Python: if more than one of three things is true, return false

Question

I'm writing a django model that allows my site to have coupons.

Coupons can have three types: lifetime account voucher, certain period of months voucher, certain number of dollars voucher.

To keep things simple, I'm only allowing coupons to have one of the three possible values (i.e. a voucher can't be for $10 and 5 months). But I want to check when a coupon is being saved to ensure this rule is true.

Currently I have:

true_count = 0
if self.months:
    true_count += 1
if self.dollars:
    true_count += 1
if self.lifetime:
    true_count += 1    

if true_count > 1:
    raise ValueError("Coupon can be valid for only one of: months, lifetime, or dollars")  

I know there's a better way to do this, but I'm not seeing it (call it coder's block).

Help is much appreciated.

In case it maters, the three types are int, int, and bool

months = models.IntegerField(default=0)
cents = models.IntegerField(default=0)
#dollars = models.FloatField(default=0.00)
#dollars replaced with integer cents per advice of group
lifetime = models.BooleanField(default=False)

Answer 1

One thing I've done in similar situations is this:

coupon_types = (self.months, self.dollars, self.lifetime,)

true_count =  sum(1 for ct in coupon_types if ct)
if true_count > 1:
    raise ValueError("Coupon can be valid for only one of: months, lifetime, or dollars")  

It's now much easier to add new coupon types to check for in the future!

Answer 2

You could also use a list comp to filter false values:

if len([x for x in [self.months, self.dollars, self.lifetime] if x]) > 1:
    raise ValueError()

Or building off MRAB's answer:

if sum(map(bool, [self.months, self.dollars, self.lifetime])) > 1:
    raise ValueErrro()