1. 什么是Hook

经常会听到钩子函数(hook function)这个概念，最近在看目标检测开源框架mmdetection，里面也出现大量Hook的编程方式，那到底什么是hook？hook的作用是什么？

• what is hook ？钩子hook，顾名思义，可以理解是一个挂钩，作用是有需要的时候挂一个东西上去。具体的解释是：钩子函数是把我们自己实现的hook函数在某一时刻挂接到目标挂载点上。

• hook函数的作用 举个例子，hook的概念在windows桌面软件开发很常见，特别是各种事件触发的机制; 比如C++的MFC程序中，要监听鼠标左键按下的时间，MFC提供了一个onLeftKeyDown的钩子函数。很显然，MFC框架并没有为我们实现onLeftKeyDown具体的操作，只是为我们提供一个钩子，当我们需要处理的时候，只要去重写这个函数，把我们需要操作挂载在这个钩子里，如果我们不挂载，MFC事件触发机制中执行的就是空的操作。

从上面可知

• hook函数是程序中预定义好的函数，这个函数处于原有程序流程当中（暴露一个钩子出来）

• 我们需要再在有流程中钩子定义的函数块中实现某个具体的细节，需要把我们的实现，挂接或者注册（register）到钩子里，使得hook函数对目标可用

• hook 是一种编程机制，和具体的语言没有直接的关系

• 如果从设计模式上看，hook模式是模板方法的扩展

• 钩子只有注册的时候，才会使用，所以原有程序的流程中，没有注册或挂载时，执行的是空（即没有执行任何操作）

本文用python来解释hook的实现方式，并展示在开源项目中hook的应用案例。hook函数和我们常听到另外一个名称：回调函数（callback function）功能是类似的，可以按照同种模式来理解。

2. hook实现例子

据我所知，hook函数最常使用在某种流程处理当中。这个流程往往有很多步骤。hook函数常常挂载在这些步骤中，为增加额外的一些操作，提供灵活性。

下面举一个简单的例子，这个例子的目的是实现一个通用往队列中插入内容的功能。流程步骤有2个

• 需要再插入队列前，对数据进行筛选 input_filter_fn

• 插入队列 insert_queue

class ContentStash(object):  
    """  
    content stash for online operation  
    pipeline is  
    1. input_filter: filter some contents, no use to user  
    2. insert_queue(redis or other broker): insert useful content to queue  
    """  

    def __init__(self):  
        self.input_filter_fn = None  
        self.broker = []  

    def register_input_filter_hook(self, input_filter_fn):  
        """  
        register input filter function, parameter is content dict  
        Args:  
            input_filter_fn: input filter function  

        Returns:  

        """  
        self.input_filter_fn = input_filter_fn  

    def insert_queue(self, content):  
        """  
        insert content to queue  
        Args:  
            content: dict  

        Returns:  

        """  
        self.broker.append(content)  

    def input_pipeline(self, content, use=False):  
        """  
        pipeline of input for content stash  
        Args:  
            use: is use, defaul False  
            content: dict  

        Returns:  

        """  
        if not use:  
            return  

        # input filter  
        if self.input_filter_fn:  
            _filter = self.input_filter_fn(content)  

        # insert to queue  
        if not _filter:  
            self.insert_queue(content)  



# test  
## 实现一个你所需要的钩子实现：比如如果content 包含time就过滤掉，否则插入队列  
def input_filter_hook(content):  
    """  
    test input filter hook  
    Args:  
        content: dict  

    Returns: None or content  

    """  
    if content.get('time') is None:  
        return  
    else:  
        return content  


# 原有程序  
content = {'filename': 'test.jpg', 'b64_file': "#test", 'data': {"result": "cat", "probility": 0.9}}  
content_stash = ContentStash('audit', work_dir='')  

# 挂上钩子函数， 可以有各种不同钩子函数的实现，但是要主要函数输入输出必须保持原有程序中一致，比如这里是content  
content_stash.register_input_filter_hook(input_filter_hook)  

