未填充Django下拉列表

# forms.py
from django import forms

class MyForm(forms.Form):
    my_choices = (
        ('option1', '选项1'),
        ('option2', '选项2'),
        ('option3', '选项3'),
    )
    my_field = forms.ChoiceField(choices=my_choices, widget=forms.Select(attrs={'class': 'form-control'}))

# views.py
from django.shortcuts import render
from .forms import MyForm

def my_view(request):
    form = MyForm()
    return render(request, 'my_template.html', {'form': form})

# my_template.html
<form method="post">
  {% csrf_token %}
  {{ form.my_field }}
  <input type="submit" value="提交">
</form>