Not sure about python function

Question

I am only starting and getting mad over this function (it gives me the wrong outputs):

def rental_car_cost(days):
 x = 40
 if days < 2:
 return days*x
 elif days >= 3:
 return days*x-20
 elif days>= 7:
 return days*x-50
 else:
 print "Please enter nr of days"

Also, how do I make sure that a number is entered for "days"?

• Henry Keiter: Give us an example of actual vs expected outputs, and explain how the function is supposed to behave. Also, ask one question per question, please.
• aaaa: rental_car_cost(2) should equal 60, but it gives me "please enter nr of days"
• twalberg: @aaa Well, 2 is not less than 2, nor is it greater than or equal to either 3 or 7, so it is only doing what you asked it to...

Answer 1

Not sure what you are expecting, however change the order of the elif conditions:

def rental_car_cost(days):
 if isinstance(days, int):
 x = 40
 if days < 2:
 return days*x
 elif days >= 7:
 return days*x-50
 elif days>= 3:
 return days*x-20
 else:
 print "Please enter nr of days"

• Ankit Jaiswal: Consider days = 15, its greater than 7 but will be caught into the previous elif days >= 3: block in your case.
• xgord: it's because any number greater than 7 is also greater than 3 so it satisfies the condition x >= 3. you want x >= 3 and x < 7

Answer 2

The days>= 7 and else clauses never trigger, because the earlier days >= 3 triggers on the same inputs. if/elif/else clauses are processed in order until one of them is triggered.

What you need are clauses for days < 2, days < 7 and else.

To detect non-numbers, start with

if not isinstance(days, int):

which does a type check for integers.

• arshajii: This is python 2.7, so it might be better to do isinstance(days, (int, long)). Although in this context it shouldn't matter I suppose.
• Fred Foo: @arshajii: buying a car is usually a better idea than renting it for more than 2**31-1 days :)
• arshajii: I was thinking for the sake of completeness, but touché :)

Answer 3

rental_car_cost(2) should equal 60

But, none of your if statements will match 2. 2 isn't less than 2, nor is it greater than or equal to 3, nor is it greater than or equal to 7. Follow the advise from the other two answers by larsmans and Ankit Jaiswal also, but I'm assuming 2 should match the days*x-20 part. Just change elif days >= 3: to elif days >= 2:.

• aaaa: thank you. should have spotted that on my own although I was looking for the mistake for some time.
• kirbyfan64sos: @aaaa: Whenever you look for a mistake for a while, you end up missing it. It happens to me all the time.