Python中的面向对象编程

Question

I am defining a class called 'car' I am comparing it to a document that runs a series of tests on my class. However, I am getting errors, and I am not sure why. Here's my code. The Drive method is supposed to take the car and move it a specified amount of miles. If the car can achieve all the miles with the given amount of fuel, then the car makes the trip and outputs the miles. If it can't, it goes the maximum amount of miles it can. The addFuel method is supposed to add fuel to the car, but if it overflows it doesn't add any fuel, and if the parameter specified isn't an integer, or isn't a positive value, it's supposed to throw an exception. And the tripRange module is supposed to be given the amount of gallons in the car, and determine how many miles you will travel. Below I have posted my code first, and then the code to test it out. Can you help me? I appreciate it.

#Define the class


class Car(object):

    def __init__(self,fuelEfficiency=0,fuelCapacity=0,fuelLevel=0,odometer=0):
        self.setCar(fuelEfficiency,fuelCapacity,fuelLevel,odometer)

    def setFuelEfficiency(self,newFuelEfficiency):
        self.setCar(fuelEfficiency = newFuelEfficiency)

    def setFuelCapacity(self,newFuelCapacity):
        self.setCar(fuelCapactity = newFuelCapacity)

    def setFuelLevel(self,newFuelLevel):
        self.setCar(fuelLevel = newFuelLevel)

    def setOdometer(self,newOdometer):
        self.setCar(odometer = newOdometer)

    def setCar(self,fuelEfficiency = None,fuelCapacity = None,fuelLevel = None,odometer = None):
        if fuelEfficiency == None:
            fuelEfficiency = self.getFuelEfficiency

        if fuelCapacity == None:
            fuelCapacity = self.getFuelCapacity

        if fuelLevel == None:
            fuelLevel = self.getFuelLevel

        if odometer == None:
            odometer = self.getOdometer

        self.fuelEfficiency = fuelEfficiency
        self.fuelCapacity = fuelCapacity
        self.fuelLevel = fuelLevel
        self.odometer = odometer

    def drive(self,miles):
        if miles < 0:
            return ("The car is not driven")

        milesDriven = miles/self.fuelEfficiency

        if milesDriven < self.fuelLevel:
            print("The car drove {} miles".format(miles))
        else:
            if self.fuelLevel == 0:
                print("The car drove 0 miles")
            else:
                newMiles = milesDriven * miles
                print("The car drove {} miles".format(newMiles))

        self.fuelLevel = self.fuelLevel - milesDriven
        self.odometer += miles

    def getCar(self):
        #Returns a tuple that has (FE,FC,FL,OD)
        return (self.fuelEfficiency,self.fuelCapacity,self.fuelLevel,self.odometer)        


    def addFuel(self,num):
        if type(num) == str:
            raise KeyError("String valued enter, an integer was expected.")
        if num < 0:
            print("Sorry, you need to enter a postive number.")
        if num + self.fuelLevel > self.fuelCapacity:
            return self.fuelLevel
        else:
            return self.fuelLevel + num

    def getFuelEfficiency(self):
        return self.getCar()[0]

    def getFuelCapacity(self):
        return self.getCar()[1]

    def getFuelLevel(self):
        return self.getCar()[2]

    def getOdometer(self):
        return self.getCar()[3]

    def tripRange(self):
        numOfMiles = self.fuelEfficiency 
        return numOfMiles

    def __str__(self):
        FE = self.getFuelEfficiency()
        FC = self.getFuelCapacity()
        FL = self.getFuelLevel()
        OD = self.getOdometer()

        string = '{}:{}:{}:{}'.format(FE,FC,FL,OD)
        return string

And here's the test code:

from car import *

def checkCar(car, expected, message):
    global errorsFound
    mpg, cap, level, odo = expected
    if car.getFuelEfficiency() != mpg:
        errorsFound = True
        print(message + ': Error efficiency. Expected ' + str(mpg))
        print('\tCar:', car)
    if car.getFuelCapacity() != cap:
        errorsFound = True
        print(message + ': Error capacity. Expected ' + str(cap))
        print('\tCar:', car)
    if car.getFuelLevel() != level:
        errorsFound = True
        print(message + ': Error level. Expected ' + str(level))
        print('\tCar:', car)
    if car.getOdometer() != odo:
        errorsFound = True
        print(message + ': Error odometer. Expected ' + str(odo))
        print('\tCar:', car)

def checkNum(value, expected, message):
    global errorsFound
    if value != expected:
        errorsFound = True
        print(message + ': Error value. Expected {}. Got {}'.format(expected, value))

