python - 为什么克隆 FloatField 的这个子类(带有添加的验证器)会抛出异常？

Question

我是 Python 和 Django 新手。

我希望我的模型具有经过范围验证的 float 。来自 this answer我写的是:

class FloatRangeField (FloatField):
    """A FloatField constrained to a given range."""

    def __init__ (self, minimum, maximum, **kwargs):

        minmax = [MinValueValidator (minimum), MaxValueValidator (maximum)]

        print ("\n\t\tFloatRangeField({},{})".format(minimum,maximum)) # (A)
        FloatField.__init__ (self, validators = minmax, **kwargs)
        print ("\t\tFINISHED\n")

这导致 python3 manage.py migrate 中出现错误，我将其范围缩小到 clone()称呼。演示:

print ("HERE 1")
tmp1 = FloatRangeField (10, 20)
print ("HERE 2")
tmp2 = FloatRangeField (10, 20)
print ("HERE 3")
tmp3 = tmp1.clone ()      # (B)
print ("HERE 4")

它从 # (B) 行抛出异常。奇怪的是，当发生这种情况时，行# (A)处的跟踪未打印。这是输出:

HERE 1

                FloatRangeField(10,20)
                FINISHED

HERE 2

                FloatRangeField(10,20)
                FINISHED

HERE 3
Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python3.5/dist-packages/django/core/management/__init__.py", line 371, in execute_from_command_line
    utility.execute()

[...snip...]

    tmp3 = tmp1.clone ()
  File "/usr/local/lib/python3.5/dist-packages/django/db/models/fields/__init__.py", line 470, in clone
    return self.__class__(*args, **kwargs)
TypeError: __init__() missing 2 required positional arguments: 'minimum' and 'maximum'

以及 # (A) 没有打印任何内容的奇怪现象，这与我在 migrate 期间看到的错误不相同。如果我取出tmp1=...填充并运行migrate ，回溯看起来像这样

Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/django/db/migrations/state.py", line 411, in from_model
    fields.append((name, field.clone()))
  File "/usr/local/lib/python3.5/dist-packages/django/db/models/fields/__init__.py", line 470, in clone
    return self.__class__(*args, **kwargs)

[...snip...]

    FloatField.__init__ (self, validators = minmax, **kwargs)
TypeError: __init__() got multiple values for keyword argument 'validators'

为什么会出现这种情况以及我应该如何实现 FloatRangeField以便封装最小/最大验证器？

Answer 1

就像documentation on custom model fields中写的那样:

If you add a new keyword argument, you need to write code to put its value into kwargs yourself (...)

所以你还应该添加一个解构函数。这是必要的，例如在迁移文件中表示此字段:

class FloatRangeField (FloatField):
    """A FloatField constrained to a given range."""

    def __init__ (self, minimum, maximum, **kwargs):
        <b>self.minimum = minimum</b>
        <b>self.maximum = maximum</b>
        minmax = [MinValueValidator (minimum), MaxValueValidator (maximum)]
        FloatField.__init__ (self, validators = minmax, **kwargs)

    def deconstruct(self):
        result = __, __, __, kwargs = super(FloatRangeField, self).deconstruct()
        <b>kwargs['minimum'] = self.minimum</b>
        <b>kwargs['minimum'] = self.maximum</b>
        <b>del kwargs['validators']</b>
        return result

请注意，您最好不要使用 validators = minmax 作为参数，因为这意味着如果用户将 validator 参数用于您的 FloatRangeField 构造函数，会出现参数冲突。

例如，可以附加我们的minmax验证器到已经存在的验证器，然后当我们想要解构它时将它们从验证器中弹出:

class FloatRangeField (FloatField):
    """A FloatField constrained to a given range."""

    def __init__ (self, minimum, maximum, **kwargs):
        self.minimum = minimum
        self.maximum = maximum
        <b>old_validators = kwargs.get('validators', [])</b>
        minmax = [MinValueValidator (minimum), MaxValueValidator (maximum)]
        <b>minmax += old_validators</b>
        <b>kwargs['validators'] = minmax</b>
        FloatField.__init__ (self, **kwargs)
    

    def deconstruct(self):
        result = __, __, __, kwargs = super(FloatRangeField, self).deconstruct()
        kwargs['minimum'] = self.minimum
        kwargs['minimum'] = self.maximum
        <b>kwargs['validators'] = kwargs['validators'][2:]</b>
        return result

因此，我们在 deconstruct(..) 函数中删除了前两个验证器(我们在 __init__(..) 函数中添加了它们)。