io.py

#   Copyright (c) 2018 PaddlePaddle Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import os
import time
import shutil

from paddle.fluid.evaluator import Evaluator
from paddle.fluid.framework import Program, Parameter, default_main_program, Variable
from . import core

__all__ = [
    'save_vars', 'save_params', 'save_persistables', 'load_vars', 'load_params',
    'load_persistables', 'save_inference_model', 'load_inference_model',
    'get_inference_program', 'save_checkpoint', 'load_checkpoint',
    'clean_checkpoint', 'load_persist_vars_without_grad',
    'save_persist_vars_without_grad', 'get_latest_checkpoint_serial'
]


def is_parameter(var):
    """
    Check whether the given variable is an instance of Parameter.

    Args:
        var(Variable): The variable to be checked.

    Returns:
        bool: True if the given `var` is an instance of Parameter,
        False if not.

    Examples:
        .. code-block:: python

            param = fluid.default_main_program().global_block().var('fc.w')
            res = fluid.io.is_parameter(param)
    """
    return isinstance(var, Parameter)


def is_persistable(var):
    """
    Check whether the given variable is persistable.

    Args:
        var(Variable): The variable to be checked.

    Returns:
        bool: True if the given `var` is persistable
        False if not.

    Examples:
        .. code-block:: python

            param = fluid.default_main_program().global_block().var('fc.w')
            res = fluid.io.is_persistable(param)
    """
    if var.desc.type() == core.VarDesc.VarType.FEED_MINIBATCH or \
            var.desc.type() == core.VarDesc.VarType.FETCH_LIST:
        return False
    return var.persistable


def _clone_var_in_block_(block, var):
    assert isinstance(var, Variable)
    return block.create_var(
        name=var.name,
        shape=var.shape,
        dtype=var.dtype,
        type=var.type,
        lod_level=var.lod_level,
        persistable=True)


def save_vars(executor,
              dirname,
              main_program=None,
              vars=None,
              predicate=None,
              filename=None):
    """
    Save variables to the given directory by executor.

    There are two ways to specify variables to be saved: The first way, list 
    variables in a list and assign it to the `vars`. The second way, assign the 
    `main_program` with an existing program, then all variables in the program 
    will be saved. The first way has a higher priority. In other words, if `vars` 
    are assigned, the `main_program` and the `predicate` will be ignored.

    The `dirname` are used to specify the folder where to save variables. 
    If you prefer to save variables in separate files in the folder `dirname`, 
    set `filename` None; if you prefer to save all variables in a single file, 
    use `filename` to specify it.

    Args:
        executor(Executor): The executor to run for saving variables.
        dirname(str): The directory path.
        main_program(Program|None): The program whose variables will be saved. 
                                    If it is None, the default main program will 
                                    be used automatically.
                                    Default: None
        vars(list[Variable]|None): The list that contains all variables to save. 
                                   It has a higher priority than the `main_program`.
                                   Default: None
        predicate(function|None): If it is not None, only variables in the 
                                  `main_program` that makes predicate(variable)==True 
                                  will be saved. It only works when we are using the 
                                  `main_program` to specify variables (In other words 
                                  `vars` is None).
                                  Default: None
        filename(str|None): The file which to save all variables. If you prefer to save 
                            variables separately, set it to None.
                            Default: None

    Returns:
        None

    Raises:
        TypeError: If `main_program` is not an instance of Program nor None.

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            param_path = "./my_paddle_model"

            # The first usage: using `main_program` to specify variables
            def name_has_fc(var):
                res = "fc" in var.name
                return res

            prog = fluid.default_main_program()
            fluid.io.save_vars(executor=exe, dirname=path, main_program=prog,
                               vars=None)
            # All variables in `main_program` whose name includes "fc" will be saved.
            # And variables are going to be saved separately.


