python 钩子函数,python钩子函数原理

  python 钩子函数,python钩子函数原理

  

Python教程栏目介绍Python中的Hook钩子函数

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  1. 什么是Hook

  经常听到钩子函数的概念。最近在看mmdetection,一个用于目标检测的开源框架。里面也有很多钩子的编程方法。那么hook到底是什么?钩子的作用是什么?

  钩子是什么?钩子,顾名思义,可以理解为钩子。它的作用是在需要的时候挂东西。具体解释就是:钩子函数就是在某个时间把我们自己的钩子函数钩到目标挂载点。

  比如hook的概念在windows桌面软件开发中很常见,尤其是触发各种事件的机制;比如在C的MFC程序中,为了监控鼠标左键按下的时间,MFC提供了onLeftKeyDown的钩子函数。显然,MFC框架并没有为我们实现onLeftKeyDown的具体操作,只是为我们提供了一个钩子。当我们需要处理它的时候,我们只需要重写这个函数,在这个钩子中挂载我们需要的操作。如果我们不挂载它,MFC事件触发机制将执行一个空操作。

  从上面可以看出

  钩子函数是程序中预定义的函数,在原程序流程中(暴露一个钩子)。

  我们需要在流程中的钩子定义的功能块中实现一个特定的细节,我们需要在钩子中挂接或注册我们的实现,使钩子功能对目标可用。

  Hook是一种编程机制,与具体语言没有直接关系。

  从设计模式的角度来看,钩子模式是模板方法的扩展。

  钩子只有在注册的时候才会用到,所以当原程序没有注册或者挂载的时候,执行是空的(也就是不执行任何操作)。

  本文用python解释了hook的实现,并展示了hook在开源项目中的应用案例。钩子函数类似于我们经常听到的另一个名字:回调函数,可以按照同样的模式来理解。

  00-1010据我所知,钩子函数最常用于某种进程。这个过程通常有许多步骤。钩子函数通常安装在这些步骤中,以便为附加操作提供灵活性。

  我们举个简单的例子。这个例子的目的是实现一个将内容插入队列的通用功能。有两个步骤。

  在将数据插入队列input_filter_fn之前过滤数据

  插入队列插入队列

  class ContentStash(object):

  用于在线操作的内容存储

  管道是

  1.过滤一些内容,对用户没有用

  2.insert_queue(redis或其他代理):将有用的内容插入到队列中

  def __init__(self):

  self.input_filter_fn=None

  self.broker=[]

  def寄存器_输入_过滤器_钩子(self,input_filter_fn):

  注册输入过滤函数,参数是内容字典

  Args:

  input_filter_fn:输入过滤功能

  返回:

  self . input _ filter _ fn=input _ filter _ fn

  def insert_queue(自身,内容):

  quot;"

   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

      

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