Polish the English documentation DynamicRNN (#20453)

* Refine the main comment of DynamicRNN.

* Refine the documentation of DynamicRNN's step_input function.

* Refine the documentation of DynamicRNN's static_input function.

* Refine the documentation of DynamicRNN's block function.

* Refine the documentation of DynamicRNN's memory function.

* Refine the documentation of DynamicRNN's update_memory and output function.

* Refine the code format and remove the method list.

* Refine the documentation of DynamicRNN's __call__ function.
test=develop
test=document_fix

* Minor modification.
test=develop
test=document_fix

* Fix some typo.

* Update API.spec.
test=develop
test=document_fix

* Refine the English according to the comments.

* Update API.spec.
test=develop
test=document_fix

* Fix some typo.

* Update API.spec.

paddle/fluid/API.spec

@@ -363,14 +363,14 @@ paddle.fluid.layers.IfElse.false_block (ArgSpec(args=['self'], varargs=None, key
 paddle.fluid.layers.IfElse.input (ArgSpec(args=['self', 'x'], varargs=None, keywords=None, defaults=None), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
 paddle.fluid.layers.IfElse.output (ArgSpec(args=['self'], varargs='outs', keywords=None, defaults=None), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
 paddle.fluid.layers.IfElse.true_block (ArgSpec(args=['self'], varargs=None, keywords=None, defaults=None), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
-paddle.fluid.layers.DynamicRNN ('paddle.fluid.layers.control_flow.DynamicRNN', ('document', 'b71e87285dbd4a43a6cc4f8a473245e6'))
+paddle.fluid.layers.DynamicRNN ('paddle.fluid.layers.control_flow.DynamicRNN', ('document', 'cc7209c675ec5841efd10c9d2f1028d9'))
 paddle.fluid.layers.DynamicRNN.__init__ (ArgSpec(args=['self', 'name'], varargs=None, keywords=None, defaults=(None,)), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
-paddle.fluid.layers.DynamicRNN.block (ArgSpec(args=['self'], varargs=None, keywords=None, defaults=None), ('document', '6d3e0a5d9aa519a9773a36e1620ea9b7'))
-paddle.fluid.layers.DynamicRNN.memory (ArgSpec(args=['self', 'init', 'shape', 'value', 'need_reorder', 'dtype'], varargs=None, keywords=None, defaults=(None, None, 0.0, False, 'float32')), ('document', '57cdd0a63747f4c670cdb9d250ceb7e1'))
-paddle.fluid.layers.DynamicRNN.output (ArgSpec(args=['self'], varargs='outputs', keywords=None, defaults=None), ('document', 'b439a176a3328de8a75bdc5c08eece4a'))
-paddle.fluid.layers.DynamicRNN.static_input (ArgSpec(args=['self', 'x'], varargs=None, keywords=None, defaults=None), ('document', '55ab9c562edd7dabec0bd6fd6c1a28cc'))
-paddle.fluid.layers.DynamicRNN.step_input (ArgSpec(args=['self', 'x', 'level'], varargs=None, keywords=None, defaults=(0,)), ('document', '4b300851b5201891d0e11c406e4c7d07'))
-paddle.fluid.layers.DynamicRNN.update_memory (ArgSpec(args=['self', 'ex_mem', 'new_mem'], varargs=None, keywords=None, defaults=None), ('document', '5d83987da13b98363d6a807a52d8024f'))
+paddle.fluid.layers.DynamicRNN.block (ArgSpec(args=['self'], varargs=None, keywords=None, defaults=None), ('document', '7935170393923803885fa461c3f3cfbc'))
+paddle.fluid.layers.DynamicRNN.memory (ArgSpec(args=['self', 'init', 'shape', 'value', 'need_reorder', 'dtype'], varargs=None, keywords=None, defaults=(None, None, 0.0, False, 'float32')), ('document', '904250e0e51498e5c52711e667c20941'))
+paddle.fluid.layers.DynamicRNN.output (ArgSpec(args=['self'], varargs='outputs', keywords=None, defaults=None), ('document', 'eccd774fb93665e51fb38bfb84d78b52'))
+paddle.fluid.layers.DynamicRNN.static_input (ArgSpec(args=['self', 'x'], varargs=None, keywords=None, defaults=None), ('document', 'a766b9ea54014d1af913b0d39d7fb010'))
+paddle.fluid.layers.DynamicRNN.step_input (ArgSpec(args=['self', 'x', 'level'], varargs=None, keywords=None, defaults=(0,)), ('document', '70081f137db4d215b302e49b5db33353'))
+paddle.fluid.layers.DynamicRNN.update_memory (ArgSpec(args=['self', 'ex_mem', 'new_mem'], varargs=None, keywords=None, defaults=None), ('document', 'b416c1c4a742a86379887b092cf7f2f0'))
 paddle.fluid.layers.StaticRNN ('paddle.fluid.layers.control_flow.StaticRNN', ('document', '4b0befc7f1ca7f11a459939f09c24d9a'))
 paddle.fluid.layers.StaticRNN.__init__ (ArgSpec(args=['self', 'name'], varargs=None, keywords=None, defaults=(None,)), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))
 paddle.fluid.layers.StaticRNN.memory (ArgSpec(args=['self', 'init', 'shape', 'batch_ref', 'init_value', 'init_batch_dim_idx', 'ref_batch_dim_idx'], varargs=None, keywords=None, defaults=(None, None, None, 0.0, 0, 1)), ('document', 'dcacaa7f1953e89ef5c9e0f9f37c5470'))

