# 如果没有_validated_data，则对initial_data验证，把验证通过的返回值给到_initial_data
# 如果验证失败，抛出_validated_data异常，_validated_data = {}，把异常给到_errors
# 如果没有异常，则self._errors = {}
if not hasattr(self, '_validated_data'):
    try:
        self._validated_data = self.run_validation(self.initial_data)
    except ValidationError as exc:
        self._validated_data = {}
        self._errors = exc.detail
    else:
        self._errors = {}

　　

先执行try内容，通不过，则看下能不能匹配到SyntaxError或者SystemError

如果都匹配不到，则print('unknow error')

try通过，则继续运行else的内容

try:
    a = b
    print(a)
except SyntaxError as e:
    print(e)
except SystemError as e:
    print(e)
except:
    print('unknow error')
else:
    print('xx')