            # The second usage: using `vars` to specify variables
            var_list = [var_a, var_b, var_c]
            fluid.io.save_vars(executor=exe, dirname=path, vars=var_list, 
                               filename="vars_file")
            # var_a, var_b and var_c will be saved. And they are going to be
            # saved in the same file named 'var_file' in the path "./my_paddle_model".
    """
    if vars is None:
        if main_program is None:
            main_program = default_main_program()
        if not isinstance(main_program, Program):
            raise TypeError("program should be as Program type or None")

        save_vars(
            executor,
            dirname=dirname,
            vars=filter(predicate, main_program.list_vars()),
            filename=filename)
    else:
        save_program = Program()
        save_block = save_program.global_block()

        save_var_map = {}
        for each_var in vars:
            # NOTE: don't save the variable which type is RAW
            if each_var.type == core.VarDesc.VarType.RAW:
                continue
            new_var = _clone_var_in_block_(save_block, each_var)
            if filename is None:
                save_block.append_op(
                    type='save',
                    inputs={'X': [new_var]},
                    outputs={},
                    attrs={'file_path': os.path.join(dirname, new_var.name)})
            else:
                save_var_map[new_var.name] = new_var

        if filename is not None:
            save_var_list = []
            for name in sorted(save_var_map.keys()):
                save_var_list.append(save_var_map[name])

            save_block.append_op(
                type='save_combine',
                inputs={'X': save_var_list},
                outputs={},
                attrs={'file_path': os.path.join(dirname, filename)})

        executor.run(save_program)


def save_params(executor, dirname, main_program=None, filename=None):
    """
    This function filters out all parameters from the give `main_program`
    and then save them to the folder `dirname` or the file `filename`.

    Use the `dirname` to specify the saving folder. If you would like to 
    save parameters in separate files, set `filename` None; if you would 
    like to save all parameters in a single file, use `filename` to specify 
    the file name.

    NOTICE: Some variables are not Parameter while they are necessary for 
    training. So you can NOT save and continue your training just by 
    `save_params()` and `load_params()`. Please use `save_persistables()` 
    and `load_persistables()` instead.

    Args:
        executor(Executor): The executor to run for saving parameters.
        dirname(str): The saving directory path.
        main_program(Program|None): The program whose parameters will be
                                    saved. If it is None, the default
                                    main program will be used automatically.
                                    Default: None
        filename(str|None): The file to save all parameters. If you prefer 
                            to save parameters in differnet files, set it 
                            to None.
                            Default: None

    Returns:
        None

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            param_path = "./my_paddle_model"
            prog = fluid.default_main_program()
            fluid.io.save_params(executor=exe, dirname=param_path, 
                                 main_program=None)
    """
    save_vars(
        executor,
        dirname=dirname,
        main_program=main_program,
        vars=None,
        predicate=is_parameter,
        filename=filename)


def save_persistables(executor, dirname, main_program=None, filename=None):
    """
    This function filters out all variables with `persistable==True` from the 
    give `main_program` and then saves these variables to the folder `dirname` 
    or file `filename`.

    The `dirname` is used to specify the folder where persistable variables 
    are going to be saved. If you would like to save variables in separate 
    files, set `filename` None; if you would like to save all variables in a 
    single file, use `filename` to specify the file name.

    Args:
        executor(Executor): The executor to run for saving persistable variables.
        dirname(str): The directory path.
        main_program(Program|None): The program whose persistbale variables will 
                                    be saved. If it is None, the default main 
                                    program will be used automatically.
                                    Default: None
        filename(str|None): The file to saved all variables. If you prefer to 
                            save variables in differnet files, set it to None.
                            Default: None

    Returns:
        None

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            param_path = "./my_paddle_model"
            prog = fluid.default_main_program()
            fluid.io.save_persistables(executor=exe, dirname=param_path, 
                                       main_program=None)
    """
    save_vars(
        executor,
        dirname=dirname,
        main_program=main_program,
        vars=None,
        predicate=is_persistable,
        filename=filename)


def load_vars(executor,
              dirname,
              main_program=None,
              vars=None,
              predicate=None,
              filename=None):
    """
    Load variables from the given directory by executor.

    There are two ways to specify variables to be loaded: The first way, list 
    variables in a list and assign it to the `vars`. The second way, assign the 
    `main_program` with an existing program, then all variables in the program 
    will be loaded. The first way has a higher priority. In other words if `vars` 
    are assigned, the `main_program` and the `predicate` will be ignored.

    The `dirname` are used to specify the folder where to load variables. 
    If variables were saved in separate files in the folder `dirname`, 
    set `filename` None; if all variables were saved in a single file, 
    use `filename` to specify it.

    Args:
        executor(Executor): The executor to run for loading variables.
        dirname(str): The directory path.
        main_program(Program|None): The program whose variables will be loaded. 
                                    If it is None, the default main program will 
                                    be used automatically.
                                    Default: None
        vars(list[Variable]|None): The list that contains all variables to load. 
                                   It has a higher priority than the `main_program`.
                                   Default: None
        predicate(function|None): If it is not None, only variables in the 
                                  `main_program` that makes predicate(variable)==True 
                                  will be loaded. It only works when we are using the 
                                  `main_program` to specify variables (In other words 
                                  `vars` is None).
                                  Default: None
        filename(str|None): The file which saved all required variables. If variables 
                            were saved in differnet files, set it to None.
                            Default: None

    Returns:
        None

    Raises:
        TypeError: If `main_program` is not an instance of Program nor None.