python/paddle/fluid/layers/control_flow.py

@@ -2002,44 +2002,68 @@ class IfElse(object):
 
 class DynamicRNN(object):
     """
-    The dynamic RNN can process a batch of sequence data. The length of each
-    sample sequence can be different. This API automatically process them in
-    batch.
+    **Note: the input of this class should be LoDTensor which holds the
+    information of variable-length sequences. If the input is fixed-length Tensor,
+    please use StaticRNN (fluid.layers.** :ref:`api_fluid_layers_StaticRNN` **) for
+    better performance.**
+
+    DynamicRNN can process a minibatch of variable-length sequences.
+    The length of each sample can be different and is recorded in LoD.
+    In DynamicRNN, an input sequence will be unfolded into time steps and users
+    can define how to process each time step in :code:`block()` .
+    The total number of time steps is determined by the longest sequence.
+    DynamicRNN will not pad all sequences to the same length, instead it will
+    sort the sequences internally by the sequence length in descending order.
+    The input sequences will be shrinked because only sequences of which the
+    length is larger than the time step will participate the remaining calculation.
+
+    If defined :code:`drnn = DynamicRNN()`, then users can call :code:`drnn()`
+    to obtain the result sequences. It is a LoDTensor gained by merging all
+    time steps's output. When RNN's input sequence x meets :code:`x.lod_level == 1`,
+    the output LoDTensor will have the same LoD with x. The result of :code:`drnn()`
+    includes RNN's outputs of all time steps, users can call
+    :ref:`api_fluid_layers_sequence_last_step` to extract the data of the last time step.
+
+    Warning:
+        Currently it is not supported to set :code:`is_sparse = True` of any
+        layers defined within DynamicRNN's :code:`block` function.
 
-    The input lod must be set. Please reference to `lod_tensor`.
-
-    The dynamic RNN will unfold sequence into timesteps. Users need to define
-    how to process each time step during the :code:`with` block.
-
-    The `memory` is used staging data cross time step. The initial value of
-    memory can be zero or another variable.
-
-    The dynamic RNN can mark multiple variables as its output. Use `drnn()` to
-    get the output sequence.
-
-    NOTES:
-        Currently it is not supported that setting is_sparse to True of any 
-        layers within DynamicRNN.
+    Args:
+        name (str, optional): The default value is None.  Normally there is no
+            need for user to set this property.  For more information,
+            please refer to :ref:`api_guide_Name` .
 
