Django迁移和可分解的字符串 [英] Django migrations and deconstructible string

问题描述

我必须创建一个实例，该实例必须满足两个条件：

I have to create a class which instances must fit two conditions:

• 成为 str 子类，以便可以将其传递给 os.listdir（）


• 是可分解的，因此不会出现字符串django生成迁移时保持原样，但 mailing.conf.StrConfRef（'another string'）

• being an str subclass so that it can be passed to os.listdir()
• being deconstructible so that the string does not appear as-is when django generates migrations, but as mailing.conf.StrConfRef('another string')

这是我尝试的内容：

class StrConfRef(str):

    def __new__(cls, name, within=None):
        value = globals()[name]
        if within:
            value = within.format(value)
        self = str.__new__(cls, value)
        self.name = name
        self.within = within
        return self

    def deconstruct(self):
        return ('{}.{}'.format(__name__, self.__class__.__name__), (self.name,),
                {'within': self.within})

尊重第一点 os.list dir（StrConfRef（...））有效。但是，在迁移中，它仍被视为标准字符串。我签出 django.db.migrations.autodetector 并注意到，执行代码后，它的StrConfRef实例到达此行（这是预期的，应该意味着StrConfRef已正确解构）。

The first point is respected os.listdir(StrConfRef(...)) works. However, it is still evaluated as a "standard string" in migrations. I checked out django.db.migrations.autodetector and noticed that when the code is executed, it StrConfRef instances reach this line (which is expected, and should mean that StrConfRef is properly deconstructed).

所以我想知道为什么它在我的迁移中显示为字符串，而不是mailing.conf.StrConfRef实例。以及如何满足我的条件。

So I wonder why it appears as a string in my migrations, and not a mailing.conf.StrConfRef instance. And how to fulfill my conditions.

PS：如果您想知道为什么我需要这种行为，请结帐这个问题。

PS: If you wonder why I need this behavior, checkout this question.

PS2：我运行的是Python 3.4和Django 1.9.2

PS2: I'm runnign Python 3.4 and Django 1.9.2

推荐答案

不幸的是，它看起来像具有 deconstruct（）方法的对象优先级低于 str 子类。

Unfortunately it looks like object that have a deconstruct() method don't have priority over str subclasses.

在这里您可以使用 django.db .migrations.writer.SettingsReference 类，它看起来比 str 具有更高的优先级。

What you could could here is use the django.db.migrations.writer.SettingsReference class which looks like it has a priority over str.

我建议您改为创建一个自定义字段子类，该子类将默认为您的 conf 值。例如，您的 Campaign.prefix_subject 可能是此类的实例：

What I suggest you do instead is create a custom field subclass that will default to your conf value. For example, your Campaign.prefix_subject could be an instance of this class:

class PrefixSubject(models.BooleanField):
    default_help_text = (
        'Wheter to prefix the subject with "{}" or not.' % conf.SUBJECT_PREFIX
    )

    def __init__(self, *args, **kwargs):
        kwargs.setdefault('help_text', default_help_text)
        super().__init__(*args, **kwargs)

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        if kwargs['help_text'] == self.default_help_text:
            kwargs.pop('help_text')
        return name, path, args, kwargs

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