如何从django select小部件中删除------

如何删除我的django modelform widgts中的-----? 文档说明使用空的\标签,但它用于SelectDateWidget

我的窗体

class ProjectForm(forms.ModelForm):

    class Meta:
        model = Project
        exclude = ('copy', 'created', 'researcher', 'keywords', 'application_area', 'predominant_area')

        widgets = {
            'title':forms.TextInput(attrs={
                'class':'form-control',
                'placeholder': 'Titulo da oportunidade'
            }),
            'conclusion_date':forms.TextInput(attrs={
                'class': 'form-control',
                'type':'text',
                'placeholder':'Data de conclusão'
        }),
            'category': forms.RadioSelect(attrs={
                'class':'form-control'
        }),
            'result':forms.Select(attrs={
                'class':'form-control'
        }),
     }