     Examples:
         .. code-block:: python
 
-          import paddle.fluid as fluid
+            import paddle.fluid as fluid
 
-          sentence = fluid.layers.data(name='sentence', shape=[1], dtype='int64', lod_level=1)
-          embedding = fluid.layers.embedding(input=sentence, size=[65536, 32], is_sparse=True)
-    
-          drnn = fluid.layers.DynamicRNN()
-          with drnn.block():
-              word = drnn.step_input(embedding)
-              prev = drnn.memory(shape=[200])
-              hidden = fluid.layers.fc(input=[word, prev], size=200, act='relu')
-              drnn.update_memory(prev, hidden)  # set prev to hidden
-              drnn.output(hidden)
-
-          # Get the last time step of rnn. It is the encoding result.
-          rnn_output = drnn()
-          last = fluid.layers.sequence_last_step(rnn_output)
+            sentence = fluid.data(name='sentence', shape=[None, 32], dtype='float32', lod_level=1)
+            encoder_proj = fluid.data(name='encoder_proj', shape=[None, 32], dtype='float32', lod_level=1)
+            decoder_boot = fluid.data(name='boot', shape=[None, 10], dtype='float32')
+
+            drnn = fluid.layers.DynamicRNN()
+            with drnn.block():
+                # Set sentence as RNN's input, each time step processes a word from the sentence
+                current_word = drnn.step_input(sentence)
+                # Set encode_proj as RNN's static input
+                encoder_word = drnn.static_input(encoder_proj)
+                # Initialize memory with boot_memory, which need reorder according to RNN's input sequences
+                memory = drnn.memory(init=decoder_boot, need_reorder=True)
+                fc_1 = fluid.layers.fc(input=encoder_word, size=30)
+                fc_2 = fluid.layers.fc(input=current_word, size=30)
+                decoder_inputs = fc_1 + fc_2
+                hidden, _, _ = fluid.layers.gru_unit(input=decoder_inputs, hidden=memory, size=30)
+                # Update memory with hidden
+                drnn.update_memory(ex_mem=memory, new_mem=hidden)
+                out = fluid.layers.fc(input=hidden, size=10, bias_attr=True, act='softmax')
+                # Set hidden and out as RNN's outputs
+                drnn.output(hidden, out)
+
+            # Get RNN's result
+            hidden, out = drnn()
+            # Get RNN's result of the last time step
+            last = fluid.layers.sequence_last_step(out)
     """
     BEFORE_RNN = 0
     IN_RNN = 1
@@ -2063,14 +2087,94 @@ class DynamicRNN(object):
 
     def step_input(self, x, level=0):
         """
-        Mark a sequence as a dynamic RNN input.
+        This function is used to set sequence x as DynamicRNN's input.
+        The maximum sequence length in x determines the number of time steps
+        the RNN unit will be executed. DynamicRNN can take multiple inputs.
+        When all inputs' :code:`lod_level` are 1, all inputs should hold the
+        same LoD. When :code:`x.lod_level >= 2` , the input sequence will be
+        unfold along specified level, and the slice of each time step is a
+        LoDTensor whose lod_level is :code:`x.lod_level - level - 1` .
+        In this case, the specified LoD level of multiple inputs should be the same.
+
+        - Case 1:
+
+        .. code-block:: text
+
+            # input, where Si is slice data of shape [1, N]
+            level = 0
+            x.lod = [[2, 1, 3]]
+            x.shape = [6, N]
+            x.data = [[S0],
+                      [S0],
+                      [S1],
+                      [S2],
+                      [S2],
+                      [S2]]
+
+            # output
+            # step 0, time step data of 3 sequences
+            out.lod = [[]]
+            out.shape = [3, N]
+            out.data = [[S2],
+                        [S0],
+                        [S1]]
+
+            # step 1, time step data of 2 sequences
+            out.lod = [[]]
+            out.shape = [2, N]
+            out.data = [[S2],
+                        [S0]]
+
+            # step 2, time step data of 1 sequences
+            out.lod = [[]]
+            out.shape = [1, N]
+            out.data = [[S2]]
+
 
         Args:
-            x (Variable): The input sequence which should have lod information.
-            level (int): The level of lod used to split steps. Default: 0.
+            x (Variable): The input LoDTensor which holds information of a
+                minibatch of variable-length sequences and should meet :code:`x.lod_level >= 1` .
+                When RNN has multiple inputs, the first dimension should match
+                across all inputs, but other shape components may differ.
+                Optional data types are: bool, float16, float32, float64, int8, int16, int32, int64, uint8.
+            level (int, optional): The level of lod used to split steps.
+                It should be in range :math:`[0, x.lod\_level)` . The default value is 0.
 
