
-            self.after_pass()
-            self._in_forward_pass = False
 
-    def before_pass(self):
-        pass
+    def sample_search(self):
+        """
+        Override to implement this method to iterate over mutables and make decisions.
+
+        Returns
+        -------
+        dict
+            A mapping from key of mutables to decisions.
+        """
+        raise NotImplementedError
+
+    def sample_final(self):
+        """
+        Override to implement this method to iterate over mutables and make decisions that is final
+        for export and retraining.
 
-    def after_pass(self):
-        pass
+        Returns
+        -------
+        dict
+            A mapping from key of mutables to decisions.
+        """
+        raise NotImplementedError
 
-    def _check_in_forward_pass(self):
-        if not hasattr(self, "_in_forward_pass") or not self._in_forward_pass:
-            raise ValueError("Not in forward pass. Did you forget to call mutator.forward_pass(), or forget to call "
-                             "super().before_pass() and after_pass() in your override method?")
+    def reset(self):
+        """
+        Reset the mutator by call the `sample_search` to resample (for search).
+
+        Returns
+        -------
+        None
+        """
+        self._cache = self.sample_search()
+
+    def export(self):
+        """
+        Resample (for final) and return results.
+
+        Returns
+        -------
+        dict
+        """
+        return self.sample_final()
 
     def on_forward_layer_choice(self, mutable, *inputs):
         """
-        Callback of layer choice forward. Override if you are an advanced user.
         On default, this method calls :meth:`on_calc_layer_choice_mask` to get a mask on how to choose between layers
         (either by switch or by weights), then it will reduce the list of all tensor outputs with the policy specified
         in `mutable.reduction`. It will also cache the mask with corresponding `mutable.key`.
@@ -54,18 +67,17 @@ def on_forward_layer_choice(self, mutable, *inputs):
         -------
         tuple of torch.Tensor and torch.Tensor
         """
-        self._check_in_forward_pass()
 
         def _map_fn(op, *inputs):
             return op(*inputs)
 
-        mask = self._cache.setdefault(mutable.key, self.on_calc_layer_choice_mask(mutable))
+        mask = self._get_decision(mutable)
+        assert len(mask) == len(mutable.choices)
         out = self._select_with_mask(_map_fn, [(choice, *inputs) for choice in mutable.choices], mask)
         return self._tensor_reduction(mutable.reduction, out), mask
 
-    def on_forward_input_choice(self, mutable, tensor_list, tags):
+    def on_forward_input_choice(self, mutable, tensor_list):
         """
-        Callback of input choice forward. Override if you are an advanced user.
         On default, this method calls :meth:`on_calc_input_choice_mask` with `tags`
         to get a mask on how to choose between inputs (either by switch or by weights), then it will reduce
         the list of all tensor outputs with the policy specified in `mutable.reduction`. It will also cache the
@@ -81,48 +93,11 @@ def on_forward_input_choice(self, mutable, tensor_list, tags):
         -------
         tuple of torch.Tensor and torch.Tensor
         """
-        self._check_in_forward_pass()
-        mask = self._cache.setdefault(mutable.key, self.on_calc_input_choice_mask(mutable, tags))
+        mask = self._get_decision(mutable)
+        assert len(mask) == mutable.n_candidates
         out = self._select_with_mask(lambda x: x, [(t,) for t in tensor_list], mask)
         return self._tensor_reduction(mutable.reduction, out), mask
 
