无法将用户定义函数的输入传递给其他函数，并对返回的值求和

Question

I'm a N00b learning Python. As part of a small project to create a simple calculator I need a function to take two inputs and determine total costs for a trip.

At the bottom of this code, I have a function called "trip_cost()" that takes two inputs, city and days. City is a string of the city you are going to visit and days is an integer value of days you are staying.

trip_cost() pass the inputs to rental_car_cost, hotel_cost, and plane_ride_cost functions and return a sum of their respective outputs.

I understand that a function can be called from another function as a variable by name but I'm confused as to how I should process the input from trip_cost() into the other functions and return three values to be summed within trip_cost().

Any advice would be greatly appreciated.

Thanks,

-- Matt

def hotel_cost(n):
    nights = n * 140
    return nights

hotel_cost(3)

def plane_ride_cost(c):
    if c =="Charlotte":
        return 183
else:
    if c == "Tampa":
        return 220
    else:
        if c == "Pittsburgh":
            return 222
        else:
            if c == "Los Angeles":
                return 475

plane_ride_cost("Tampa")

def rental_car_cost(d):
    days = 40
    if d >= 7:
    return d * days - 50
elif d >= 3:
    return d * days - 20
else:
    return d * days

rental_car_cost(3)

def trip_cost(c,d):
    return sum (hotel_cost) + sum(rental_car_cost) + sum (plane_ride_cost)

trip_cost("Tampa",3)

Answer 1

I've created a "better" version of yours, sice you are learning python, I've put a lot of things here:

def plane_ride_cost(city):  # First, in this case, I used a dictionary
    return {                # to get a value of a city
        'Charlotte': 183,
        'Tampa': 220,
        'Pittsburgh': 222,
        'Los Angeles': 475
    }[city]

def hotel_cost(nights):     # You can always use expressions in
    return nights * 140     # the return statement

def rental_car_cost(days):  # If it is possible, use long variable
    cost = 40               # names, not just one letters -> readability counts
    if days >= 7:
        return days * cost - 50
    elif days >= 3:
        return days * cost - 20
    else:
        return days * cost

def trip_cost(city, days):  # You can just add the values together, that is SUM
    return hotel_cost(days) + rental_car_cost(days) + plane_ride_cost(city)

Demo:

print trip_cost('Tampa', 3) # Call the 'main' function, with variables

And this will return:

# 740

Answer 2

You don't really need sum() here because the following would work and require little overhead:

def trip_cost(c, d):
    return hotel_cost(d) + rental_car_cost(d) + plane_ride_cost(c)

However, you could use sum() with something like the following, which creates a tuple of the results of calling each cost function, and then pass that object to sum() to be totaled.

def trip_cost(c, d):
    return sum((hotel_cost(d), rental_car_cost(d), plane_ride_cost(c)))

Another alternative would be to use a generator expression , but it would require each of the functions to accept the same input arguments even if they didn't use both of them. If they were rewritten to be that way, it would allow you to do something along these lines:

def trip_cost(c, d):
    cost_functions = (hotel_cost, rental_car_cost, plane_ride_cost)
    return sum(cf(c, d) for cf in cost_functions)

With this, the calls to each cost function would occur at the same time sum() was executing. Something like that might be useful if you wanted the tuple of functions to vary, in which case you could pass them to the trip_cost() function as an additional argument instead of hardcoding them. However based on your current code and apparent needs, the first way would probably be the best way to go.

Answer 3

Resolve:

def hotel_cost(nights):
    return 140*nights

def plane_ride_cost(city):
    if city=='Charlotte':
        return 183
    elif city=='Tampa':
        return 220
    elif city=='Pittsburgh':
        return 222
    elif city=='Los Angeles':
        return 475

def rental_car_cost(days):
    day = 40
    cost = days*day
    if days >= 7:
        cost = cost - 50
    elif days >= 3:
        cost = cost - 20
    return cost

def trip_cost(city,days):
    city = plane_ride_cost(city)
    return city+rental_car_cost(days)+hotel_cost(days)