问Django十进制精度损失

EN

worth = Decimal(request.POST.get('worth'))
print(request.user.profile.cash) # -> prints 10000.000000000000000
print(worth) # -> prints 10000
Profile.objects.filter(user=request.user).update(cash=F('cash')-worth)
request.user.profile.refresh_from_db()
if request.user.profile.cash < 0:
     ###!!!!####
     print(request.user.profile.cash) # -> -1.819E-12

#model definition:
class Profile(models.Model):
    cash = models.DecimalField(default=10000, max_digits=30, decimal_places=15)