Mmcv Config class introduction

Prepare information

  This series mainly introduces mmcv And mmdetection Source code interpretation . therefore , It is suggested that the reader should install it locally first mmdetection Environmental Science . Installation tutorial ：mmdet2.8 The latest installation tutorial ！

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Preface

  This is mmcv Source code interpretation Config class Introduce . The code address is mmcv/utils/config.py In file .

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1、FasterRcnn For example

  Most online Config Class explanation is a particularly dry code introduction , Lack of a concrete example to understand . therefore , This article takes mmdetection Medium FasterRcnn Network as an example Config class .（ Of course ,Config Class also has many functions , Still need to learn to explore , Welcome to exchange :Q2541612007）.
  Stick it down FasterRcnn Configuration file for mmdetection/configs/faster_rcnn/faster_rcnn_r50_fpn_1x_coco.py）：

_base_ = [
    '../_base_/models/faster_rcnn_r50_fpn.py',
    '../_base_/datasets/coco_detection.py',
    '../_base_/schedules/schedule_1x.py', '../_base_/default_runtime.py'
]

  In the above configuration file ,_base_ The length is 4 Of list： Model , Data sets , Optimizer , How to train . Here under Config Class action ：“ take The fields in the configuration file are converted into a dictionary ”. A simple example ： With coco_detection.py Some file fields are taken as examples ：

dataset_type = 'CocoDataset'
data_root = 'data/coco/'
img_norm_cfg = dict(
    mean=[123.675, 116.28, 103.53], std=[58.395, 57.12, 57.375], to_rgb=True)

  Emotionally ： Will be Above “=” The one on the left is regarded as a dictionary key;“=” The content on the right is regarded as value. I call it “ Equal sign rule ”. The final course Config Class returns an instantiated cfg object , use _cfg_dict initialization Config class , Become an attribute . actually _cfg_dict Still a dictionary .

2、 Implementation process

  As can be seen from the first part ：Config Class actually does two things ：
 （1） Set the configuration file as “ Equal sign rule “ Put the contents of the configuration file into a dictionary _cfg_dict.
 （2） Using a dictionary _cfg_dict complete Config Class initialization , Make it a Config An attribute of a class .
  therefore , The first part of the work actually belongs to Config Class . therefore , It can be used Python Static methods in @staticmethod complete . The second part is direct init The class can . Next, I will use two parts to introduce . The process is shown in the figure below ：

2.1. The configuration file is transferred to the dictionary (_file2dict function )

  from faster_rcnn From the configuration file of ： In fact, one with _base_ List at the beginning of the field . The elements in the list are four paths . therefore , The function program needs to traverse 4 A path , Then add it to a dictionary . The overall logic of the function is as follows （ use ppt Painted , Forgive me ~）：

  After the overall logic is clarified , It's much easier to look at the code . I only post some here The core Of （ I can't understand many details , What the boss wrote is so awesome ）.

    @staticmethod
    def _file2dict(filename, use_predefined_variables=True):
        filename = osp.abspath(osp.expanduser(filename))      # filename: py The absolute path to the file 
                #  take file Go to a dictionary 
                cfg_dict = {
    
                    name: value
                    for name, value in mod.__dict__.items()
                    if not name.startswith('__')
                }
        if BASE_KEY in cfg_dict:   #  To determine if there is _base_ Field 
            base_filename = cfg_dict.pop(BASE_KEY)      # base_filename = ['../_base_/models/faster_rcnn_r50_fpn.py','../_base_/datasets/coco_detection.py','','']
            cfg_dict_list = list()                      #  Store final results 
            cfg_text_list = list()
            for f in base_filename:
                _cfg_dict, _cfg_text = Config._file2dict(osp.join(cfg_dir, f))  #  Traversal recursive call file2path
                cfg_dict_list.append(_cfg_dict)
                cfg_text_list.append(_cfg_text)
            #  After the traversal is over ,cfg_dict_list=[{},{},{},{}] , cfg_text_list = ['','','','']  What's in it is   Contents in the four configuration files 
            base_cfg_dict = dict()
            for c in cfg_dict_list:
                if len(base_cfg_dict.keys() & c.keys()) > 0:
                    raise KeyError('Duplicate key is not allowed among bases')
                base_cfg_dict.update(c)
        return cfg_dict, cfg_text

  Probably no problem . Easier to understand .

2.2. Config Class initialization

 2.1 Got it cfg_dict, Then you can use it to complete initialization .

   def __init__(self, cfg_dict=None, cfg_text=None, filename=None):
        super(Config, self).__setattr__('_cfg_dict', ConfigDict(cfg_dict))# Config Class to set a _cfg_dict attribute , And will cfg_dict Example is ConfigDict class .
        super(Config, self).__setattr__('_filename', filename)            # Config Class to set a _filename attribute .
        super(Config, self).__setattr__('_text', text)                    # Config Adding a '_text' attribute 
    def __setattr__(self, name, value):
        if isinstance(value, dict):
            value = ConfigDict(value)
        self._cfg_dict.__setattr__(name, value)

  The code rewrites __setattr__ Method . Simple is to cfg_dict Become a ConfigDict class . The essence is a dictionary , It just rewrites __getattr__ Method , On the basis of key Some logical judgments are added to the index （keyerror What? ）, Here it is still understood as a common dictionary .

class ConfigDict(Dict):

    def __missing__(self, name):
        raise KeyError(name)

    def __getattr__(self, name):
        try:
            value = super(ConfigDict, self).__getattr__(name)
        except KeyError:
            ex = AttributeError(f"'{
      self.__class__.__name__}' object has no "
                                f"attribute '{
      name}'")
        except Exception as e:
            ex = e
        else:
            return value
        raise ex

summary

  This article mainly introduces Config The design concept of class puts . actually Config Class has many other methods , such as _merge_a_to_b etc. . But they can't escape those two parts ： Change the field to according to the equal sign rule dict, And then use it dict initialization Config class .
  If you have any questions, welcome +vx：wulele2541612007, Pull you into the group to discuss communication .