detectron2 中yacs的使用解读

在阅读Detectron2的源代码是发现，代码中对模型的配置大量使用了yacs这个模块。
yacs是一个python库，用于为一个系统构建config文件

安装

$ pip install yacs

导入

from yacs.config import CfgNode as CN

使用
创建配置节点
需要创建CN()这个作为容器来装载我们的参数，这个容器可以嵌套

from yacs.config import CfgNode as CN
__C = CN()
__C.name = 'test'
__C.model = CN()  # 嵌套使用
__C.model.backbone = 'resnet'
__C.model.depth = 18

print(__C)  
'''
  name: test
  model:
      backbone: resnet
      depth: 18
'''

API reference
使用__C 作为创建的配置文件

1. clone()
   return a copy config file, so the defaults will not be altered

def get_cfg_defaults():
	return __C.clone()

2. clear()
   clear your config file, you will get None as the result

print(__C.clear())  # None

3. merge_from_file()
   对于不同的模型配置，你有不同的超参设置，所以你可以使用yaml文件来管理不同的configs，然后使用merge_from_file()这个方法，这个会比较每个模型特有的config和默认参数的区别，会将默认参数与特定参数不同的部分，用特定参数覆盖。

__C.merge_from_file("./test_config.yaml")

4. merge_from_list()
   可以用list来传递参数

from yacs.config import CfgNode as CN
__C = CN()
__C.name = 'test'
__C.model = CN()
__C.model.backbone = 'resnet'
__C.model.depth = 18
print(__C)
'''
model:
  backbone: resnet
  depth: 18
name: test
'''

opts = ["name", 'test_name', "model.backbone", "vgg"]
__C.merge_from_list(opts)
print(__C)
'''
model:
  backbone: vgg
  depth: 18
name: test_name
'''

5. merge_from_other_cfg()
   the same as merge_from_file and merge_from_list, the only difference is that the merged file is also a CfgNode class

6. freeze()
   freeze the configs, and you can not change the value after this operation

from yacs.config import CfgNode as CN
__C = CN()
__C.name = 'test'
__C.model = CN()
__C.model.backbone = 'resnet'
__C.model.depth = 18

# freeze the config
__C.freeze()
# try to change the name's value, raise an error
__C.name = 'test2'  # error

7. defrost()
   reverse operation of freeze()

from yacs.config import CfgNode as CN
__C = CN()
__C.name = 'test'
__C.model = CN()
__C.model.backbone = 'resnet'
__C.model.depth = 18

# freeze the config
__C.freeze()
# try to change the name's value, raise an error
__C.name = 'test2'  # error

__C.defrost()  # not freeze cfgs, after this operation you can change the value
__C.name = 'test2'  # work

参考博客：https://zhuanlan.zhihu.com/p/366289700