为什么UserCreationForm在Django的模型类中明确定义了用户名字段？

我正在查看

django.contrib.auth.forms

我注意到以下情况：

class Meta:
            model = User
            fields = ("username",)

为什么需要在

字段

中明确提到这一点（“用户名”）。因为在

用户创建表单

中再次定义了

用户名

字段。为什么？为什么上面的

元类定义中不包括
密码字段

class UserCreationForm(forms.ModelForm):
    """
    A form that creates a user, with no privileges, from the given username and
    password.
    """
    error_messages = {
        'duplicate_username': _("A user with that username already exists."),
        'password_mismatch': _("The two password fields didn't match."),
    }
    username = forms.RegexField(label=_("Username"), max_length=30,
        regex=r'^[\w.@+-]+$',
        help_text=_("Required. 30 characters or fewer. Letters, digits and "
                      "@/./+/-/_ only."),
        error_messages={
            'invalid': _("This value may contain only letters, numbers and "
                         "@/./+/-/_ characters.")})
    password1 = forms.CharField(label=_("Password"),
        widget=forms.PasswordInput)
    password2 = forms.CharField(label=_("Password confirmation"),
        widget=forms.PasswordInput,
        help_text=_("Enter the same password as above, for verification."))

    class Meta:
        model = User
        fields = ("username",)

    def clean_username(self):
        # Since User.username is unique, this check is redundant,
        # but it sets a nicer error message than the ORM. See #13147.
        username = self.cleaned_data["username"]
        try:
            User._default_manager.get(username=username)
        except User.DoesNotExist:
            return username
        raise forms.ValidationError(
            self.error_messages['duplicate_username'],
            code='duplicate_username',
        )

    def clean_password2(self):
        password1 = self.cleaned_data.get("password1")
        password2 = self.cleaned_data.get("password2")
        if password1 and password2 and password1 != password2:
            raise forms.ValidationError(
                self.error_messages['password_mismatch'],
                code='password_mismatch',
            )
        return password2

    def save(self, commit=True):
        user = super(UserCreationForm, self).save(commit=False)
        user.set_password(self.cleaned_data["password1"])
        if commit:
            user.save()
        return user

fields
属性之所以存在，是因为如果没有它，该模型中的所有字段都会显示在表单上（ModelForm的默认行为是在没有“fields”或“exclude”属性的情况下显示模型中的所有字段）

password
字段不在其中，因为表单上显示的密码字段实际上不是存储在模型中的密码字段——模型中的字段是散列密码，而表单上显示的字段是普通文本字段。因此，处理此表单的代码采用这些文本密码，确保它们相同，然后创建“真实”密码并将其存储在模型中