sample.py

class Car(models.Model):
    RED = 'r'
    WHITE = 'w'
    BLUE = 'b'

    COLOR_CHOICES = (
        (RED, 'red'),
        (WHITE, 'white'),
        (BLUE, 'blue'),
    )

    color = models.CharField(max_length=5, choices=COLOR_CHOICES, default=RED)
    
>>> Car.objects.filter(color=Car.RED)