# 执行流程  
content_stash.input_pipeline(content)  

3. hook在开源框架中的应用

3.1 keras

在深度学习训练流程中，hook函数体现的淋漓尽致。

一个训练过程（不包括数据准备），会轮询多次训练集，每次称为一个epoch，每个epoch又分为多个batch来训练。流程先后拆解成：

• 开始训练

• 训练一个epoch前

• 训练一个batch前

• 训练一个batch后

• 训练一个epoch后

• 评估验证集

• 结束训练

这些步骤是穿插在训练一个batch数据的过程中，这些可以理解成是钩子函数，我们可能需要在这些钩子函数中实现一些定制化的东西，比如在训练一个epoch后我们要保存下训练的模型，在结束训练时用最好的模型执行下测试集的效果等等。

keras中是通过各种回调函数来实现钩子hook功能的。这里放一个callback的父类，定制时只要继承这个父类，实现你过关注的钩子就可以了。

@keras_export('keras.callbacks.Callback')  
class Callback(object):  
  """Abstract base class used to build new callbacks.  

  Attributes:  
      params: Dict. Training parameters  
          (eg. verbosity, batch size, number of epochs...).  
      model: Instance of `keras.models.Model`.  
          Reference of the model being trained.  

  The `logs` dictionary that callback methods  
  take as argument will contain keys for quantities relevant to  
  the current batch or epoch (see method-specific docstrings).  
  """  

  def __init__(self):  
    self.validation_data = None  # pylint: disable=g-missing-from-attributes  
    self.model = None  
    # Whether this Callback should only run on the chief worker in a  
    # Multi-Worker setting.  
    # TODO(omalleyt): Make this attr public once solution is stable.  
    self._chief_worker_only = None  
    self._supports_tf_logs = False  

  def set_params(self, params):  
    self.params = params  

  def set_model(self, model):  
    self.model = model  

 @doc_controls.for_subclass_implementers  
 @generic_utils.default  
  def on_batch_begin(self, batch, logs=None):  
    """A backwards compatibility alias for `on_train_batch_begin`."""  

 @doc_controls.for_subclass_implementers  
 @generic_utils.default  
  def on_batch_end(self, batch, logs=None):  
    """A backwards compatibility alias for `on_train_batch_end`."""  

 @doc_controls.for_subclass_implementers  
  def on_epoch_begin(self, epoch, logs=None):  
    """Called at the start of an epoch.  

    Subclasses should override for any actions to run. This function should only  
    be called during TRAIN mode.  

    Arguments:  
        epoch: Integer, index of epoch.  
        logs: Dict. Currently no data is passed to this argument for this method  
          but that may change in the future.  
    """  

 @doc_controls.for_subclass_implementers  
  def on_epoch_end(self, epoch, logs=None):  
    """Called at the end of an epoch.  

    Subclasses should override for any actions to run. This function should only  
    be called during TRAIN mode.  

    Arguments:  
        epoch: Integer, index of epoch.  
        logs: Dict, metric results for this training epoch, and for the  
          validation epoch if validation is performed. Validation result keys  
          are prefixed with `val_`.  
    """  

 @doc_controls.for_subclass_implementers  
 @generic_utils.default  
  def on_train_batch_begin(self, batch, logs=None):  
    """Called at the beginning of a training batch in `fit` methods.  

    Subclasses should override for any actions to run.  

    Arguments:  
        batch: Integer, index of batch within the current epoch.  
        logs: Dict, contains the return value of `model.train_step`. Typically,  
          the values of the `Model`'s metrics are returned.  Example:  
          `{'loss': 0.2, 'accuracy': 0.7}`.  
    """  
    # For backwards compatibility.  
    self.on_batch_begin(batch, logs=logs)  

 @doc_controls.for_subclass_implementers  
 @generic_utils.default  
  def on_train_batch_end(self, batch, logs=None):  
    """Called at the end of a training batch in `fit` methods.  

    Subclasses should override for any actions to run.  

    Arguments:  
        batch: Integer, index of batch within the current epoch.  
        logs: Dict. Aggregated metric results up until this batch.  
    """  
    # For backwards compatibility.  
    self.on_batch_end(batch, logs=logs)  

 @doc_controls.for_subclass_implementers  
 @generic_utils.default  
  def on_test_batch_begin(self, batch, logs=None):  
    """Called at the beginning of a batch in `evaluate` methods.  

    Also called at the beginning of a validation batch in the `fit`  
    methods, if validation data is provided.  

    Subclasses should override for any actions to run.  