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            param_path = "./my_paddle_model"

            # The first usage: using `main_program` to specify variables
            def name_has_fc(var):
                res = "fc" in var.name
                return res

            prog = fluid.default_main_program()
            fluid.io.load_vars(executor=exe, dirname=path, main_program=prog,
                               vars=None)
            # All variables in `main_program` whose name includes "fc" will be loaded.
            # And all the variables are supposed to have been saved in differnet files.


            # The second usage: using `vars` to specify variables
            var_list = [var_a, var_b, var_c]
            fluid.io.load_vars(executor=exe, dirname=path, vars=var_list, 
                               filename="vars_file")
            # var_a, var_b and var_c will be loaded. And they are supposed to haven 
            # been saved in the same file named 'var_file' in the path "./my_paddle_model".
    """
    if vars is None:
        if main_program is None:
            main_program = default_main_program()
        if not isinstance(main_program, Program):
            raise TypeError("program's type should be Program")

        load_vars(
            executor,
            dirname=dirname,
            vars=filter(predicate, main_program.list_vars()),
            filename=filename)
    else:
        load_prog = Program()
        load_block = load_prog.global_block()

        load_var_map = {}
        for each_var in vars:
            assert isinstance(each_var, Variable)
            if each_var.type == core.VarDesc.VarType.RAW:
                continue
            new_var = _clone_var_in_block_(load_block, each_var)
            if filename is None:
                load_block.append_op(
                    type='load',
                    inputs={},
                    outputs={'Out': [new_var]},
                    attrs={'file_path': os.path.join(dirname, new_var.name)})
            else:
                load_var_map[new_var.name] = new_var

        if filename is not None:
            load_var_list = []
            for name in sorted(load_var_map.keys()):
                load_var_list.append(load_var_map[name])

            load_block.append_op(
                type='load_combine',
                inputs={},
                outputs={"Out": load_var_list},
                attrs={'file_path': os.path.join(dirname, filename)})

        executor.run(load_prog)


def load_params(executor, dirname, main_program=None, filename=None):
    """
    This function filters out all parameters from the give `main_program`
    and then try to load these parameters from the folder `dirname` or
    the file `filename`.

    Use the `dirname` to specify the folder where parameters were saved. If 
    parameters were saved in separate files in the folder `dirname`, set 
    `filename` None; if all parameters were saved in a single file, use 
    `filename` to specify the file name.

    NOTICE: Some variables are not Parameter while they are necessary for 
    training. So you can NOT save and continue your training just by 
    `save_params()` and `load_params()`. Please use `save_persistables()` 
    and `load_persistables()` instead. 

    Args:
        executor(Executor): The executor to run for loading parameters.
        dirname(str): The directory path.
        main_program(Program|None): The program whose parameters will be
                                    loaded. If it is None, the default
                                    main program will be used automatically.
                                    Default: None
        filename(str|None): The file which saved all parameters. If parameters 
                            were saved in differnet files, set it to None.
                            Default: None

    Returns:
        None

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            param_path = "./my_paddle_model"
            prog = fluid.default_main_program()
            fluid.io.load_params(executor=exe, dirname=param_path, 
                                main_program=None)
    """
    load_vars(
        executor,
        dirname=dirname,
        main_program=main_program,
        predicate=is_parameter,
        filename=filename)


def load_persistables(executor, dirname, main_program=None, filename=None):
    """
    This function filters out all variables with `persistable==True` from the 
    give `main_program` and then trys to load these variables from the folder 
    `dirname` or the file `filename`.

    Use the `dirname` to specify the folder where persistable variables were 
    saved. If variables were saved in separate files, set `filename` None; 
    if all variables were saved in a single file, use `filename` to specify 
    the file name.