-    def on_calc_layer_choice_mask(self, mutable):
-        """
-        Recommended to override. Calculate a mask tensor for a layer choice.
-
-        Parameters
-        ----------
-        mutable: LayerChoice
-            Corresponding layer choice object.
-
-        Returns
-        -------
-        torch.Tensor
-            Should be a 1D tensor, either float or bool. If float, the numbers are treated as weights. If bool,
-            the numbers are treated as switch.
-        """
-        raise NotImplementedError("Layer choice mask calculation must be implemented")
-
-    def on_calc_input_choice_mask(self, mutable, tags):
-        """
-        Recommended to override. Calculate a mask tensor for a input choice.
-
-        Parameters
-        ----------
-        mutable: InputChoice
-            Corresponding input choice object.
-        tags: list of string
-            The name of labels of input tensors given by user. Usually it's a
-            :class:`~nni.nas.pytorch.mutables.MutableScope` marked by user.
-
-        Returns
-        -------
-        torch.Tensor
-            Should be a 1D tensor, either float or bool. If float, the numbers are treated as weights. If bool,
-            the numbers are treated as switch.
-        """
-        raise NotImplementedError("Input choice mask calculation must be implemented")
-
     def _select_with_mask(self, map_fn, candidates, mask):
         if "BoolTensor" in mask.type():
             out = [map_fn(*cand) for cand, m in zip(candidates, mask) if m]
@@ -146,3 +121,20 @@ def _tensor_reduction(self, reduction_type, tensor_list):
         if reduction_type == "concat":
             return torch.cat(tensor_list, dim=1)
         raise ValueError("Unrecognized reduction policy: \"{}\"".format(reduction_type))
+
+    def _get_decision(self, mutable):
+        """
+        By default, this method checks whether `mutable.key` is already in the decision cache,
+        and returns the result without double-check.
+
+        Parameters
+        ----------
+        mutable: Mutable
+
+        Returns
+        -------
+        any
+        """
+        if mutable.key not in self._cache:
+            raise ValueError("\"{}\" not found in decision cache.".format(mutable.key))
+        return self._cache[mutable.key]
diff --git a/src/sdk/pynni/nni/nas/pytorch/pdarts/mutator.py b/src/sdk/pynni/nni/nas/pytorch/pdarts/mutator.py
index 6e385b1170..da31b3cc69 100644
--- a/src/sdk/pynni/nni/nas/pytorch/pdarts/mutator.py
+++ b/src/sdk/pynni/nni/nas/pytorch/pdarts/mutator.py
@@ -11,14 +11,14 @@
 
 class PdartsMutator(DartsMutator):
 
-    def __init__(self, model, pdarts_epoch_index, pdarts_num_to_drop, switches=None):
+    def __init__(self, pdarts_epoch_index, pdarts_num_to_drop, switches=None):
         self.pdarts_epoch_index = pdarts_epoch_index
         self.pdarts_num_to_drop = pdarts_num_to_drop
         self.switches = switches
 
-        super(PdartsMutator, self).__init__(model)
+        super(PdartsMutator, self).__init__()
 
-    def before_build(self, model):
+    def before_build(self):
         self.choices = nn.ParameterDict()
         if self.switches is None:
             self.switches = {}
diff --git a/src/sdk/pynni/nni/nas/pytorch/pdarts/trainer.py b/src/sdk/pynni/nni/nas/pytorch/pdarts/trainer.py
index 4d9c231143..d4ef2bbb8e 100644
--- a/src/sdk/pynni/nni/nas/pytorch/pdarts/trainer.py
+++ b/src/sdk/pynni/nni/nas/pytorch/pdarts/trainer.py
@@ -35,8 +35,7 @@ def train(self):
             layers = self.layers+self.pdarts_num_layers[epoch]
             model, loss, model_optim, _ = self.model_creator(
                 layers, n_nodes)
-            mutator = PdartsMutator(
-                model, epoch, self.pdarts_num_to_drop, switches)
+            mutator = PdartsMutator(model, epoch, self.pdarts_num_to_drop, switches)  # pylint: disable=too-many-function-args
 
             self.trainer = DartsTrainer(model, loss=loss, optimizer=model_optim,
                                         mutator=mutator, **self.darts_parameters)
diff --git a/src/sdk/pynni/nni/nas/pytorch/trainer.py b/src/sdk/pynni/nni/nas/pytorch/trainer.py
index ab18e6c6e5..a4954a0747 100644
--- a/src/sdk/pynni/nni/nas/pytorch/trainer.py
+++ b/src/sdk/pynni/nni/nas/pytorch/trainer.py
@@ -1,24 +1,39 @@
+import json
+import logging
 from abc import abstractmethod
 
 import torch
 
 from .base_trainer import BaseTrainer
 
+_logger = logging.getLogger(__name__)
+
+
+class TorchTensorEncoder(json.JSONEncoder):
+    def default(self, o):  # pylint: disable=method-hidden
+        if isinstance(o, torch.Tensor):
+            olist = o.tolist()
+            if "bool" not in o.type().lower() and all(map(lambda d: d == 0 or d == 1, olist)):
+                _logger.warning("Every element in %s is either 0 or 1. "
+                                "You might consider convert it into bool.", olist)
+            return olist
+        return super().default(o)
+
 
 class Trainer(BaseTrainer):
-    def __init__(self, model, loss, metrics, optimizer, num_epochs,
-                 dataset_train, dataset_valid, batch_size, workers, device, log_frequency,
-                 mutator, callbacks):
+    def __init__(self, model, mutator, loss, metrics, optimizer, num_epochs,
+                 dataset_train, dataset_valid, batch_size, workers, device, log_frequency, callbacks):
         self.device = torch.device("cuda" if torch.cuda.is_available() else "cpu") if device is None else device
         self.model = model
+        self.mutator = mutator
         self.loss = loss
+
         self.metrics = metrics
         self.optimizer = optimizer
-        self.mutator = mutator
 
         self.model.to(self.device)
-        self.loss.to(self.device)
         self.mutator.to(self.device)
+        self.loss.to(self.device)
 
         self.num_epochs = num_epochs
         self.dataset_train = dataset_train
@@ -38,7 +53,7 @@ def train_one_epoch(self, epoch):
     def validate_one_epoch(self, epoch):
         pass
 
