我创建棉花糖自定义字段如下：

from marshmallow.fields import Field

class CustomFiled(Field):
    def _serialize(self, value, attr, obj, **kwargs):
        pass

    def _deserialize(self, value, attr, data, **kwargs):
        pass

然后创建custom validator，如下所示：

from marshmallow import ValidationError

def customValidation(value):
    # why value is None, not "some value here" ??
    print("value:", value)

    if value is None:
        raise ValidationError("custom_field cannot be None!")

和开始验证的主代码：

from marshmallow import ValidationError, Schema

def main():
    try:
        data = {
            "custom_field": "some value here"
        }
        schema = {
            "custom_field": CustomFiled(required=True, validate=customValidation)
        }

        # validation begin
        ma_schema = Schema.from_dict(schema)
        ma_schema().load(data)

        # if no error occurs, then validation success
        print("Validation Success! :)")
    except ValidationError as e:
        print(e.messages)

'''
output on terminal:
    value: None
    {'custom_field': ['custom_field cannot be None!']}
'''
main()

这是完整的源代码：

from marshmallow.fields import Field
from marshmallow import ValidationError, Schema

class CustomFiled(Field):
    def _serialize(self, value, attr, obj, **kwargs):
        pass

    def _deserialize(self, value, attr, data, **kwargs):
        pass

def customValidation(value):
    # why value is None, not "some value here" ??
    print("value:", value)

    if value is None:
        raise ValidationError("custom_field cannot be None!")

def main():
    try:
        data = {
            "custom_field": "some value here"
        }
        schema = {
            "custom_field": CustomFiled(required=True, validate=customValidation)
        }

        # validation begin
        ma_schema = Schema.from_dict(schema)
        ma_schema().load(data)

        # if no error occurs, then validation success
        print("Validation Success! :)")
    except ValidationError as e:
        print(e.messages)

'''
output on terminal:
    value: None
    {'custom_field': ['custom_field cannot be None!']}
'''
main()

为什么函数customValidation()上的arg value是None而不是"some value here"？
我想终端的输出：
“value：这里有一些值”
“验证成功！：）”
但是当我尝试使用内置验证器（Str）. arg value on customValidator(value)时，会打印出来。下面是代码：

from marshmallow.fields import Str
from marshmallow import ValidationError, Schema

def supposed():
    def customValidator(value):
        print("value: ", value)

    try:
        data = {
            "name": "candra"
        }
        schema = {
            "name": Str(validate=customValidator)
        }

        # validation begin
        ma_schema = Schema.from_dict(schema)
        ma_schema().load(data)

        # if no error occurs, then validation success
        print("Validation Success! :)")
    except ValidationError as e:
        print(e.messages)

'''
Terminal output:
    value:  candra
    Validation Success! :)
'''
supposed()

端子上的输出：
值：坎德拉
验证成功！：）
这里，candra取自schema = { "name": Str(validate=customValidator) }经过的data = {'name': 'candra'}