In order to be sure I force users to pick a valid value from a dropdown (rather than unknowingly leaving the first option in the list set without changing it to the correct value), I am inserting the blank choice field in many required form fields.

为了确保强制用户从下拉列表中选择一个有效的值(而不是无意中保留列表集中的第一个选项而不将其更改为正确的值)，我在许多必填表单域中插入了空白的选项域。

models.py

Models.py

class MyModel(models.Model):
    gender = models.CharField(max_length=1, null=True, blank=False, choices=GENDER_CHOICES, default='')

Original forms.py (Explicitly defining the form field. This works.)

原始forms.py(显式定义表单域。这是可行的。)

from django.db.models.fields import BLANK_CHOICE_DASH
class MyForm(forms.ModelForm):
    gender = forms.TypedChoiceField(required=True, choices=BLANK_CHOICE_DASH+MyModel._meta.get_field('gender').choices)
    class Meta:
        model = MyModel
        fields = '__all__'

Now, because I am doing this for many fields, I tried to revise my earlier code in forms.py:

现在，因为我要对许多字段执行此操作，所以我尝试在forms.py中修改前面的代码：

Revised forms.py (Setting the choices in __init__(). Does not work.)

修订了forms.py（在__init__（）中设置选项。不起作用）。

class MyForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        for type in ('gender', ...):
            self.fields[type].choices.insert(0, (u'',u'---------' ))
    # Not needed anymore.. replaced by code in __init__()
    # gender = forms.TypedChoiceField(required=True, choices=BLANK_CHOICE_DASH+MyModel._meta.get_field('gender').choices)
    class Meta:
        model = MyModel
        fields = '__all__'

In this case, I do not get the dashes as the first choice in the dropdown.

在本例中，我没有将破折号作为下拉列表中的第一选择。

I tried to look into the problem by using pdb in the template to inspect the form field right before it was outputted. The debugger shows element.field.choices to be (u'', u'---------'), ('f', 'Female'), ...] in both cases. Nevertheless, the code that outputs the form field inserts the dashes only in the original code, not in the revised code.

我试图通过在模板中使用PDB在输出之前检查表单域来研究这个问题。调试器显示element.field。选项为(u‘’，u‘-’)，(‘f’，‘Female’)，...]在这两种情况下。但是，输出表单域的代码只在原始代码中插入破折号，而不在修订后的代码中插入。

I tried stepping through the Django code that renders the form field to figure out if it was using some other field besides .choices that I'm not aware of, but I never stepped into the right part of the code.

我尝试单步执行呈现表单域的Django代码，以确定它是否使用了我不知道的其他字段，但我从未进入代码的正确部分。

How can I accomplish this? Although it works fine the first way, the revised code is much more DRY. If only I could make it work!

我怎样才能做到这一点呢？尽管第一种方法运行良好，但修订后的代码要枯燥得多。要是我能让它起作用就好了！

This appears to be a bug in ChoiceWidget.__deepcopy__().

这似乎是ChoiceWidget.__DeepCopy__()中的错误。

These work:

这些工作包括：

# self.fields[name].choices.insert(0, (u'', u'---------'))
self.fields[name].widget.choices.insert(0, (u'', u'---------'))
# or
self.fields[name].choices = BLANK_CHOICE_DASH + self.fields[name].choices

Explanation

TypedChoiceField.choices, inherited from ChoiceField, is a property:

从ChoiceField继承而来的TyedChoiceField.Chotions是一个属性：

class TypedChoiceField(ChoiceField):

class ChoiceField(Field):
    ...

    def _get_choices(self):
        return self._choices
    
    def _set_choices(self, value):
        # Setting choices also sets the choices on the widget.
        # ...
        if callable(value):
            value = CallableChoiceIterator(value)
        else:
            value = list(value)
    
        self._choices = self.widget.choices = value
    
    choices = property(_get_choices, _set_choices)

• _set_choices() assigns self.widget.choices in addition to self._choices.
  
  
  
  • The widget choices is the one used in the rendering.
  
  
  • The widget choices is supposed to be the same instance as that in the field.
  
  
  
  

The field and widget are deepcopy'ed as follows:

字段和小工具的深度复制如下：

class ChoiceField(Field):
    widget = Select
    ...

    def __deepcopy__(self, memo):
        result = super().__deepcopy__(memo)
        result._choices = copy.deepcopy(self._choices, memo)
        return result

class Select(ChoiceWidget):

class ChoiceWidget(Widget):
    ...

    def __deepcopy__(self, memo):
        obj = copy.copy(self)
        obj.attrs = self.attrs.copy()
        obj.choices = copy.copy(self.choices)
        memo[id(self)] = obj
        return obj

• ChoiceWidget.__deepcopy__() does not memoize the copy of self.choices.
  
  
  
  • The copy of choices is then unavailable to the deepcopy in ChoiceField.
  
  
  • The widget choices is no longer the same instance as that in the field.