title: Why does random in django rest serializer always return the same field?
body: <p>I have a simple serializer</p>
  <pre><code>class Serializer(serializers.Serializer):
      name = serializers.CharField(required=False, default='someName')
      id = serializers.IntegerField(required=False, default=random.randrange(100, 200))
  </code></pre>
  <p>And when I create multiple instances of the serializer it always returns the same field</p>
  <pre><code>a = Serializer(data={})
  a.is_valid(data={})
  data = a.data
  data['id'] // return  for example 150
   
  b = Serializer(data={})
  b.is_valid(data={})
  b_data = b.data
  b_data['id'] // return also 150
  </code></pre>
  <p>Why it happens? How to get rid of this?</p>