    Args:
        executor(Executor): The executor to run for loading persistable variables.
        dirname(str): The directory path.
        main_program(Program|None): The program whose persistbale variables will 
                                    be loaded. If it is None, the default main 
                                    program will be used automatically.
                                    Default: None
        filename(str|None): The file which saved all variables. If variables were 
                            saved in differnet files, set it to None.
                            Default: None

    Returns:
        None

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            param_path = "./my_paddle_model"
            prog = fluid.default_main_program()
            fluid.io.load_persistables(executor=exe, dirname=param_path, 
                                       main_program=None)
    """
    load_vars(
        executor,
        dirname=dirname,
        main_program=main_program,
        predicate=is_persistable,
        filename=filename)


def get_inference_program(target_vars, main_program=None):
    if main_program is None:
        main_program = default_main_program()
    if not isinstance(target_vars, list):
        target_vars = [target_vars]
    vars = []
    for var in target_vars:
        if isinstance(var, Evaluator):
            vars.extend(var.states)
            vars.extend(var.metrics)
        else:
            vars.append(var)
    pruned_program = main_program.prune(targets=vars)
    inference_program = pruned_program.inference_optimize()
    return inference_program


def prepend_feed_ops(inference_program,
                     feed_target_names,
                     feed_holder_name='feed'):
    if len(feed_target_names) == 0:
        return

    global_block = inference_program.global_block()
    feed_var = global_block.create_var(
        name=feed_holder_name,
        type=core.VarDesc.VarType.FEED_MINIBATCH,
        persistable=True)

    for i, name in enumerate(feed_target_names):
        out = global_block.var(name)
        global_block.prepend_op(
            type='feed',
            inputs={'X': [feed_var]},
            outputs={'Out': [out]},
            attrs={'col': i})


def append_fetch_ops(inference_program,
                     fetch_target_names,
                     fetch_holder_name='fetch'):
    global_block = inference_program.global_block()
    fetch_var = global_block.create_var(
        name=fetch_holder_name,
        type=core.VarDesc.VarType.FETCH_LIST,
        persistable=True)

    for i, name in enumerate(fetch_target_names):
        global_block.append_op(
            type='fetch',
            inputs={'X': [name]},
            outputs={'Out': [fetch_var]},
            attrs={'col': i})


def save_inference_model(dirname,
                         feeded_var_names,
                         target_vars,
                         executor,
                         main_program=None,
                         model_filename=None,
                         params_filename=None):
    """
    Prune the given `main_program` to build a new program especially for inference,
    and then save it and all related parameters to given `dirname` by the `executor`.

    Args:
        dirname(str): The directory path to save the inference model.
        feeded_var_names(list[str]): Names of variables that need to be feeded data 
                                     during inference.
        target_vars(list[Variable]): Variables from which we can get inference 
                                     results.
        executor(Executor): The executor that saves the inference model.
        main_program(Program|None): The original program, which will be pruned to 
                                    build the inference model. If is setted None, 
                                    the default main program will be used.
                                    Default: None.
        model_filename(str|None): The name of file to save the inference program 
                                  itself. If is setted None, a default filename 
                                  `__model__` will be used.
        params_filename(str|None): The name of file to save all related parameters. 
                                   If it is setted None, parameters will be saved 
                                   in separate files .

    Returns:
        None

    Raises:
        ValueError: If `feed_var_names` is not a list of basestring.
        ValueError: If `target_vars` is not a list of Variable.

    Examples:
        .. code-block:: python
            exe = fluid.Executor(fluid.CPUPlace())
            path = "./infer_model"
            fluid.io.save_inference_model(dirname=path, feeded_var_names=['img'],
                         target_vars=[predict_var], executor=exe)

            # In this exsample, the function will prune the default main program 
            # to make it suitable for infering the `predict_var`. The pruned 
            # inference program is going to be saved in the "./infer_model/__model__" 
            # and parameters are going to be saved in separate files under folder
            # "./infer_model". 

    """
    if isinstance(feeded_var_names, basestring):
        feeded_var_names = [feeded_var_names]
    else:
        if len(feeded_var_names) > 0:
            if not (bool(feeded_var_names) and all(
                    isinstance(name, basestring) for name in feeded_var_names)):
                raise ValueError("'feed_var_names' should be a list of str.")

    if isinstance(target_vars, Variable):
        target_vars = [target_vars]
    else:
        if not (bool(target_vars) and all(
                isinstance(var, Variable) for var in target_vars)):
            raise ValueError("'target_vars' should be a list of Variable.")

    if main_program is None:
        main_program = default_main_program()
    copy_program = main_program.clone()

    if not os.path.isdir(dirname):
        os.makedirs(dirname)