    Arguments:  
        batch: Integer, index of batch within the current epoch.  
        logs: Dict, contains the return value of `model.test_step`. Typically,  
          the values of the `Model`'s metrics are returned.  Example:  
          `{'loss': 0.2, 'accuracy': 0.7}`.  
    """  

 @doc_controls.for_subclass_implementers  
 @generic_utils.default  
  def on_test_batch_end(self, batch, logs=None):  
    """Called at the end of a batch in `evaluate` methods.  

    Also called at the end of a validation batch in the `fit`  
    methods, if validation data is provided.  

    Subclasses should override for any actions to run.  

    Arguments:  
        batch: Integer, index of batch within the current epoch.  
        logs: Dict. Aggregated metric results up until this batch.  
    """  

 @doc_controls.for_subclass_implementers  
 @generic_utils.default  
  def on_predict_batch_begin(self, batch, logs=None):  
    """Called at the beginning of a batch in `predict` methods.  

    Subclasses should override for any actions to run.  

    Arguments:  
        batch: Integer, index of batch within the current epoch.  
        logs: Dict, contains the return value of `model.predict_step`,  
          it typically returns a dict with a key 'outputs' containing  
          the model's outputs.  
    """  

 @doc_controls.for_subclass_implementers  
 @generic_utils.default  
  def on_predict_batch_end(self, batch, logs=None):  
    """Called at the end of a batch in `predict` methods.  

    Subclasses should override for any actions to run.  

    Arguments:  
        batch: Integer, index of batch within the current epoch.  
        logs: Dict. Aggregated metric results up until this batch.  
    """  

 @doc_controls.for_subclass_implementers  
  def on_train_begin(self, logs=None):  
    """Called at the beginning of training.  

    Subclasses should override for any actions to run.  

    Arguments:  
        logs: Dict. Currently no data is passed to this argument for this method  
          but that may change in the future.  
    """  

 @doc_controls.for_subclass_implementers  
  def on_train_end(self, logs=None):  
    """Called at the end of training.  

    Subclasses should override for any actions to run.  

    Arguments:  
        logs: Dict. Currently the output of the last call to `on_epoch_end()`  
          is passed to this argument for this method but that may change in  
          the future.  
    """  

 @doc_controls.for_subclass_implementers  
  def on_test_begin(self, logs=None):  
    """Called at the beginning of evaluation or validation.  

    Subclasses should override for any actions to run.  

    Arguments:  
        logs: Dict. Currently no data is passed to this argument for this method  
          but that may change in the future.  
    """  

 @doc_controls.for_subclass_implementers  
  def on_test_end(self, logs=None):  
    """Called at the end of evaluation or validation.  

    Subclasses should override for any actions to run.  

    Arguments:  
        logs: Dict. Currently the output of the last call to  
          `on_test_batch_end()` is passed to this argument for this method  
          but that may change in the future.  
    """  

 @doc_controls.for_subclass_implementers  
  def on_predict_begin(self, logs=None):  
    """Called at the beginning of prediction.  

    Subclasses should override for any actions to run.  

    Arguments:  
        logs: Dict. Currently no data is passed to this argument for this method  
          but that may change in the future.  
    """  

 @doc_controls.for_subclass_implementers  
  def on_predict_end(self, logs=None):  
    """Called at the end of prediction.  

    Subclasses should override for any actions to run.  

    Arguments:  
        logs: Dict. Currently no data is passed to this argument for this method  
          but that may change in the future.  
    """  

  def _implements_train_batch_hooks(self):  
    """Determines if this Callback should be called for each train batch."""  
    return (not generic_utils.is_default(self.on_batch_begin) or  
            not generic_utils.is_default(self.on_batch_end) or  
            not generic_utils.is_default(self.on_train_batch_begin) or  
            not generic_utils.is_default(self.on_train_batch_end))  

这些钩子的原始程序是在模型训练流程中的

keras源码位置： tensorflow\python\keras\engine\training.py

部分摘录如下(## I am hook)：

# Container that configures and calls `tf.keras.Callback`s.  
      if not isinstance(callbacks, callbacks_module.CallbackList):  
        callbacks = callbacks_module.CallbackList(  
            callbacks,  
            add_history=True,  
            add_progbar=verbose != 0,  
            model=self,  
            verbose=verbose,  
            epochs=epochs,  
            steps=data_handler.inferred_steps)  

