Django自定义YamlField实现过程解析

import yaml
from django.db import models

class YamlField(models.TextField):
  def to_python(self, value): # 将数据库内容转为python对象时调用
    if not value:
      value = {}
    if isinstance(value, (list, dict)):
      return value
    return yaml.safe_load(value)

  def get_prep_value(self, value): # create时插入数据, 转为字符串存储
    return value if value is None else yaml.dump(value, default_flow_style=False)

  def from_db_value(self, value, expression, connection): # 从数据库读取字段是调用
    return self.to_python(value)

import django
from django.db import models
from django import forms
from django.core.exceptions import ValidationError
import yaml


class YamlWidget(forms.Textarea):
  def render(self, name, value, attrs=None, renderer=None):
    if value is None:
      value = ""
    if not isinstance(value, str):
      value = yaml.safe_dump(value, default_flow_style=False)
    if django.VERSION < (2, 0):
      return super().render(name, value, attrs)
    return super().render(name, value, attrs, renderer)


class YamlFormField(forms.CharField):
  empty_values = [None, '']

  def __init__(self, *args, **kwargs):
    if 'widget' not in kwargs:
      kwargs['widget'] = YamlWidget
    super().__init__(*args, **kwargs)

  def to_python(self, value):
    if isinstance(value, str) and value:
      try:
        return yaml.safe_load(value)
      except Exception as exc:
        raise forms.ValidationError('Yaml decode error: %s' % (exc.args[0],))
    else:
      return value

  def validate(self, value):
    if value in self.empty_values and self.required:
      raise forms.ValidationError(self.error_messages['required'], code='required')


class YamlField(models.Field):
  description = "Yaml object"

  def get_internal_type(self):
    return 'TextField'

  def formfield(self, **kwargs):
    defaults = {
      'form_class': YamlFormField,
      'widget': YamlWidget
    }
    defaults.update(**kwargs)
    return super().formfield(**defaults)

  def to_python(self, value: str): # 将数据库内容转为python对象时调用
    if value is None:
      if not self.null and self.blank:
        return ""
      return None
    if isinstance(value, (list, dict)):
      return value
    value = yaml.safe_load(value)
    return value

  def validate(self, value, model_instance): # 验证从接受到字典格式
    if not self.null and value is None:
      raise ValidationError(self.error_messages['null'])
    try:
      self.get_prep_value(value)
    except ValueError:
      raise ValidationError(self.error_messages['invalid'] % value)

  def get_prep_value(self, value: (list, dict)): # 保存时插入数据, 转为字符串存储
    if value is None:
      return None
    value = yaml.safe_dump(value, default_flow_style=False)
    return value

  def from_db_value(self, value: str, expression, connection, *args, **kwargs): # 从数据库读取字段是调用
    return self.to_python(value)

  def value_to_string(self, obj): # Rest Framework调用时
    return self.value_from_object(obj)