    # Clear the is_target information and remove the existed feed and fetch op
    global_block = copy_program.global_block()
    for i, op in enumerate(global_block.ops):
        op.desc.set_is_target(False)
        if op.type == "feed" or op.type == "fetch":
            global_block.remove_op(i)
    copy_program.desc.flush()

    pruned_program = copy_program.prune(targets=target_vars)
    inference_program = pruned_program.inference_optimize()
    fetch_var_names = [v.name for v in target_vars]

    prepend_feed_ops(inference_program, feeded_var_names)
    append_fetch_ops(inference_program, fetch_var_names)

    if model_filename is not None:
        model_filename = os.path.basename(model_filename)
    else:
        model_filename = "__model__"
    model_filename = os.path.join(dirname, model_filename)

    if params_filename is not None:
        params_filename = os.path.basename(params_filename)

    with open(model_filename, "wb") as f:
        f.write(inference_program.desc.serialize_to_string())

    save_persistables(executor, dirname, inference_program, params_filename)


def load_inference_model(dirname,
                         executor,
                         model_filename=None,
                         params_filename=None):
    """
    Load inference model from a directory

    Args:
        dirname(str): The directory path
        executor(Executor): The executor to run for loading inference model.
        model_filename(str|None): The name of file to load inference program.
                                  If it is None, the default filename 
                                  '__model__' will be used.
                                  Default: None
        params_filename(str|None): The name of file to load all parameters.
                                   It is only used for the case that all 
                                   parameters were saved in a single binary 
                                   file. If parameters were saved in separate 
                                   files, set it as 'None'.

    Returns:
        tuple: The return of this function is a tuple with three elements:
        (program, feed_target_names, fetch_targets). The `program` is a 
        Program, it's the program for inference. The `feed_target_names` is 
        a list of str, it contains Names of variables that need to feed 
        data in the inference program. The `fetch_targets` is a list of 
        Variable. It contains variables from which we can get inference 
        results.

    Raises:
        ValueError: If `dirname` is not a existing directory.

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            path = "./infer_model"
            [inference_program, feed_target_names, fetch_targets] = 
                fluid.io.load_inference_model(dirname=path, executor=exe)
            results = exe.run(inference_program,
                          feed={feed_target_names[0]: tensor_img},
                          fetch_list=fetch_targets)

            # In this exsample, the inference program is saved in the 
            # "./infer_model/__model__" and parameters were saved in 
            # separate files in ""./infer_model". 
            # After getting inference program, feed target names and 
            # fetch targets, we can use an Executor to run the inference 
            # program to get the inference result.

    """
    if not os.path.isdir(dirname):
        raise ValueError("There is no directory named '%s'", dirname)

    if model_filename is not None:
        model_filename = os.path.basename(model_filename)
    else:
        model_filename = "__model__"
    model_filename = os.path.join(dirname, model_filename)

    if params_filename is not None:
        params_filename = os.path.basename(params_filename)

    with open(model_filename, "rb") as f:
        program_desc_str = f.read()

    program = Program.parse_from_string(program_desc_str)
    load_persistables(executor, dirname, program, params_filename)

    feed_target_names = program.desc.get_feed_target_names()
    fetch_target_names = program.desc.get_fetch_target_names()
    fetch_targets = [
        program.global_block().var(name) for name in fetch_target_names
    ]

    return [program, feed_target_names, fetch_targets]


def get_parameter_value(para, executor):
    """
    Get the LoDTensor value of the given parameter.

    Args:
        para(Parameter): The parameter to get value from.
        executor(Executor): The executor to run for retrieving the value.

    Returns:
        numpy.array: The given parameter's values.

    Raises:
        AssertionError: If the `para` is not an instance of Parameter.

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            param = fluid.default_main_program().global_block().var('fc.w')
            p = fluid.io.get_parameter_value(param, exe)

    """
    assert is_parameter(para)

    get_program = Program()
    block = get_program.global_block()
    new_var = _clone_var_in_block_(block, para)
    return executor.run(get_program, feed={}, fetch_list=[new_var])[0]


def get_parameter_value_by_name(name, executor, program=None):
    """
    Get the LoDTensor value of a certain parameter by its name.

    Args:
        name(str): The parameter's name.
        executor(Executor): The executor to run for retrieving the value.
        program(Program | None): The program where to find the parameter.
                               If it's set to be None, the function will
                               try to find the parameter in the default
                               main program.

    Returns:
        numpy.array: The parameter's values.

