DRF序列化器自定义验证失效：validate方法为何不被调用？

class LoginSerializer(serializers.Serializer):
    """
    登陆与注册
    """
    id = serializers.IntegerField(read_only=True)
    username = serializers.CharField(max_length=20, error_messages={
        'required': '用户名不能为空',
        'max_length': '用户名不能超过20个字符'
    })
    openid = serializers.CharField(source='open_id', max_length=255, write_only=True)
    unionid = serializers.CharField(source='union_id', max_length=255, write_only=True)
    avatar = serializers.CharField(max_length=255)
    login_type = serializers.IntegerField(min_value=1, max_value=1)
    user_type = serializers.IntegerField(read_only=True)
    score = serializers.IntegerField(read_only=True)
    token = serializers.CharField(read_only=True)

    def create(self, validated_data):
        return UserProfile.objects.create(**validated_data)

    def validate(self, attrs):
        print(self.context.get('attrs'))
        print(self.context.get('user'))
        attrs['token'] = get_tokens(self.context.get('user'))
        return attrs