         Returns:
-            The current timestep in the input sequence.
+            Variable: The current time step in the input sequence. If there are :code:`num_sequences` \
+                sequences in x whose length is larger than :code:`step_idx` , the returned Variable \
+                will only hold the :code:`step_idx` -th time step of those `num_sequences` sequences. \
+                The data type is the same as input. If :code:`x.lod_level == 1` , the return value is \
+                a Tensor of shape :math:`\{num\_sequences, x.shape[1], ...\}` , or it will \
+                be a variable-length LoDTensor.
+
+        Raises:
+            ValueError: When :code:`step_input()` is called outside :code:`block()` .
+            TypeError: When x is not a Variable.
+
+        Examples:
+            ..  code-block:: python
+
+                import paddle.fluid as fluid
+
+                sentence = fluid.data(name='sentence', shape=[None, 1], dtype='int64', lod_level=1)
+                embedding = fluid.layers.embedding(input=sentence, size=[65536, 32], is_sparse=True)
+
+                drnn = fluid.layers.DynamicRNN()
+                with drnn.block():
+                    # Set embedding as RNN's input, each time step processes a word from the sentence
+                    word = drnn.step_input(embedding)
+                    # Initialize memory to a Tensor whose value is 0, shape=[batch_size, 200],
+                    # where batch_size is the number of sequences in embedding.
+                    memory = drnn.memory(shape=[200])
+                    hidden = fluid.layers.fc(input=[word, memory], size=200, act='relu')
+                    # Update memory to hidden
+                    drnn.update_memory(ex_mem=memory, new_mem=hidden)
+                    # Set hidden as RNN's output
+                    drnn.output(hidden)
+
+                # Get RNN's result
+                rnn_output = drnn()
         """
         self._assert_in_rnn_block_("step_input")
         if not isinstance(x, Variable):
@@ -2117,38 +2221,129 @@ class DynamicRNN(object):
 
     def static_input(self, x):
         """
-        Mark a variable as a RNN input. The input will not be scattered into
-        time steps. It is optional.
+        This function is used to set x as DynamicRNN's static input. It is optional.
+
+        - Case 1, set static input with LoD
+
+        .. code-block:: text
+
+            # RNN's input is the same as the case listed in step_input
+            # static input, where Si is slice data of shape [1, M]
+            x.lod = [[3, 1, 2]]
+            x.shape = [6, M]
+            x.data = [[S0],
+                      [S0],
+                      [S0],
+                      [S1],
+                      [S2],
+                      [S2]]
+
+            # step 0, batch data corresponding to the 3 input sequences
+            out.lod = [[2, 3, 1]]
+            out.shape = [6, M]
+            out.data = [[S2],
+                        [S2],
+                        [S0],
+                        [S0],
+                        [S0],
+                        [S1]]
+
+            # step 1, batch data corresponding to the 2 input sequences
+            out.lod = [[2, 3]]
+            out.shape = [5, M]
+            out.data = [[S2],
+                        [S2],
+                        [S0],
+                        [S0],
+                        [S0]]
+
+            # step 2, batch data corresponding to the 1 input sequences
+            out.lod = [[2]]
+            out.shape = [2, M]
+            out.data = [[S2],
+                        [S2]]
+
+
+        - Case 2, set static input without LoD
+
+        .. code-block:: text
+
+            # RNN's input is the same as the case listed in step_input
+            # static input, where Si is slice data of shape [1, M]
+            x.lod = [[]]
+            x.shape = [3, M]
+            x.data = [[S0],
+                      [S1],
+                      [S2]]
+
+            # step 0, batch data corresponding to the 3 input sequences
+            out.lod = [[]]
+            out.shape = [3, M]
+            out.data = [[S2],
+                        [S0],
+                        [S1]]
+
+            # step 1, batch data corresponding to the 2 input sequences
+            out.lod = [[]]
+            out.shape = [2, M]
+            out.data = [[S2],
+                        [S0]]
+
+            # step 2, batch data corresponding to the 1 input sequences
+            out.lod = [[]]
+            out.shape = [1, M]
+            out.data = [[S2]]
+
 
         Args:
-            x (Variable): The input variable.
+            x (Variable): The static input LoDTensor which should hold the same number of sequences
+                as RNN's input (the input LoDTensor set by :code:`step_input()` ). If the LoD is None,
+                the input x will be treated as a minibatch with :code:`x.shape[0]` sequences of length 1.
+                Optional data types are: bool, float16, float32, float64, int8, int16, int32, int64, uint8.
 