    Raises:
        TypeError: If given `name` is not an instance of basestring.
        TypeError: If the parameter with the given name doesn't exist.
        AssertionError: If there is a varibale named `name` in the
                        given program but it is not a Parameter.

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            p = fluid.io.get_parameter_value('fc.w', exe)
    """
    if program is None:
        program = default_main_program()
    var = program.global_block().var(name)
    return get_parameter_value(var, executor)


SUCCESS_MARK_FILENAME = "_SUCCESS"
CHECKPOINT_PREFIX = "checkpoint"
MODEL_DIR = "__model__"
TRAINER_PREFIX = "trainer"
CHECKPOINT_SEPARATOR = "_"


def save_checkpoint(executor,
                    checkpoint_dir,
                    trainer_id,
                    trainer_args=None,
                    main_program=None,
                    max_num_checkpoints=3):
    """"
    This function filters out all checkpoint variables from the give
    main_program and then saves these variables to the 'checkpoint_dir' 
    directory.

    In the training precess, we generally save a checkpoint in each
    iteration. So there might be a lot of checkpoints in the 
    'checkpoint_dir'. To avoid them taking too much disk space, the 
    `max_num_checkpoints` are introduced to limit the total number of 
    checkpoints. If the number of existing checkpints is greater than 
    the `max_num_checkpoints`, the oldest ones will be scroll deleted.

    A variable is a checkpoint variable and will be loaded if it meets
    all the following conditions:
        1. It's persistable.
        2. It's type is not FEED_MINIBATCH nor FETCH_LIST nor RAW.
        3. It's name contains no "@GRAD" nor ".trainer_" nor ".block".

    Args:
        executor(Executor): The executor to run for save checkpoint.
        checkpoint_dir(str): The folder where to save checkpoints.
        trainer_id(int): currect trainer id, if id is equal to 0, the trainer 
            is chief.
        trainer_args(dict|None): Current training arguments. Such as 'epoch_id' 
            and 'step_id'.
            Defaut: None
        main_program(Program|None): The program whose checkpoint variables will
            be saved. If it is None, the default main program will be used.
        max_num_checkpoints(int): The max number of total number of existing 
            checkpoints.
            Default: 3

    Returns:
        None

    Raises:
        ValueError: If `checkpoint_dir` is None.
        AssertionError: If `trainer_args` is not a dict.

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            path = "./checkpoints"
            prog = fluid.default_main_program()
            trainer_args = {"epoch_id": 200,
                            "step_id": 20} # just an example
            fluid.io.save_checkpoint(executor=exe,
                                     checkpoint_dir=path,
                                     trainer_id=0,
                                     trainer_args=trainer_args,
                                     main_program=prog,
                                     max_num_checkpoints=3)
    """
    if checkpoint_dir is None:
        raise ValueError("'checkpoint_dir' should not be None")

    if trainer_args:
        assert isinstance(trainer_args, dict)

    if not os.path.isdir(checkpoint_dir):
        os.makedirs(checkpoint_dir)

    serial = get_latest_checkpoint_serial(checkpoint_dir) + 1
    cur_dir = _get_serial_dir(checkpoint_dir, serial)

    save_trainer_args(cur_dir, trainer_id, trainer_args)

    if trainer_id == 0:
        save_persist_vars_without_grad(executor, cur_dir, main_program)

    _scroll_delete(checkpoint_dir, max_num_checkpoints)


def load_checkpoint(executor, checkpoint_dir, serial, main_program):
    """
    This function filters out all checkpoint variables from the give
    main_program and then try to load these variables from the
    'checkpoint_dir' directory.

    In the training precess, we generally save a checkpoint in each
    iteration. So there are more than one checkpoint in the
    'checkpoint_dir'(each checkpoint has its own sub folder), use
    'serial' to specify which serial of checkpoint you would like to
    load.

    A variable is a checkpoint variable and will be loaded if it meets
    all the following conditions:
        1. It's persistable.
        2. It's type is not FEED_MINIBATCH nor FETCH_LIST nor RAW.
        3. It's name contains no "@GRAD" nor ".trainer_" nor ".block".

    Args:
        executor(Executor): The executor to run for loading checkpoint.
        checkpoint_dir(str): The folder where all checkpoints are.
        serial(int): The serial of checkpoint you would like to load.
        main_program(Program): The program whose checkpoint variables will
                               be loaded.

    Returns:
        None

    Raises:
        ValueError: If `checkpoint_dir` is None.
        ValueError: If `serial` is None or `serial` is less than 0.
        ValueError: If `main_program` is None.