-    def _train(self, validate):
+    def train(self, validate=True):
         for epoch in range(self.num_epochs):
             for callback in self.callbacks:
                 callback.on_epoch_begin(epoch)
@@ -55,11 +70,13 @@ def _train(self, validate):
             for callback in self.callbacks:
                 callback.on_epoch_end(epoch)
 
-    def train_and_validate(self):
-        self._train(True)
-
-    def train(self):
-        self._train(False)
-
     def validate(self):
         self.validate_one_epoch(-1)
+
+    def export(self, file):
+        mutator_export = self.mutator.export()
+        with open(file, "w") as f:
+            json.dump(mutator_export, f, indent=2, sort_keys=True, cls=TorchTensorEncoder)
+
+    def checkpoint(self):
+        raise NotImplementedError("Not implemented yet")
diff --git a/src/sdk/pynni/nni/nas/pytorch/utils.py b/src/sdk/pynni/nni/nas/pytorch/utils.py
new file mode 100644
index 0000000000..d3a4292155
--- /dev/null
+++ b/src/sdk/pynni/nni/nas/pytorch/utils.py
@@ -0,0 +1,107 @@
+from collections import OrderedDict
+
+_counter = 0
+
+
+def global_mutable_counting():
+    global _counter
+    _counter += 1
+    return _counter
+
+
+class AverageMeterGroup:
+
+    def __init__(self):
+        self.meters = OrderedDict()
+
+    def update(self, data):
+        for k, v in data.items():
+            if k not in self.meters:
+                self.meters[k] = AverageMeter(k, ":4f")
+            self.meters[k].update(v)
+
+    def __str__(self):
+        return "  ".join(str(v) for _, v in self.meters.items())
+
+
+class AverageMeter:
+    """Computes and stores the average and current value"""
+
+    def __init__(self, name, fmt=':f'):
+        self.name = name
+        self.fmt = fmt
+        self.reset()
+
+    def reset(self):
+        self.val = 0
+        self.avg = 0
+        self.sum = 0
+        self.count = 0
+
+    def update(self, val, n=1):
+        self.val = val
+        self.sum += val * n
+        self.count += n
+        self.avg = self.sum / self.count
+
+    def __str__(self):
+        fmtstr = '{name} {val' + self.fmt + '} ({avg' + self.fmt + '})'
+        return fmtstr.format(**self.__dict__)
+
+
+class StructuredMutableTreeNode:
+    """
+    A structured representation of a search space.
+    A search space comes with a root (with `None` stored in its `mutable`), and a bunch of children in its `children`.
+    This tree can be seen as a "flattened" version of the module tree. Since nested mutable entity is not supported yet,
+    the following must be true: each subtree corresponds to a ``MutableScope`` and each leaf corresponds to a
+    ``Mutable`` (other than ``MutableScope``).
+    """
+
+    def __init__(self, mutable):
+        self.mutable = mutable
+        self.children = []
+
+    def add_child(self, mutable):
+        self.children.append(StructuredMutableTreeNode(mutable))
+        return self.children[-1]
+
+    def type(self):
+        return type(self.mutable)
+
+    def __iter__(self):
+        return self.traverse()
+
+    def traverse(self, order="pre", deduplicate=True, memo=None):
+        """
+        Return a generator that generates a list of mutables in this tree.
+
+        Parameters
+        ----------
+        order: str
+            pre or post. If pre, current mutable is yield before children. Otherwise after.
+        deduplicate: bool
+            If true, mutables with the same key will not appear after the first appearance.
+        memo: dict
+            An auxiliary variable to make deduplicate happen.
+
+        Returns
+        -------
+        generator of Mutable
+        """
+        if memo is None:
+            memo = set()
+        assert order in ["pre", "post"]
+        if order == "pre":
+            if self.mutable is not None:
+                if not deduplicate or self.mutable.key not in memo:
+                    memo.add(self.mutable.key)
+                    yield self.mutable
+        for child in self.children:
+            for m in child.traverse(order=order, deduplicate=deduplicate, memo=memo):
+                yield m
+        if order == "post":
+            if self.mutable is not None:
+                if not deduplicate or self.mutable.key not in memo:
+                    memo.add(self.mutable.key)
+                    yield self.mutable