         Returns:
-            The input variable that can access in RNN.
+            Variable: The input LoDTensor after sorted and shrinked. If there are :code:`num_sequences` \
+                sequences in RNN's input LoDTensor whose length is larger than :code:`step_idx` , \
+                the static input Tensor will be sorted to the same order as RNN's input and \
+                will only retain data corresponding to those :code:`num_sequences` sequences. \
+                The data type is the same as input. If :code:`x.lod == None` , the return value is \
+                a Tensor of shape :math:`\{num\_sequences, x.shape[1], ...\}` , or it will \
+                be a variable-length LoDTensor.
+
+        Raises:
+            ValueError: When :code:`static_input()` is called outside :code:`block()` .
+            TypeError: When x is not a Variable.
+            RuntimeError: When :code:`static_input()` is called before :code:`step_input()` .
 
         Examples:
             .. code-block:: python
 
-              import paddle.fluid as fluid
-
-              sentence = fluid.layers.data(name='sentence', dtype='float32', shape=[32], lod_level=1)
-              encoder_proj = fluid.layers.data(name='encoder_proj', dtype='float32', shape=[32], lod_level=1)
-              decoder_boot = fluid.layers.data(name='boot', dtype='float32', shape=[10], lod_level=1)
-
-              drnn = fluid.layers.DynamicRNN()
-              with drnn.block():
-                  current_word = drnn.step_input(sentence)
-                  encoder_word = drnn.static_input(encoder_proj)
-                  hidden_mem = drnn.memory(init=decoder_boot, need_reorder=True)
-                  fc_1 = fluid.layers.fc(input=encoder_word, size=30, bias_attr=False)
-                  fc_2 = fluid.layers.fc(input=current_word, size=30, bias_attr=False)
-                  decoder_inputs = fc_1 + fc_2
-                  h, _, _ = fluid.layers.gru_unit(input=decoder_inputs, hidden=hidden_mem, size=30)
-                  drnn.update_memory(hidden_mem, h)
-                  out = fluid.layers.fc(input=h, size=10, bias_attr=True, act='softmax') 
-                  drnn.output(out)
-
-              rnn_output = drnn()
+                import paddle.fluid as fluid
+
+                sentence = fluid.data(name='sentence', shape=[None, 32], dtype='float32', lod_level=1)
+                encoder_proj = fluid.data(name='encoder_proj', shape=[None, 32], dtype='float32', lod_level=1)
+                decoder_boot = fluid.data(name='boot', shape=[None, 10], dtype='float32')
+
+                drnn = fluid.layers.DynamicRNN()
+                with drnn.block():
+                    # Set sentence as RNN's input, each time step processes a word from the sentence
+                    current_word = drnn.step_input(sentence)
+                    # Set encode_proj as RNN's static input
+                    encoder_word = drnn.static_input(encoder_proj)
+                    # Initialize memory with boot_memory, which need reorder according to RNN's input sequences
+                    memory = drnn.memory(init=decoder_boot, need_reorder=True)
+                    fc_1 = fluid.layers.fc(input=encoder_word, size=30)
+                    fc_2 = fluid.layers.fc(input=current_word, size=30)
+                    decoder_inputs = fc_1 + fc_2
+                    hidden, _, _ = fluid.layers.gru_unit(input=decoder_inputs, hidden=memory, size=30)
+                    # Update memory with hidden
+                    drnn.update_memory(ex_mem=memory, new_mem=hidden)
+                    out = fluid.layers.fc(input=hidden, size=10, bias_attr=True, act='softmax')
+                    # Set out as RNN's output
+                    drnn.output(out)
+
+                # Get RNN's result
+                rnn_output = drnn()
         """
         self._assert_in_rnn_block_("static_input")
         if not isinstance(x, Variable):
@@ -2172,7 +2367,12 @@ class DynamicRNN(object):
     @signature_safe_contextmanager
     def block(self):
         """
-        The block for user to define operators in RNN.
+        The function is used to list the operations executed during
+        each time step in RNN. The operation list will be executed :code:`max_sequence_len`
+        times (where :code:`max_sequence_len` is the maximum length of RNN's input sequences).
+
+        Raises:
+            ValueError: When :code:`block()` is called multi-times.
         """
         if self.status != DynamicRNN.BEFORE_RNN:
             raise ValueError("rnn.block() can only be invoke once")
@@ -2201,7 +2401,16 @@ class DynamicRNN(object):
 