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            path = "./checkpoints"
            prog = fluid.default_main_program()
            fluid.io.load_checkpoint(executor=exe, checkpoint_dir=path,
                    serial=9, main_program=prog)

            # In this example, `load_checkpoint` function
            # will first filters out all checkpoint variables in the default
            # main program, and then try to load these variables form the
            # folder "./checkpoints/checkpoint_9/__model__".
    """

    if checkpoint_dir is None:
        raise ValueError("'checkpoint_dir' should not be None")

    if serial is None or serial < 0:
        raise ValueError("'serial' should not be None or <0 ")

    if main_program is None:
        raise ValueError('main_program should not be None.')

    cur_dir = _get_serial_dir(checkpoint_dir, serial)
    load_persist_vars_without_grad(executor, cur_dir, main_program, True)


def clean_checkpoint(checkpoint_dir, delete_dir=False):
    """
    clean the checkpoint dir, when the train exits normally, the trainer will call clean_checkpoint to delete checkpoint directory saved before.
    delete_dir only works when the directory is empty, otherwise, OSError is raised.

    : param checkpoint_dir
    : param delete_dir
    """

    if checkpoint_dir is None:
        raise ValueError("'checkpoint_dir' should not be None")
    _scroll_delete(checkpoint_dir, max_num_checkpoints=0)

    if delete_dir and not os.listdir(checkpoint_dir):
        os.rmdir(checkpoint_dir)


def load_persist_vars_without_grad(executor,
                                   dirname,
                                   program,
                                   has_model_dir=False):
    """
    This function filters out all checkpoint variables from the give
    program and then try to load these variables from the given directory.

    A variable is a checkpoint variable if it meets all the following
    conditions:
        1. It's persistable.
        2. It's type is not FEED_MINIBATCH nor FETCH_LIST nor RAW.
        3. It's name contains no "@GRAD" nor ".trainer_" nor ".block".

    Args:
        executor(Executor): The executor to run for loading variables.
        dirname(str): The directory path.
        program(Program): The program whose checkpoint variables will
                          be loaded.
        has_model_dir(bool): if True, the function loads variables
                             from a sub directory named '__model__'.
                             Default: False

    Returns:
        None

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            param_path = "./my_paddle_model"
            prog = fluid.default_main_program()
            fluid.io.load_persist_vars_without_grad(executor=exe,
                    dirname=param_path, program=prog, has_model_dir=True)

            # In this example, `load_persist_vars_without_grad` function
            # will first filters out all checkpoint variables in the default
            # main program, and then trys to load these variables form the
            # folder "./my_paddle_model/__model__".
    """

    if has_model_dir:
        dirname = _get_model_dir(dirname)

    load_vars(
        executor,
        dirname=dirname,
        main_program=program,
        predicate=_is_checkpoint_var,
        filename=None)


def save_persist_vars_without_grad(executor, dirname, program):
    """
    This function filters out all checkpoint variables from the give
    program and then save these variables to a sub-folder '__model__' of 
    the given directory.

    A variable is a checkpoint variable if it meets all the following
    conditions:
        1. It's persistable.
        2. It's type is not FEED_MINIBATCH nor FETCH_LIST nor RAW.
        3. It's name contains no "@GRAD" nor ".trainer_" nor ".block".

    Args:
        executor(Executor): The executor to run for saving variables.
        dirname(str): The directory path.
        program(Program): The program whose checkpoint variables will
                          be saved.

    Returns:
        None

    Examples:
        .. code-block:: python

            exe = fluid.Executor(fluid.CPUPlace())
            param_path = "./my_paddle_model"
            prog = fluid.default_main_program()
            fluid.io.save_persist_vars_without_grad(executor=exe,
                    dirname=param_path, program=prog)

            # In this example, `save_persist_vars_without_grad` function
            # will first filters out all checkpoint variables in the default
            # main program, and then saves these variables to the folder 
            # "./my_paddle_model/__model__".
    """
    cur_dir = _get_model_dir(dirname)
    save_vars(
        executor,
        dirname=cur_dir,
        main_program=program,
        vars=None,
        predicate=_is_checkpoint_var,
        filename=None)
    _write_success(cur_dir)


def save_trainer_args(dirname, trainer_id, trainer_args):
    assert isinstance(trainer_args, dict)

    cur_dir = _get_trainer_dir(dirname, trainer_id)

