根据另一个属性设置属性中的默认值

// HTML
<select id="gender">
  <option value="male">男性</option>
  <option value="female">女性</option>
</select>
<input type="number" id="age">

// JavaScript
const genderSelect = document.getElementById('gender');
const ageInput = document.getElementById('age');

genderSelect.addEventListener('change', function() {
  if (genderSelect.value === 'male') {
    ageInput.value = 18;
  } else if (genderSelect.value === 'female') {
    ageInput.value = 20;
  }
});

from django.db import models

class Person(models.Model):
    GENDER_CHOICES = [
        ('male', '男性'),
        ('female', '女性'),
    ]
    gender = models.CharField(max_length=6, choices=GENDER_CHOICES)
    age = models.IntegerField(default=18)

    def save(self, *args, **kwargs):
        if self.gender == 'female':
            self.age = 20
        super().save(*args, **kwargs)