     def __call__(self, *args, **kwargs):
         """
-        Get the output of RNN. This API should only be invoked after RNN.block()
+        This function is used to get the output  sequneces of DynamicRNN.
+
+        Args:
+            None
+
+        Returns:
+            Variable or Variable list: RNN's output sequences.
+
+        Raises:
+            ValueError: When :code:`__call__()` is called before :code:`block()` .
         """
         if self.status != DynamicRNN.AFTER_RNN:
             raise ValueError(("Output of the dynamic RNN can only be visited "
@@ -2218,62 +2427,89 @@ class DynamicRNN(object):
                need_reorder=False,
                dtype='float32'):
         """
-        Create a memory variable for dynamic rnn.
+        Create a memory Variable for DynamicRNN to deliver data cross time steps.
+        It can be initialized by an existing Tensor or a constant Tensor of given
+        dtype and shape.
 
-        If the :code:`init` is not None, :code:`memory` will be initialized by
-        this variable. The :code:`need_reorder` is used to reorder the memory as
-        the input variable. It should be set to true when the initialized memory
-        depends on the input sample.
+        Args:
+            init (Variable, optional): LoDTensor used to initialize the memory.
+                If init is not None, it should hold the same number of sequences
+                as RNN's input (the input LoDTensor set by :code:`step_input()` )
+                and the memory will be initialized to it. If init's LoD is None,
+                it will be treated as a minibatch with :code:`init.shape[0]` sequences
+                of length 1. The default value is None.
+            shape (list|tuple, optional): When init is None, it is used to specify
+                the memory's shape. Note that the shape does not include the batch_size.
+                If setting shape to :math:`\{D_1, D_2, ...\}` , the shape of memory Tensor
+                will be :math:`\{batch\_size, D_1, D_2, ...\}` , where batch_size is
+                determined by RNN's input sequences. The default value is None.
+            value (float, optional): When init is None, it is used as initalized value
+                of memory. The default value is 0.0.
+            need_reorder (bool, optional): When init is not None, it determines whether
+                the memory needs to reorder like the RNN's input sequeneces. It should be
+                set to True when the initialized memory depends on the order of input samples.
+                The default value is False.
+            dtype (str|numpy.dtype, optional): When init is None, it is used to set the
+                data type of memory. The default value is "float32". Optional data types
+                are: "float32", "float64", "int32", "int64".
+
+        Returns:
+            Variable: The memory LoDTensor after shrinked.  If there are :code:`num_sequences` \
+                sequences in RNN's input LoDTensor whose length is larger than :code:`step_idx` , \
+                the memory Tensor also need to be shrinked and will only retain data \
+                corresponding to those :code:`num_sequences` sequences.
+
+        Raises:
+            ValueError: When :code:`memory()` is called outside :code:`block()` .
+            TypeError: When init is set and is not a Variable.
+            ValueError: When :code:`memory()` is called before :code:`step_input()` .
 
         Examples:
             .. code-block:: python
 
-              import paddle.fluid as fluid
+                import paddle.fluid as fluid
 
-              sentence = fluid.layers.data(name='sentence', shape=[32], dtype='float32', lod_level=1)
-              boot_memory = fluid.layers.data(name='boot', shape=[10], dtype='float32', lod_level=1)
-              
-              drnn = fluid.layers.DynamicRNN()
-              with drnn.block():
-                  word = drnn.step_input(sentence)
-                  memory = drnn.memory(init=boot_memory, need_reorder=True)
-                  hidden = fluid.layers.fc(input=[word, memory], size=10, act='tanh')
-                  drnn.update_memory(ex_mem=memory, new_mem=hidden)
-                  drnn.output(hidden)
+                sentence = fluid.data(name='sentence', shape=[None, 32], dtype='float32', lod_level=1)
+                boot_memory = fluid.data(name='boot', shape=[None, 10], dtype='float32')
 
-              rnn_output = drnn()
+                drnn = fluid.layers.DynamicRNN()
+                with drnn.block():
+                    # Set sentence as RNN's input, each time step processes a word from the sentence
+                    word = drnn.step_input(sentence)
+                    # Initialize memory with boot_memory, which need reorder according to RNN's input sequences
+                    memory = drnn.memory(init=boot_memory, need_reorder=True)
+                    hidden = fluid.layers.fc(input=[word, memory], size=10, act='tanh')
+                    # Update memory with hidden
+                    drnn.update_memory(ex_mem=memory, new_mem=hidden)
+                    # Set hidden as RNN's output
+                    drnn.output(hidden)
 
+                # Get RNN's result
+                rnn_output = drnn()
 
-        Otherwise, if :code:`shape`, :code:`value`, :code:`dtype` are set, the
-        :code:`memory` will be initialized by this :code:`value`.
 
