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
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')这些钩子的原始程序是在模型训练流程中的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))
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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