def main():
    c = Car(25, 15)
    checkNum(c.tripRange(), 0, 'Test 1')    

    expected = (25, 15, 0, 0)
    checkCar(c, expected, 'Test 2')

    c.addFuel(-1)
    checkCar(c, expected, 'Test 3')

    c.addFuel(1000)
    checkCar(c, expected, 'Test 4')

    c.addFuel('doctor')
    checkCar(c, expected, 'Test 5')

    c.addFuel(0)
    checkCar(c, expected, 'Test 6')

    c.addFuel(15)
    expected = (25, 15, 15, 0)
    checkCar(c, expected, 'Test 7')

    c.drive(50)
    expected = (25, 15, 13, 50)
    checkCar(c, expected, 'Test 8')

    c.drive(100000)
    expected = (25, 15, 0, 375)
    checkCar(c, expected, 'Test 9')

    c.drive(5)
    expected = (25, 15, 0, 375)
    checkCar(c, expected, 'Test 10')

    c.addFuel(10)
    expected = (25, 15, 10, 375)
    checkCar(c, expected, 'Test 11')

    c.drive(-1)
    expected = (25, 15, 10, 375)
    checkCar(c, expected, 'Test 12')

    c.drive(0)
    expected = (25, 15, 10, 375)
    checkCar(c, expected, 'Test 13')

    checkNum(c.tripRange(), 250, 'Test 14')

    if not errorsFound:
        print('No Errors Found')



errorsFound = False
main()

Answer 1

Your code works as expected. Altough I found a number of indentation errors in the car.py file So that may be why. specifically the __init__ method for the Car object you have def__init__(..) , theres no space.

If your talking about the exception that get thrown at you in the terminal output:

Test 1: Error value. Expected 0. Got 25
Sorry, you need to enter a postive number.
Traceback (most recent call last):
  File "/home/crispycret/Desktop/testcase.py", line 89, in <module>
    main()
  File "/home/crispycret/Desktop/testcase.py", line 42, in main
    c.addFuel('doctor')
  File "/home/crispycret/Desktop/car.py", line 64, in addFuel
    raise KeyError("String valued enter, an integer was expected.")
KeyError: 'String valued enter, an integer was expected.'
[Finished in 0.1s with exit code 1]

That's to be expected. As for the reason in the main() function in your test.py on line 42, your trying to do c.addFuel('doctor') , where inside the Car.addFuel() method you raise KeyError("String valued enter, an integer was expected.") when the type of input is a string.

By the way you should use proper exceptions when raising errors, in this case you should use a TypeError("String valued enter, an integer was expected.") to express that the variable type was wrong.

https://docs.python.org/2/tutorial/errors.html

Answer 2

Python is not Java, so don't write like it is.

Getters and Setters are often not needed, so don't add them, you can always do it easily later if needed. There is no enforced instance member protection either, but the underscore in self._odometer signals to the reader that _odometer is private.

Abandon type declaration and embrace duck-typing. In addFuel() you check to see if the argument is of type str and proceed if it isn't. However, the argument could be of type file, module, dict, list, and so on. You can't check them all, so it is best not to try.

Of what remains, drive() is broken, but now obviously so because so much junk has been removed and tripRange() was redundant. This leaves one working method addFuel and it is hard to make the case for testing two simple lines of code.

class Car(object):
    def __init__(self, efficiency=0, capacity=0, fuel=0, odometer=0):
        self.efficiency = efficiency
        self.capacity = capacity
        self.fuel = fuel
        self.odometer = odometer

    # four useless setters removed

def drive(self, miles):
    # argument test removed

    # miles / (miles / 1 gallon) = miles * (1 gallon / miles) = 1 gallon
    # so this expression and thus method are broken
    one_gallon = miles / self.efficiency

    if one_gallon < self.fuel:
        print("The car drove {} miles".format(miles))
    elif self.fuel == 0:
            print("The car drove 0 miles")
    else:
       newMiles = milesDriven * miles
       print("The car drove {} miles".format(newMiles))

    self.fuel -= one_gallon
    self.odometer += miles      

def addFuel(self, volume):
    # Even more argument tests removed.
    if volume + self.fuel <= self.capacity:
        self.fuel += volume

# The method "tripRange()" is equivalent to "car.efficiency" so is removed

def __str__(self):
    # trivial getters don't exist; the format template is now conventional
    return '<Car {}, {}, {}, {}>'.format(self.efficiency, self.capacity,
        self.fuel, self.odometer)

You are getting errors in your tests probably because 2/3 of your methods were broken, not even considering the peculiar getCar() and all the trivial setters and getters. You need to write much less code than you have.