         Examples:
             .. code-block:: python
 
-              import paddle.fluid as fluid
-
-              sentence = fluid.layers.data(name='sentence', dtype='float32', shape=[32], lod_level=1)
-              
-              drnn = fluid.layers.DynamicRNN()
-              with drnn.block():
-                  word = drnn.step_input(sentence)
-                  memory = drnn.memory(shape=[10], dtype='float32', value=0)
-                  hidden = fluid.layers.fc(input=[word, memory], size=10, act='tanh')
-                  drnn.update_memory(ex_mem=memory, new_mem=hidden)
-                  drnn.output(hidden)
-
-              rnn_output = drnn()
-
-
-        Args:
-            init(Variable|None): The initialized variable.
-            shape(list|tuple): The memory shape. The shape does not contain batch_size.
-            value(float): the initalized value.
-            need_reorder(bool): True if the initialized memory depends on the input sample.
-            dtype(str|numpy.dtype): The data type of the initialized memory.
-
-        Returns:
-            The memory variable.
+                import paddle.fluid as fluid
+
+                sentence = fluid.data(name='sentence', shape=[None, 32], dtype='float32', lod_level=1)
+
+                drnn = fluid.layers.DynamicRNN()
+                with drnn.block():
+                    # Set sentence as RNN's input, each time step processes a word from the sentence
+                    word = drnn.step_input(sentence)
+                    # Initialize memory to a Tensor whose value is 0, shape=[batch_size, 10],
+                    # where batch_size is the number of sequences in sentence.
+                    memory = drnn.memory(shape=[10], dtype='float32', value=0)
+                    hidden = fluid.layers.fc(input=[word, memory], size=10, act='tanh')
+                    # Update memory with hidden
+                    drnn.update_memory(ex_mem=memory, new_mem=hidden)
+                    # Set hidden as RNN's output
+                    drnn.output(hidden)
+
+                # Get RNN's result
+                rnn_output = drnn()
         """
         self._assert_in_rnn_block_('memory')
         self._init_zero_idx_()
@@ -2344,15 +2580,21 @@ class DynamicRNN(object):
 
     def update_memory(self, ex_mem, new_mem):
         """
-        Update the memory from ex_mem to new_mem. NOTE that the shape and data
-        type of :code:`ex_mem` and :code:`new_mem` must be same.
-        
+        Update the memory which need to be delivered across time steps.
+
         Args:
-            ex_mem(Variable): the memory variable.
-            new_mem(Variable): the plain variable generated in RNN block.
+            ex_mem (Variable): The memory data of previous time step.
+            new_mem (Variable): The new memory data produced in current time step.
+                The shape and data type of ex_mem and new_mem should be the same.
 
         Returns:
             None
+        
+        Raises:
+            ValueError: When :code:`update_memory()` is called outside :code:`block()` .
+            TypeError: When :code:`ex_mem` or :code:`new_mem` is not a Variable.
+            ValueError: When :code:`ex_mem` is defined by :code:`memory()` .
+            ValueError: When :code:`update_memory()` is called before :code:`step_input()` .
         """
         self._assert_in_rnn_block_('update_memory')
         if not isinstance(ex_mem, Variable):
@@ -2372,13 +2614,17 @@ class DynamicRNN(object):
 
     def output(self, *outputs):
         """
-        Mark the RNN output variables.
+        This function is used to set :code:`outputs` as RNN's output.
 
         Args:
-            outputs: The output variables.
+            *outputs (Variable ...): The output Tensor. DynamicRNN can mark multiple
+                Variables as its output.
 
         Returns:
             None
+
+        Raises:
+            ValueError: When :code:`output()` is called outside :code:`block()` .
         """
         self._assert_in_rnn_block_('output')
         parent_block = self._parent_block_()