如何仅当对象存在时才在Django模型表单中添加额外的字段？

from django import forms
from .models import YourModel

class YourModelForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super(YourModelForm, self).__init__(*args, **kwargs)
        if self.instance:
            # 在对象存在时添加额外的字段
            self.fields['extra_field'] = forms.CharField()

    class Meta:
        model = YourModel
        fields = '__all__'