驱动成本 - python 中的函数 - 如果 __name__ == '__main__' 中的 EOFerror：

Question

I am stuck with this question:

Write a function driving_cost() with input parameters miles_per_gallon, dollars_per_gallon, and miles_driven, that returns the dollar cost to drive those miles. All items are of type float. The function called with arguments (20.0, 3.1599, 50.0) returns 7.89975.

Define that function in a program whose inputs are the car's miles per gallon and the price of gas in dollars per gallon (both float). Output the gas cost for 10 miles, 50 miles, and 400 miles, by calling your driving_cost() function three times.

Output each floating-point value with two digits after the decimal point, which can be achieved as follows: print(f'{your_value:.2f}')

Ex: If the input is:

20.0 3.1599 the output is:

1.58 7.90 63.20 Your program must define and call a function: def driving_cost(miles_per_gallon, dollars_per_gallon, miles_driven)

The question also gets evaluated using a direct call of the function like this: driving_cost(20.0,3.1599, 50.0) which is the part that is broken.

Here is my code so far:

def driving_cost(miles_per_gallon, dollars_per_gallon, miles_driven):
    return dollars_per_gallon/miles_per_gallon*miles_driven
    
miles_per_gallon=float(input())
dollars_per_gallon=float(input())

miles=[10, 50, 400]
for i in miles:
    print(f'{driving_cost(miles_per_gallon, dollars_per_gallon, i):.2f}')

if __name__ == '__main__':
    miles_driven=float(input());
    cost=driving_cost(miles_per_gallon, dollars_per_gallon, miles_driven);
    print(f'{cost:.2f}');

If I just input values, the top section executes but if I call the function like this driving_cost(20.0,3.1599, 50.0) I get:

Traceback (most recent call last):
  File "main.py", line 4, in <module>
    miles_per_gallon=float(input())
ValueError: could not convert string to float: 'driving_cost(20.0, 3.1599, 50.0)' 

I don't completely grasp the if __name__ == '__main__': thing either but I really don't understand why this error is occurring. Even when I include the other inputs in the if __name__ == '__main__': part of the program, I get that error. Help please?

Answer 1

The traceback is telling you the problem happens on line 4 and shows you the line that caused the problem:

miles_per_gallon=float(input())

Even though you probably added driving_cost(20.0,3.1599, 50.0) somewhere further down, you still have this line in your code and it still takes input, which still has to be a string representing a valid floating point number, or you will get that error.

As for the if __name__ == '__main__': part: your script gets executed top to bottom when you run it. But if your script contains some useful functions you'd like to use from some other script, you may way to import my_script or from my_script import driving_cost .

However, whenever you import a module, Python actually runs the module. If all the code in the module lives inside functions, that's OK, nothing gets run until a function gets called. But your script does execute all sorts of code.

That's where if __name__ == '__main__': comes in. Every script has a __name__ variable defined. If your script is the main script being run by Python, its __name__ will be '__main__' . So, that line basically says "if this script is being run as the main script, run some code, otherwise (if this script is being run as a module) don't".

As a result, this would be a better way to organise your script:

def driving_cost(miles_per_gallon, dollars_per_gallon, miles_driven):
    return dollars_per_gallon / miles_per_gallon * miles_driven


def input_compute_and_print():
    miles_per_gallon = float(input())
    dollars_per_gallon = float(input())

    miles = [10, 50, 400]
    for i in miles:
        print(f'{driving_cost(miles_per_gallon, dollars_per_gallon, i):.2f}')


def print_example():
    cost = driving_cost(20.0,3.1599, 50.0)
    print(f'{cost:.2f}')


if __name__ == '__main__':
    input_compute_and_print()
    # print_example()

Note that this is just your own code reorganised (without the later changes). By commenting out the calls in the bottom section, you can get separate parts to run.