    for name, value in trainer_args.iteritems():
        args_file = os.path.join(cur_dir, name)
        with open(args_file, 'w') as f:
            f.write(str(value))
    _write_success(cur_dir)


def load_trainer_args(checkpoint_dir, serial, trainer_id, trainer_args):
    assert isinstance(trainer_args, list)

    cur_dir = _get_serial_dir(checkpoint_dir, serial)
    cur_dir = _get_trainer_dir(cur_dir, trainer_id)

    ret_values = []

    for arg in trainer_args:
        cur_file = os.path.join(cur_dir, arg)
        with open(cur_file, 'r') as f:
            contents = f.read()
            ret_values.append(contents.strip())
    return ret_values


def _is_checkpoint_var(var):
    """
    the checkpoint will not save or load all the variables.
    var type is FEED_MINIBATCH/FETCH_LIST/RAW or var name ends with @GRAD are discarded.

    : param var
    """
    if var.desc.type() == core.VarDesc.VarType.FEED_MINIBATCH or \
            var.desc.type() == core.VarDesc.VarType.FETCH_LIST or \
            var.desc.type() == core.VarDesc.VarType.RAW:
        return False
    # @GRAD are named for gradient variables, checkpoint will not save it.
    if "@GRAD" in var.name:
        return False
    # .trainer_ are named for distribute train variables, checkpoint will not save it.
    if ".trainer_" in var.name:
        return False

    # .block is named for distribute train variables, checkpoint will not save it.
    if ".block" in var.name:
        return False

    return var.persistable


def _get_dir_serial(dirname):
    _, serial = dirname.split(CHECKPOINT_SEPARATOR)

    try:
        serial_num = int(serial)
    except ValueError:
        serial_num = -1
    return serial_num


def _get_serial_dir(dirname, serial):
    serial_folder = CHECKPOINT_PREFIX + CHECKPOINT_SEPARATOR + str(serial)
    serial_dir = os.path.join(dirname, serial_folder)

    if not os.path.isdir(serial_dir):
        os.makedirs(serial_dir)

    return serial_dir


def _get_model_dir(dirname):
    model_dir = os.path.join(dirname, MODEL_DIR)

    if not os.path.isdir(model_dir):
        os.makedirs(model_dir)

    return model_dir


def _get_trainer_dir(dirname, trainer_id):
    trainer_folder = TRAINER_PREFIX + CHECKPOINT_SEPARATOR + str(trainer_id)
    trainer_dir = os.path.join(dirname, trainer_folder)

    if not os.path.isdir(trainer_dir):
        os.makedirs(trainer_dir)

    return trainer_dir


def _scroll_delete(dirname, max_num_checkpoints=3):
    dirs = os.listdir(dirname)
    serial_map = {}
    for serial in dirs:
        serial_num = _get_dir_serial(serial)
        serial_map[serial_num] = serial

    if len(serial_map.keys()) <= max_num_checkpoints:
        return

    serials = serial_map.keys()
    serials.sort(reverse=True)
    serials = serials[max_num_checkpoints:]
    for serial in serials:
        cur_dir = _get_serial_dir(dirname, serial)
        shutil.rmtree(cur_dir)


def _write_success(dirname):
    """
    write an empty file named "_SUCCESS" in checkpoint dir, indicate this checkpoint is correct.

    : param dirname
    """
    success_file = os.path.join(dirname, SUCCESS_MARK_FILENAME)
    with open(success_file, 'a') as f:
        now = time.ctime()
        f.write(now)


def get_latest_checkpoint_serial(checkpoint_dir):
    """
    get the latest file in checkpoint directory, the _SUCCESS file must exist in the directory

    : param checkpoint_dir
    """
    if not checkpoint_dir:
        return -1

    def has_success(checkpoint_dir, cur_dir):
        """
        is _SUCCESS in this dir
        """

        serial = _get_dir_serial(cur_dir)
        if serial == -1 or not os.path.isdir(
                os.path.join(checkpoint_dir, cur_dir)):
            return -1

        success_path = os.path.join(
            _get_serial_dir(checkpoint_dir, serial), MODEL_DIR,
            SUCCESS_MARK_FILENAME)
        if os.path.isfile(success_path):
            return serial

    if not os.path.isdir(checkpoint_dir):
        return -1

    current_dir = -1
    dirs = os.listdir(checkpoint_dir)
    for cur_dir in dirs:
        success_num = has_success(checkpoint_dir, cur_dir)
        if success_num > current_dir:
            current_dir = success_num
    return current_dir