Django:重用表单字段而不继承？

Question

如果我有两个基于不同基类(例如 Form 和 ModelForm)的表单，但我想在两者中使用一些字段，我可以以 DRY 方式重用它们吗？

考虑以下场景:

class AfricanSwallowForm(forms.ModelForm):
    airspeed_velocity = forms.IntegerField(some_important_details_here)
    is_migratory = forms.BooleanField(more_important_details)

    class Meta:
        model = AfricanBird

class EuropeanSwallowForm(forms.Form):
    airspeed_velocity = forms.IntegerField(some_important_details_here)
    is_migratory = forms.BooleanField(more_important_details)

....有没有一种方法可以重复使用 airspeed_velocity 和 is_migratory 字段？想象一下，我有几十种这样的表格。如果我一遍又一遍地写这些代码，代码会很累。

(假设，就这个问题而言，我不能或不会将 airspeed_velocity 和 is_migratory 转换为非洲鸟模型的字段。)

Answer 1

您可以使用多重继承(又名mixin)来分解在 Form 和 ModelForm 中使用的字段。

class SwallowFormFields:
    airspeed_velocity = forms.IntegerField( ... )
    is_migratory = forms.BooleanField( ... )

class AfricanSwallowForm(forms.ModelForm, SwallowFormFields):
    class Meta:
        model = AfricanBird

class EuropeanSwallowForm(forms.Form, SwallowFormFields):
    pass

更新:

由于这不适用于 Django 元编程，您需要创建一个自定义 __init__将继承的字段添加到对象的字段列表的构造函数，或者您可以在类定义中显式添加引用:

class SwallowFormFields:
    airspeed_velocity = forms.IntegerField()
    is_migratory = forms.BooleanField()

class AfricanSwallowForm(forms.ModelForm):
    airspeed_velocity = SwallowFormFields.airspeed_velocity
    is_migratory = SwallowFormFields.is_migratory
    class Meta:
        model = AfricanSwallow

class EuropeanSwallowForm(forms.Form):
    airspeed_velocity = SwallowFormFields.airspeed_velocity
    is_migratory = SwallowFormFields.is_migratory

更新 :

当然，您不必将共享字段嵌套到类中——您也可以简单地将它们定义为全局变量......

airspeed_velocity = forms.IntegerField()
is_migratory = forms.BooleanField()

class AfricanSwallowForm(forms.ModelForm):
    airspeed_velocity = airspeed_velocity
    is_migratory = is_migratory
    class Meta:
        model = AfricanSwallow

class EuropeanSwallowForm(forms.Form):
    airspeed_velocity = airspeed_velocity
    is_migratory = is_migratory

更新:

好吧，如果你真的想最大限度地干燥，你必须使用 元类 .

所以你可以这样做:

from django.forms.models import ModelForm, ModelFormMetaclass
from django.forms.forms import get_declared_fields, DeclarativeFieldsMetaclass
from django.utils.copycompat import deepcopy

class MixinFormMetaclass(ModelFormMetaclass, DeclarativeFieldsMetaclass):
    def __new__(cls, name, bases, attrs):

        # default __init__ that calls all base classes
        def init_all(self, *args, **kwargs):
            for base in bases:
                super(base, self).__init__(*args, **kwargs)
        attrs.setdefault('__init__', init_all)

        # collect declared fields
        attrs['declared_fields'] = get_declared_fields(bases, attrs, False)

        # create the class
        new_cls = super(MixinFormMetaclass, cls).__new__(cls, name, bases, attrs)
        return new_cls

class MixinForm(object):
    __metaclass__ = MixinFormMetaclass
    def __init__(self, *args, **kwargs):
        self.fields = deepcopy(self.declared_fields)

您现在可以从 MixinForm 派生您的表单域集合，如下所示:

class SwallowFormFields(MixinForm):
    airspeed_velocity = forms.IntegerField()
    is_migratory = forms.BooleanField()

class MoreFormFields(MixinForm):
    is_endangered = forms.BooleanField()

然后将它们添加到基类列表中，如下所示:

class EuropeanSwallowForm(forms.Form, SwallowFormFields, MoreFormFields):
    pass

class AfricanSwallowForm(forms.ModelForm, SwallowFormFields):
    class Meta:
        model = AfricanSwallow

那么它有什么作用呢？

元类收集在 MixinForm 中声明的所有字段

然后添加自定义 __init__构造函数，以确保 __init__ MixinForm 的方法被神奇地调用。 (否则你将不得不显式调用它。)

MixinForm.__init__将声明的字段复制到字段属性

请注意，我既不是 Python 大师也不是 django 开发人员，而且元类是危险的。因此，如果您遇到奇怪的行为，最好坚持使用上面更详细的方法:)

祝你好运!