title: unsupported operand type(s) for /: ‘map’ and 'int’
body: <pre><code>##models.py
      from django.db import models
      from django.contrib.auth.models import User
      import numpy as np
  
      class Wine(models.Model):
          name=models.CharField(max_length=200)        
          def average_rating(self):
              all_ratings = map(lambda x: x.rating, self.review_set.all())
              return np.mean(all_ratings)  
          def __unicode__(self):
              return self.name
  </code></pre>
  
  <p><em>Why is this error showing?What is wrong at line 7?</em>   </p>