      ## I am hook  
      callbacks.on_train_begin()  
      training_logs = None  
      # Handle fault-tolerance for multi-worker.  
      # TODO(omalleyt): Fix the ordering issues that mean this has to  
      # happen after `callbacks.on_train_begin`.  
      data_handler._initial_epoch = (  # pylint: disable=protected-access  
          self._maybe_load_initial_epoch_from_ckpt(initial_epoch))  
      for epoch, iterator in data_handler.enumerate_epochs():  
        self.reset_metrics()  
        callbacks.on_epoch_begin(epoch)  
        with data_handler.catch_stop_iteration():  
          for step in data_handler.steps():  
            with trace.Trace(  
                'TraceContext',  
                graph_type='train',  
                epoch_num=epoch,  
                step_num=step,  
                batch_size=batch_size):  
              ## I am hook  
              callbacks.on_train_batch_begin(step)  
              tmp_logs = train_function(iterator)  
              if data_handler.should_sync:  
                context.async_wait()  
              logs = tmp_logs  # No error, now safe to assign to logs.  
              end_step = step + data_handler.step_increment  
              callbacks.on_train_batch_end(end_step, logs)  
        epoch_logs = copy.copy(logs)  

        # Run validation.  

        ## I am hook  
        callbacks.on_epoch_end(epoch, epoch_logs)  

3.2 mmdetection

mmdetection是一个目标检测的开源框架，集成了许多不同的目标检测深度学习算法（pytorch版），如faster-rcnn, fpn, retianet等。里面也大量使用了hook，暴露给应用实现流程中具体部分。

详见https://github.com/open-mmlab/mmdetection

这里看一个训练的调用例子(摘录)（https://github.com/open-mmlab/mmdetection/blob/5d592154cca589c5113e8aadc8798bbc73630d98/mmdet/apis/train.py）

def train_detector(model,  
                   dataset,  
                   cfg,  
                   distributed=False,  
                   validate=False,  
                   timestamp=None,  
                   meta=None):  
    logger = get_root_logger(cfg.log_level)  

    # prepare data loaders  

    # put model on gpus  

    # build runner  
    optimizer = build_optimizer(model, cfg.optimizer)  
    runner = EpochBasedRunner(  
        model,  
        optimizer=optimizer,  
        work_dir=cfg.work_dir,  
        logger=logger,  
        meta=meta)  
    # an ugly workaround to make .log and .log.json filenames the same  
    runner.timestamp = timestamp  

    # fp16 setting  
    # register hooks  
    runner.register_training_hooks(cfg.lr_config, optimizer_config,  
                                   cfg.checkpoint_config, cfg.log_config,  
                                   cfg.get('momentum_config', None))  
    if distributed:  
        runner.register_hook(DistSamplerSeedHook())  

    # register eval hooks  
    if validate:  
        # Support batch_size > 1 in validation  
        eval_cfg = cfg.get('evaluation', {})  
        eval_hook = DistEvalHook if distributed else EvalHook  
        runner.register_hook(eval_hook(val_dataloader, **eval_cfg))  

    # user-defined hooks  
    if cfg.get('custom_hooks', None):  
        custom_hooks = cfg.custom_hooks  
        assert isinstance(custom_hooks, list), \  
            f'custom_hooks expect list type, but got {type(custom_hooks)}'  
        for hook_cfg in cfg.custom_hooks:  
            assert isinstance(hook_cfg, dict), \  
                'Each item in custom_hooks expects dict type, but got ' \  
                f'{type(hook_cfg)}'  
            hook_cfg = hook_cfg.copy()  
            priority = hook_cfg.pop('priority', 'NORMAL')  
            hook = build_from_cfg(hook_cfg, HOOKS)  
            runner.register_hook(hook, priority=priority)  

4. 总结

本文介绍了hook的概念和应用，并给出了python的实现细则。希望对比有帮助。总结如下：

• hook函数是流程中预定义好的一个步骤，没有实现

• 挂载或者注册时， 流程执行就会执行这个钩子函数

• 回调函数和hook函数功能上是一致的

• hook设计方式带来灵活性，如果流程中有一个步骤，你想让调用方来实现，你可以用hook函数

作者简介：wedo实验君, 数据分析师；热爱生活，热爱写作

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